Moving into widefield astrophotography

[Martin Pugh]

Hello everyone
best place I thought to pose this question would be in IIS given the amount of very talented DSLR users.

I am going to buy the 90mm Solarmax II scope (once Meade ships), and while I look forward to observing with this, I will certainly want to take photos.
Now - I also have a very nice f6.6 500mm Borg 76ED scope, which is redundant, and can be reduced to 430mm/f5. The reducer is designed for DSLRs.

Of course - I dont quite yet understand how I will attach a camera to the Solarmax, but I want to be able to use the camera interchangeably between solar and deep-sky/widefield imaging.

Been looking at a lot web sites - what would make the Canon 550D at just $880 a bad choice? Afterall, it seems to have all of the same capabilities, except perhaps it is not modded to be more sensitive at 656nm. It also has very nice 4.3micron pixels.

My other thoughts were the ST8300 OSC - makes things nice and light, and its cooled.

Grateful for some suggestions.

cheers
Martin

[renormalised]

Even though it is quite a bit dearer, I'd go for the OSC. Why, because it has a better dynamic range spectrum wise, better QE, far less hot pixels, less dark current, higher full well and it's already cooled. Pixels aren't as small as the DSLR's but you'll be able to use it on a wider range of scopes than the DSLR because of that. One good thing about the DSLR....you can run it off batteries. The ST needs a power supply, which means extra cables/cords to deal with.

To be very useful for solar observing, you'll have to mod the 550D so it can pick up the Ha from the Sun, which means extra money spent and more time wasted waiting for it to be modded.

Mind you, 18 megs a pickle versus 8.3, it's a bit of a choice!!!.

[seeker372011]

umm i dont think you need a modded dslr to image through a narrow band telescope
But i dont see many people doing that,the dmk or similar seems to be the preferred instrument ...large number of frames etc

Dont know if you can get a full disc even with a dslr?

[Paul Haese]

Martin,

Monochrome for the solar scope. Colour is useless through one of these scopes. You really need something like a 41DMK or one of the smaller SBIG, FLI or QSI cameras.

For the Borg, why not stick with monochrome and filters? If it is easy of use then a OSC is going to be better overall than a DSLR. However, there is some really nice work being done with a DSLR, but it would need modding and cooling.

Hope that helps.

[avandonk]

What do you define as wide field Martin? Three degrees? Seven degrees or far higher?

I have been battling with all the various parameters for years to get decent resolution wide fields.

The combination of big sensor with shorter FL very high quality optic. This leads to vignetting and CA and astigmatism due to off axis aberrations. We are trying to beat the laws of Physics here and will not ever win. This can only be overcome by careful processing and control by fine tuning of the light that actually enters the system.

Very short focal length optic will limit the resolution.

There is no easy answer.

All the methods I have worked out for wide fields where the stars are inherently undersampled seem to be getting somewhere.

If the weather permits will apply these methods to data at about 90mm FL and see what happens.

Welcome to the matrix! Take the blue (OSC) or the red (Mono) sensor?

Bert

[Phil Hart]

hey martin

sounds like fun!

for the solar stuff, i'll defer to the high-res imaging experts, but i assume that you'd get higher res results from a video solution rather than DSLR but i you trade that against a much wider field of view with the DSLR. not sure if the large format video mode on the DSLR is sensitive enough for that kind of work but it would sure result in a lot of data!

for the widefield stuff, the biggest consideration is probably whether you plan to use this mainly at a powered/permanent site or whether you want it to be easily portable and quick to setup and run when you're away camping somewhere. if the former, then i would also lean towards OSC CCD. if the latter, then DSLR has some more appeal.

of course the small pixels are attractive for the high res solar work, but less so for SNR characteristics on the faint fuzzies. will be interested to follow your choice and hear how it goes.

Phil

[ptc]

why not use your standard imaging camera with medium format camera lenses like the pentax 6x7?

they work great for me and the lenses are widely available on the used market. you can use the largest or smallest sensors and have plenty of back focus to use a camera + filter wheel + inline focuser such as the PDF

here are some recent examples

PL39000M
http://www.narrowbandimaging.com/ngc7K_P300ed_pl39km_fli_rsrch_s2hao 2_page.htm

http://www.narrowbandimaging.com/antares_p300ed_pl39km_baader_rgb_pa ge.htm

PL39000C (one shot color) includes photo of imaging system
http://www.narrowbandimaging.com/gamma_cygni_p300_pl39k_page.htm

ML8300
http://www.narrowbandimaging.com/ngc2244_rosette_P400ed_ml8300_cs4_H a_o3_page.htm

http://www.narrowbandimaging.com/horse_p400ed_ml8300_rgb_ha_page.htm

hardware:

http://www.narrowbandimaging.com/mounting_system_cla_page.htm

http://www.narrowbandimaging.com/mark_1_widefield_system_page.htm

other examples of images

http://www.narrowbandimaging.com/widefield_page.htm

One shot color is OK but it always gives poorer results than color sequential unless you downsample after processing. I did it for a year and concluded that the monochrome always gave better results

I have plenty of ONE shot Color images from 2009 on my website under the "latest images" tab that has everything in reverse chronology

best regards
Richard

[Martin Pugh]

Thanks everybody for the replies -I knew I would get sensible answers here.

I hear all the pros and cons, and I think I will drop the idea of an OSC.

Paul - why do you say the DSLRs are useless for solar imaging? I say that having never hooked up a DSLR to a solar scope but in theory, I can see great detail on the surface of the sun, or I can tune it to focus in on prominences. So, the latest DSLRs have full HD video capability. So, why would I not be abe to capture the same view to 30fps HD video via the DSLR? That's the theory any way, but as I say, never done it and I have a while to wait before the solar scope arrives.

Bert - I dont think I am looking to substitute my regular FSQ/STX combination for widefield stuff....that is already 4degs x 4degs, but simply to add to imaging options for very wide field - you know, wide area shots to capture meteorites or large asterims etc.

I will let you all know what I opt for and post results!

cheers
Martin

[Paul Haese]

Martin, my reasons are as follows:

1. Hydrogen alpha views of the sun are full monochrome and using a DSLR cannot possibly show all the detail or image quality that monochrome can.

2. When I used a DSLR as a try out the image was totally red and very dark, no matter how much ISO I applied. It contained little detail too.

3. Trying to process this out is very difficult to say the least, and I was not successful.

4. All the great images in Ha Solar come from monochrome cameras.

Still you could try with the HD DSLR.

[[1ponders]]

I'll second Paul's comment re DSLR. Why go for 1/4 resolution (BGGR Baayer matrix, with nothing showing through the B and G, though you might get some leakage across the wells, happens with some cameras) when with a monochrome camera you can make every pixel count. :thumbsup:

[chunky]

My 2c worth.

I remember a guy from Holland (PM Heden), that you could look up - some incredible solar shots from a DSLR.

http://www.clearskies.se/Gallery%20Deepsky.htm

Ciao
Clinton

[Martin Pugh]

Thanks Paul for the additional comments.

Clintn - thanks for the link...great stuff on that site.

cheers
Martin

[Martin Pugh]

Hello everyone
I said I would come back and let you know what I opted for.
I bought the new Canon T2i Rebel (550D), and sent it off to Hap Griffin straight away to have it modded while I worked through the manual.

I then did a lot of reading about lenses and opted for a fixed FL, the 85mm f1.8 USM lens.

I received that yesterday, and mounted the camera next to my FSQ and off I went.

I have to say I am shockingly disappointed. The first images (60 secs, ISO 800) were just terrible. Huge coma. I also thought I had achieved a good focus, but the images were dreadfully out of focus, suggesting th lens focus position moved during a slew.

I stopped the lens down 2 stops to f2.2, to no avail. So I will try f4 next.

I thought the fixed focal length lens were supposed to be far superior to telephoto lens. I also understand this is an APS-C format camera, so it physically isnt full 35mm frame format, like the 1D etc.

So, right now, very disappointed.

tips anyone?

cheers
Martin

[Octane]

Martin,

The 85mm f/1.8 USM is an excellent portrait photography lens, however, it's not designed for astrophotography.

The longer focal length L-series lenses such as the 200mm f/2.8L II USM, or 300m f/2.8L IS USM, and even the 300mm f/4L USM are better suited to flat fields.

And, yes, you must stop down these lenses, unless you wish to use an external aperture mask -- the benefit of the external mask is that it prevents diffraction spikes caused by lens diaphragm.

Here's a field that I did with the 200mm f/2.8L II USM on my old 40D (now sold; same sensor size as your 550D): http://members.optusnet.com.au/mrozycka/Cootamundra/Cootamundra_20091024_Sword_Belt_Ori on.html

H

[Phil Hart]

I'm a little surprised by your experience Martin.. I would have thought that lens would be up to the job (but I've never tried it).

Here's a few thoughts..

With my 24mm f1.4, 50mm f1.4 and 200mm f2.8 lenses, I image at about f3.5. I don't expect to produce sharp stars at f2.2 with any lens (and technically, going from f1.8 to f2.2 is not quite even one stop?). Certainly in my experience, the fixed focal length lenses are better than zoom lenses but I've never used the 85mm.

The only recent lens I've ever had trouble with was a 50mm f1.8 lens which must have got knocked or something and had a tilted lens element so displayed different focus across the frame. I would guess that you would have noticed if the image quality was different across the frame rather than just the same coma in all corners?

I assume you have liveview focus on that camera? Once you've slewed to the scene you want to shoot, you should have a star bright enough to liveview on and focus there. But slewing shouldn't be enough to change focus unless something like a dew heater strap is causing it.

Maybe you can borrow a lens to try something different. A lot of places hire Canon gear.. perhaps you could try that if nobody near you has one you can try?

Phil

[dugnsuz]

Hi Martin,
I'm having the same feelings towards my 24-70mm f2.8L lens - great for daytime, woeful under the stars...but I knew that on the way into the purchase so there's no excuses really - like H said, great portrait lens.
My other lens is another story altogether - contrary to the primes are better than zooms argument, the 70-200mm f4L IS is working out very well in the dark - Flat field, nice colour correction.
Here's my initial test...
http://s327.photobucket.com/albums/k461/doug-robertson/?action=view&current=19.jpg
Check out the MTF scores then compare the zoom range with its equivalent L primes...
http://www.photozone.de/canon-eos/196-canon-ef-70-200mm-f4-usm-l-is-test-report--review?start=1
And last, compare the 70-200 @85mm f4 with your 85mm prime @f4...
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=404&Camera=453&Sample=0&FLI=1&API=0&LensComp=397&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=5

All is not lost Martin:)
Sell the 85 and get a 70-200 f4L IS!!

All the best
Doug:thumbsup:

[h0ughy]

do you have a few of the shots to show. dew straps are a bugger at times because as you slew they move the focus - many a time has this happened. It could well be that the mod you had done was not placed back in the right position? when you use autofocus is it sharp - it may need adjustment? i use a bhatnov focus mask - which works very well with the live view

[Martin Pugh]

Thanks for the additional comments there. Useful indeed, since I did have a heater belt on it. I will remove it next time out.

I kind of expected things not to be perfect of course, but the results were just rubbish.

While the 85mm might be a great portrait image, I cant understand why it would not be good for astrophotography given the MTF curves.

Doug - I did see your initial tests before I bought the 85mm, and almost opted for that same lens, but I wanted to keep the cost down a little, as this is not my main line of astrophotograhy. So, the 85mm lens, with the reviews it had, was just $386.

I do not use Liveview for focussing. I connect to the camera from within Maxim, draw a crop around a bright star, and then take continuous 1 second images using the "View Large Statistics" menu item. I continue to focus until I reach the lowest achievable FWHM I can. Of course, doing this by hand is not perfect either.

I assume that to stop these lenses down, you can only do it via software or external aperture mask. I do not see any physical adjustment on the lens itself.

I will take some daytime shots with the 85mm lens and post them for comment, then I will stop the lens down some more and post some nightime shots also for you DSLR experts to analyse.

thanks for the links and help

appreciate it very much.

Martin

[gbeal]

Martin,
while everyone will have their favourites, I have had good results with a couple. My first was the Leica R series 180/4, stunningly sharp, and well made. Well priced too.
Where it failed was the back-focus side, as the CCD and CFW added up to more than the distance from the rear flange to chip. Yes I could have possibly worked something, but the easier option was to go to Plan B.
Plan B was a couple of Pentax 67 lenses, currently a 200/4, and a 55/4. Both have as you would expect gobs of back-focus, and are sharp as I feel I need. Bonus here is they are well priced too.
Gary

[dugnsuz]

Martin - sorry misread your post. Thought you had bought the L version of the 85mm lens.
Doug

[Octane]

Martin,

If imaging with a CCD, to stop the lens down, attach to EOS camera, select desired aperture, press the Depth-of-Field Preview button, and remove the lens from the camera whilst the button is pressed.

Furthermore, if imaging with a DSLR, if you use the EOS Utility's Remote Live View feature, you are able to nudge the focus on the lens through the software; you need to put the lens in AF mode first. Once focus is achieved, flick back to MF, disconnect and shutdown EOS Utility, connect to Maxim DL, and you should be good to go.

Hope this helps.

H

[Martin Pugh]

Thanks again.

Gary - I think its Marco that uses a Pentax EDIF lens, which are hard to get I understand but absolute pristine for astrophotography. I also understand its impossible to get a Pentax lens to CCD camera adapter, but Pentax Lens to Canon adapters are readily available.

Humi - thanks for that tip - I hadnt realised you could alter focus from within the EOS utility. (I am very new to DSLRs!). That's a great tip - saves running out to the scope at every focus frame.

Doug - So, let me understand clearly. The 70-200 f4L IS USM lens is a much better performer than its f2.8L equivalent?
Questions: Why did you opt for the more expensive IS version - is IS really necessary when the lens is used for astrophotography?

Do DSLR astrophotographers use any sort of filter on the front of the lens, like a UV cut? Clearly, the more expensive ED lens might not need it but perhaps the cheaper ones do.

cheers
Martin

[dugnsuz]

The range in order of performance/sharpness (as reviewed by daytime photographers)...
1. 70-200mm f2.8L IS
2. 70-200mm f4L IS
3. 70-200mm f4L
4. 70-200mm f2.8L

I went for the f4L IS as I wanted a very good daytime lens too where the IS will be handy.
The 2.8L IS was out of my price range and all reviews of the f4L IS found it be very sharp from edge to centre. MTF performance (pic below) at 135mm and 200mm is almost comparable with the equivalent primes
Kind of counter-intuitive in a way due to the extra lens elements used for the IS - one would think it would be less sharp than it's non IS sibling, but it's consistently reviewed as one of the sharpest zooms out there.

As you've stated, the IS circuitry is useless for astro work and should be turned off.
Doug

[Octane]

Hi Martin,

Yep, EOS Utility Remote Shooting/Live View is indispensible.

The way I've focused in the past is to monitor the chromatic aberration on the star in Live View. With each nudge you give the lens, you'll see colour change from red to blue. Once you're in the middle, you know that you're pretty much in focus. It only takes a couple of minutes to do it. Of course, you could us a Bahtinov Mask and the Bahtinov Grabber software to take all the guesswork out of it. I don't have a mask small enough for a lens, but, I know Chris (Omaroo) does and his works beautifully.

And, yep, just disable IS whilst shooting; if you're running on batteries, you'll drain them pretty quickly with IS enabled!

H

[dugnsuz]

I use a Hutech IDAS LPS2 light pollution filter in front of the lens - a must have piece of kit!

[Phil Hart]

You'll generally need UV/IR cut somewhere in the optical train otherwise you'll get bloated stars with just about any lens I think. But you probably have that already with your rear filter (but less 'cut' than the unmodded camera) unless you opted for the 'clear' filter option.

I still use a UV/IR filter on front but that's mainly to keep dew, dust and anything else off the front lens element. I don't mind wiping a filter clean but I prefer not to touch the lens itself as much as possible.

[iceman]

Unless you have a bright moon in the scene - then you're likely to get internal reflections and a second ghost blue/green moon image.

Then you'll need to remove the front filter.

But I know it's unlikely Martin will be photographing the moon :)

[h0ughy]

You must be lucky as i always get a reflection of a bright star etc in the image when i use a filter - i now remove them but use a dew strap and a microfibre lens cloth if needed.

[Martin Pugh]

Thanks for all the input.

I took up the suggestion of renting, and searched the local area for a supplier. I have a shop in town who will rent me the 70-200mm f4L or the 70-200mm f2.8L lens for $45 a week. Its a pity they do not have any the primes.

But this is an excellent way to check out these lenses before I spend any more money. I will probably try and sell the 85mm once I have tried these out.

I will probably pick up a filter, used, and give that a try also.

cheers
Martin

[dugnsuz]

Good luck Martin - look forward to viewing your results.
Doug

[gregbradley]

Gary - I think its Marco that uses a Pentax EDIF lens, which are hard to get I understand but absolute pristine for astrophotography. I also understand its impossible to get a Pentax lens to CCD camera adapter, but Pentax Lens to Canon adapters are readily available.


Hi Martin,

Ashley at Precise Parts can make one of these. I have already talked to him.

He wants one of those Canon to Pentax adapters and he will machine it or attach it to another adapter that fits the FLI Filterwheel.

I have purchased several Pentax lenses for use with my Proline 16803.
I have the non-ED 300mm F4 67 lens, 105mm and 165mm.

The Pentax 67 300mm EDIF F4 is rare and I haven't even been able to find one for sale. The 400mm F4 ED - there was one available on Ebay but it was expensive - about US$2800 or more.

Greg.

[astrospotter]

So more options are given I'll pipe in and say I am having a ton of fun and very happy with the Canon 200mm L series prime focus lens that I had already very much enjoyed for years for daytime shots. It yields 5.3 degree wide shots in front of a Qsi583 camera with filter wheel so this lens can do narrowband or LRGB with the wheel.

They say a picture is worth a thousand words so I'll post a link after a word more. The focus is extremely touchy but once obtained on a bright star and home made bahtinov mask the lens holds rock solid for hours.

The pictures are with this lens on a Qsi583 but that has just about same sensor size as Canon 40D so you would end up with about 5+ degree field wide open at f/2.8 so tighter stars can be had by stopping down 1 stop but I have been happy so far so as not to double my exposure time for the added stop (life is a trade off ... always).

Here is a shot on my site but basically all shots there are this lens and camera. (site in in north so hope link works) I don't know how to post pics to Ice In Space.

http://astrospotter.zenfolio.com/p6664130/h3eabe09b#h3eabe09b

Good luck in whatever you end up using.

For stopping down the lens I use step-down rings which are cheap and very accurate. They are real cheap so you can easily pick up a few different sizes and do the stop down that way so that your stars remain perfectly round without the Canon 8-point stars from the 8-blade stop in canon lenses. With a step down to 37mm I can obtain very tight focus to do RGB shots without re-focus (seen in the M31 image on the same website). This too is tradeoff but not bad looking IMHO. See below

http://astrospotter.zenfolio.com/p314501843/h2a1e57a0#h2a1e57a0

[gbeal]

While not wanting to cut Ashley out of the equation, I am too miserable to have something custom made, not unless there is absolutely no other way.
In my case a simple e bay buy found me a Pentax 67 to Sony DSLR adaptor (I have a Sony), and then got a mate to thread the inner section of the adaptor to accept a T thread. Then a simple male T thread section was threaded on, leaving the amount required sticking proud to accept the camera. Quirky as it sounds, the protruding T thread actually fits within the Sony adaptor if and when I have the bayonet section attached (4 very very small screws). This gives me the ability to use this one adaptor with both the CCD and the DSLR, via those minuscule 4 screws.
Gary

[luigi]

I sold the 85 1.8 I had because I didn't like it at night.
Instead I'm using a 100mm F2.8 Macro and it works well.
The 70-200 are also great and the 200 F2.8L prime it's fantastic and cheap too.
The 550D is a great camera for astro, the HD video modes are very interesting for stacking purposes.

[Martin Pugh]

Hi all, and thanks again for the input. I shall certainly go after those step-down rings now that I have seen the result of the internal diaphragm and its effect.

Well, I managed to rent the 70-200mm f4L lens for this weekend, and got out with it tonight. This is only an assessment, so the image is short on exposure, high on noise etc. But I am pleased with the lens.

The image here (http://tinyurl.com/2v68k3v) is 11 x 3 minutes, shot at f6.3, FL was 100mm, ISO800.

The image is 50% of its original size, and the lens had an UV filter on the front of it.

Instead of dark subtracting, I used a high dither rate, and used the power of CCDSTACK to remove the dark noise (it did an okay job).

I accept its noisy, and needs more exposure, but I like the lens. There is almost no cropping on this image except for minor alignment areas, and the stars are pretty good.

I have some more questions for you DSLR experts later.

Comments on this image very welcome.

cheers
Martin

[Octane]

Looks like you're up and running, Martin, top stuff.

I would imagine that f/5.6 would have been enough (closed down one stop).

Also, get rid of the UV filter on the lens! You'll only degrade the quality of the image by having light go through another layer of glass (typically, they're nowhere near the quality of the Canon glass, either).

Incidentally, I'm picking up the 70-200mm f/2.8L USM (non-IS) on Wednesday. I already have the 200mm f/2.8L II USM for astro, and, it is superb. Perhaps that's one you should look at, too.

H

[mick pinner]

what about the 135mm f/2.

[Octane]

The 135mm f/2L is superb.

But, I think Martin's after something a bit longer.

H

[Martin Pugh]

Thanks guys.

actually - what I am looking for is a lens that can do (in priority order)

1. Milky Way, large asterism shots.
2. star trails
3. Meteorite showers.

So, I think about 100mm is good for Milky Way, but think that about 200mm is optimum for star trails.

100mm is probably too long for meteorite showers.

So, can 1 thru 3 be done with a single lens? Probably a zoom lens like 70-200mm would cut it?

any other thoughts?
cheers
Martin

[luigi]

Martin: Very nice image!!

The problem is that a zoom lens won't be as fast as a prime for the same price. You can get a 70-200 F2.8L II lens but is very expensive.

I'd consider:

The 100mm F2 lens is really cheap and underrated, if you don't shoot macro the F2 aperture at 100mm really works nice for astro.

The 200mm F2.8 L II prime is also cheap and has excellent IQ.

I think 100mm is kind of large for the Milky Way but I'm not sure what kind of MW shots you like to do, to get a good span of the MW I'm using focals from 14 to 28mm on my 5DII.

[Phil Hart]

hey Martin

I think your idea of widefield must be different to mine! ;)

This meteor shower pic was taken with a 24mm f1.4 lens on 5DmkII:
Geminid Meteor Shower (http://apod.nasa.gov/apod/ap091218.html)

You *really* need fast lenses for meteors and I couldn't conceive of using anything longer than 50mm. There are very fast 24, 35 and 50mm prime lenses which is mainly where I would be looking. For a cropped sensor camera you 'could' consider the 10-22mm zoom but it's nowhere near as fast.

For widefield milky way scenes/landscapes I also use the 24mm (and my new-ish 14mm ultra wide).

For classic 'widefield' shots I have heavily used my 50mm lens. These two milky way mosaics are both three parts on a 40D with 50mm 1.4 lens:

Southern Cross (http://philhart.com/gallery/Astrophotography/Milky_Way/CruxMosaic_resize.jpg.html)

Centre of the Milky Way (http://philhart.com/gallery/Astrophotography/Milky_Way/Milky_Way_Mosaic.jpg.html)

For comparison, this is four part mosaic with 200mm f2.8 lens on 40D:

Antares, Rho Ophiuchi and the Blue Horsehead (http://philhart.com/gallery/Astrophotography/Nebulae/RhoOphMosaic_rz.jpg.html)

And this is a single image with 200mm f2.8 lens on 5DmkII:

Large Magellenic Cloud (http://philhart.com/gallery/Astrophotography/Galaxies/Large_Magellenic_Cloud.jpg.html)

Hopefully that gives you some food for thought.

cheers
Phil

[iceman]

I'd agree with Phil - for widefield, you really want between 15 and 50mm - not 100mm or 200mm.

Star trails look best when there's some foreground interest/scene. At 200mm, you won't get anything in the foreground.

Mine aren't nearly as good as Phil's, but this Southern Cross and Eta Carinae (http://www.mikesalway.com.au/blog/2010/04/22/the-southern-cross-pointers-and-eta-carinae-widefield/) was at 35mm, and this Scorpius (http://www.mikesalway.com.au/blog/2010/04/20/scorpius-and-the-milky-way-widefield/) one was at 24mm.

[Martin Pugh]

Thanks again all.

I see that I was unwittingly setting myself up to do multi-panel mosaic shots!

All of your sound advice has been taken in...but clearly there are differing opinions about covering the lens with a filter - I guess the reality is that they are not terribly expensive to add.

I will go off and price up some of these lenses you mention and I hadnt looked to see what I can rent in the low fl range.

I appreciate all the advice thus far.

Next question:
I kind of noticed that the quality of the images, when captured in RAW format (using the EOS utility) and then importing into Photoshop via the Canon DPP utility was somewhat better than acquiring via Maxim.

Maxim only gives you the option to capture in RAW Mono or RAW Colour, and I understand RAW Mono is better for dark subtraction. However, the image size is massively less than the .CR2 format coming off the camera. Indeed, Maxim saves them as FITS files.

have you found this? What procedure do you use to capture your lights?

At least you can set up a sequence in Maxim, I have not found that yet in the EOS utility.

cheers
Martin

[Martin Pugh]

Phil

one of your links is double up to the same image.

Hadnt realised you had an APOD with your meteorite shot! Congrats

cheers
Martin

[Octane]

Martin,

You can use the EOS Utility to set up a sequence using the Remote Timer shooting. Click on the icon with the stopwatch on it. Set the number of shots you want to take, the bulb exposure and the interval. Note, if your bulb exposure is 5 minutes, set your interval to 5:10 which will allow 10 seconds for the image to download to the PC (or save to the card).

There's a couple of very clever guys on this forum who have written their own software for controlling EOS cameras. One of the programs is called APT (Astro Photography Tool) and another one is BackyardEOS. Check out the software and computers sub-forum for links. Both of those will allow you to program multiple sets of exposures of differing lengths, etc.

Myself, I prefer to keep things as simple as possible and just shoot RAW with the EOS Utility, using Digital Photo Professional for chimping.

I process my images in IRIS and it does its own conversion from CR2 to its proprietory PIC format.

H

[Phil Hart]

sorry.. have fixed the LMC image link above.

i'm pretty old school on this.. i use liveview focus on the camera and capture to the camera memory card and then calibrate, stack and digital develop the RAW files in ImagesPlus with final processing in Photoshop.

that means my laptop is free to guide and control the CCD camera running on a separate mount at the same time. the widefield happily looks after itself unguided and i spend all my efforts and frustration on the CCD ;)

phil

[dannat]

some great reading in here - i am thinking of a lens myself -the tamron 90mm macro -anyone used one?

[Martin Pugh]

Hello again all

I am returning to this post, because I am about to buy a fixed focal length lens to go on to the ST8300 and/or the 550D.

I can see that 24mm or 50mm is the way to go for what I want to do, but there is this one intermedite lens - the 35mm f1.4L.

It kind of sits in the middle and would appear to be a good choice.

any comments or owners have a view?

cheers
Martin

[troypiggo]

I have that lens, but haven't used it for astrophotography. It is an awesome lens, certainly one of the top-of-the-line Canon primes.

[hotspur]

Had a look at the reviews-this will be my next Canon prime lens.You cannot beat a prime L lens.

[Phil Hart]

It might be good to rent or borrow some of these to try first. They will all be capable of very sharp images at f2.8-4.

One problem I find at the very wide end is that star halos and chromatic aberration have a greater effect on the total image and as you lose the resolving ability bright star fields start losing the crisp appearance you can still get with a 50mm lens. The 35mm would certainly be better than the 24mm in these respects, but I'd still be more tempted by the 50mm depending on your goals.

Creating mosaics out of frames with the 24/35 also very difficult due to sky gradients but certainly possible with the 50.

Phil

[gregbradley]

I think Phil is right on the money here. I have used quite a few lenses over the years with CCD cameras.

Nikkor 50mm F1.8, Nikkor 180mm F2.8 ED Canon FD 85 and 105mm, on an STL11, a Pentax 67 55mmF4, 165mm F2.8, 300mm F4 and Canon 18-55mm on a Proline 16803.

The Nikkor 50mm was definitely one of the better ones. The bigger the sensor though as you'd expect the tougher it is on the lens.

The Pentax 67 55mm F4 showed quite a bit of coma but cleaned up mostly with 2x2 binning. The 165 though was quite aberration free fully open at F2.8 as is the 300mm on the Proline 16803 which is quite a feat.

I found in an old camera bag a Pentax 50mm F1.2 which I would like to use on my ML8300. The 8300 should not be so demanding on the lens being aberration free but unless you want to do manual focusing you need something with some decent backfocus.

My imaging train with the Pentax 67 lenses is Proline, filter wheel, PDF focuser, Precise Parts adapter, Pentax 67 lens. I notice focus is like a telescope and quite particular and I think it would be hard to adjust manually.

In the case of FLI the 8300 chip is about 22mm inside the body and the Proline 30. I imagine the ST8300 is similar to the FLI in chip placement.

Another way of focusing I have seen is a rubber belt around the focus ring of the lens attached to a wheel run by a Robofocus.

The Canon FD lenses were a bit disappointing but I did a couple of shots that were OK.

You really need ED lenses in there otherwise there will be some chromatic aberration. In the case of the Pentax 67 it is minor and you can process it out mainly being bright stars look magenta.

The 55mm F4 gives a very widefield view with the Proline 16803 so that 2 images would cover 1/2 the Milky Way approximately.

I am beginning to feel a Canon 5D would be the best camera for this type of shot with a nice fast short focal length lens like Phil uses or Alex uses with his Nikon D3

The Nikon 14-24 would be very tempting if you have the budget for it.
I am waiting for the 5D Mark 111 to be released before I get a 5D camera but it may not be that different for wide field images than the current model.


I found this guide to Pentax 67 lenses very helpful:

http://www.antiquecameras.net/pentax6x7lenses.html

There was another table I read which listed the backfocus of many different lenses. There are only a few that match this 87mm or so backfocus. The Pentax 67 lenses are fairly plentiful on ebay as are adapters to make them fit Canon cameras.

I am not sure how they compare to Canon L lenses but they are very good for CCD cameras.

So in summary the Nikkor 50mm and the Pentax 67 165mm F2.8 so far have been the standouts. The Nikkor 180mm F2.8 ED as I recall was also good but too long for what you have in mind.

Of course any lens you get for an APS sized sensor you need to multiply the focal length by 1.6 to match a full frame 5D or Nikon D3 etc. Full frame will be wider field for any lens. So when Phil mentions a particular lens his camera is a 5D and the same on your 550D will be a factor of 1.6 less in FOV.

Best,

Greg.

[Martin Pugh]

Thanks for the input and comprehensive replies.

Greg - my 550D will be relegated to daytime photography, as I am pretty disappointed with it. It is no use on my Solarmax DS90 and too noisy to be used in astro way. Obviously I will try out the lens on the camera, but the primary use will be with the ST8300/CFW8. I have no idea at this point whether I will need extra adapters to come to focus....I obviously have the one adapter that interfaces the CFW to the lens.

Phil - thanks a lot. I think I will follow your advice and opt for the 50mm.

cheers
Martin

[gregbradley]

With my setup I can easily substitute the ML8300 for the Proline 16803.

I'll try that out at some point but unfortunately the Milky Way is fast disappearing soon until next year.

But I should try it on some other target. Perhaps Vela.

Greg.

[Martin Pugh]

Hello all
thought I would pick up on this thread from last year.
I am back in Australia and have set up the Canon 50mm f1.2 L lens and the ST8300. Given the expense of this lens, I expected super results, but instead, they are very disappointing.

I have tried the lens at the native f1.2, 1.4 and 1.6, and the off-axis aberrations are terrible.

I am not getting this? These results should be really good. Is it something to do with the size of the ST8300 chip? However, when I review Phil Hart's post, and I am now curious as to whether f2.8-f4 is where this lens should operate?

This is a brand new lens, so what is a simple test to see that the lens itself is ok?

cheers
Martin

[Paul Haese]

Probably you are too wide open. Maybe trying 2.8 or even at 3.5. I generally use 3.5 now for most of my DSLR imaging with primes. Do you have an image to take a look at Martin?

[Martin Pugh]

Thanks for the post Paul

image attached.

cheers
Martin

[gregbradley]

I have used quite a few lens with CCD cameras over the years. Nikkor 50mm F1.8 ( a good one), Canon FD 85mm, 135mm these were quite good. Pentax 67 55mm F4, 165mm F2.8 (the best).Nikon 180mm F2.8 ED (good) and a few others.

As Paul said most need to be stopped down and it makes you wonder how some of these lenses go being marketed as being really good and you see how poorly they perform.

Of these only the Pentax 67 105mm F2.8 (I think that is what it is) works fine at F2.8 with a Proline 16803 camera. The 55mm does all right as well but best stopped down 2 stops. Also using 2x2 binning helps a lot as well with some. It doesn't affect the resolution much anyway as they are so widefield. So the 55mm I used at F5.6 and 2x2 binning and similar aberrations as your sample image went away.

Greg.

[h0ughy]

to me it looks quite simple - the lens is not orthagonal to the imaging plane. how you fix that - no idea but when i was using the 127 i bought a hotech field flattener which worked wonders as it was self centering. For this you might need to get an adaptive collar that is correctable - the qhy10 came with one (not that i have used that the camera is still BNIB) http://www.gamaelectronics.com.au/QHY10.html if you look here you can see it here. maybe you can get something like that to correct for the imaging plane:question:

[Octane]

You have to note that these lenses are primarily designed for terrestrial use. The bokeh at f/1.2 on the 50mm is amazing. I have taken some gorgeous portraits with it. I ended up selling it back to Leon, though.

You might want to try the 200mm f/2.8L II USM or the 135mm f/2L USM for astrophotography. Much, much better suited and once stopped down a stop, aberration free.

H

[gregbradley]

You're right there is some tilt but it is mainly coma as it radiates away from the centre in all 4 corners with the bottom left being the worst where there is also tilt.

If you stop down and use 2x2 binning you may be surprised.

I doubt these things are made to the same standards as astronomical telescopes. They are mass produced in a factory.

Greg.

[multiweb]

I think that what you're showing is to be expected from a lens. No surprises there. Some are better than other, regardless of the brand. It's a bit of a lucky pick IMHO with lenses and certainly sensor size too, the smaller surface the better.

[Paul Haese]

I have a 135 F2 DC Nikkor which is very sharp and pretty much aberration free at f3.5. F2 is fine but it needs just a little stopping down to have an impact.

Martin I think looking for renticular lenses will help in your quest of wide field.

[Martin Pugh]

Thanks for the various replies.

I dont want to go to the 135/200mm range. My intent remains to get a lens good for Milky Way shots/meteors. I selected the 50mm L lens based on the advice here, based on that requirement. So I will look to finding a way to null out the tilt, as well as stop the lens down further, and bin 2x2 as Greg suggests to see what this does.

At the end of the day, my main line of imaging is hi-res deep-sky, so if I do not produce from this lens, I will just sell it on.

cheers
Martin

[bojan]

Have a look at Christian Buil's evaluation of canon lenses:
http://www.astrosurf.com/buil/50mm/test_us.htm
I modified older mechanical Canon F1.4 FD to fit EOS for wide fields, and I have to close it down to F3.5 or more. Then it is pin-point sharp, even at corners (of my 400d - full frame sensor, being larger, may show more coma).
http://www.iceinspace.com.au/forum/showthread.php?t=39293
The whole job cost me <~100$.

[Martin Pugh]

Thank you very much for that last link....very interesting indeed, and somewhat encouraging. I will certainly put this lens to f2.8 or more and retry.

thanks again
Martin

[Phil Hart]

hi Martin

hmm... I'm thinking your expectations were a little high :-).

Camera lenses are not like telescopes with fixed apertures. An f2.8 imaging scope is expected to produce sharp stars to the corners of its imaging field because f2.8 is all it can be. Some scopes can even deliver on that promise.

Camera lenses with variable aperture offer a wide open aperture that is a bit of a compromise.. if you're shooting portraits or whatever, you'll often accept a bit of softness in the corners (maybe hardly noticeable on regular images) in exchange for the awesome amount of light you get at f1.2 or whatever it is.

With my 50mm f1.4 lens, I typically shoot widefield images at f3.2. That's about where it becomes tack sharp on my 40D. Using a mono sensor is even more demanding. There are not many optical systems that can deliver tack sharp stars from a wide field of view at anything much faster than that.

If your 1.2 lens can't do any better than f3.2 (noting extra demand of mono sensor) then you could sell that and save yourself money with the 1.4 lens. For most peope I think that would be better value.

I had the f1.8 lens previously and had serious image tilt with it. Whether it came like that or was the result of a knock I'll never know. The tolerance on tilt at f1.2 is stupendousy small.. I don't how anything in the camera can be manufactured to that kind of tolerance, let alone all the pieces working together.

Can't tell a lot from your sample image, but the star bloat/halos look like what I expect wide open. Show me f3 and I reckon we'll be talking. You'll still get deep exposures real quick!

Phil

[Phil Hart]

oh.. and if you do find there is tilt, there's a 90% chance it's tilt of your sensor rather than tilt in the lens optics. you should be able to find out by rotating the lens relative to sensor and see whether the problem area shifts or not.

the KAF8300 sensor in my QHY9 was tilted.. tolerance spec is +/-0.25mm which is *enormous* compared to tolerance at f1.2. unless SBIG specifically calibrate the sensor in your 8300 then you could easily be dealing with same problem. good chance they can't calibrate to the required tolerance anyway. at f3 you might stand a chance.

Phil

[gregbradley]

Martin another point may be the spacing between your lens and the camera. I am sure the Canon engineers designed the lens to be a certain distance away from the sensor. Perhaps you should measure that and make sure your lens is in that zone.

Also I think the Pentax 67 lenses are good bang for buck here. They have outstanding backfocus at around 89mm (among the highest of any camera lens).

I use FLI PDF focuser, FLI CFW and Proline and an adapter made by Precise Parts that fits on the PDF.

I am using Pentax 67 55mm F4, 165mm F2.8 and 300mm F4.
All are non-ED and relatively cheap and plentiful on ebay.

The 55mm I use at F5.6 and 2x2 binning. The 165mm can be done wide open at F2.8 and on a demanding Proline 16803 chip is sharp corner to corner. The 300mm also is sharp.

Examples of the 55mm at F5.6 and the Proline 16803 are here:

http://www.pbase.com/gregbradley/image/140786051

http://www.pbase.com/gregbradley/image/140793803

There is some chromatic aberration that shows up as a bit of magenta around brighter stars but its easily fixed in Photoshop.

Also at 2x2 the stars are a bit larger than at 1x1( needs a small amount of deconvolution) but I was getting awful seagull outer stars at 1x1 on the Proline. 2x2 though magically stopped that.

There is also a 45mm F4 Pentax 67. They go for a few hundred US dollars on ebay.

An 8300 chipped camera would be a piece of cake for these lenses.

Your Canon F1.2 is probably worth a lot and is best used on a 5D mark iii!

Greg.

[Martin Pugh]

Thanks again for the info, all v.useful. I will be testing at f2 and up this weekend.

I will have to research the designed backfocus distance between a Canon L lens and the EFS cameras (I have the 550D) and see how that differs.

cheres
Martin

[troypiggo]

Shouldn't be an issue using the lens on a Canon camera. Got the impression Greg was talking about using it on a CCD with adapters etc.

[gregbradley]

Just one other point as well. I have been reading lens reviews for the last couple of months to get up to speed on Nikon and Canon lenses.

I often read in reviews of fast lenses that they are often soft in the corners and edges. It usually isn't that much of an issue as they are often used for portraits etc where the attention is on the centre part of the image and the outside corners etc are often intended to be blurred using the shallow depth of field of fast lenses.

That said, it may be that a really fast lens like F1.2 may not be your best choice for widefield images where corner sharpness is paramount.

The 50mm lenses from both Canon and Nikon usually get a near perfect score. Its like a 50mm lens must be the easiest to make and both make them to perfection especially F1.8s. Perhaps that is not wide enough for your intentions but any 50mm especially F1.8 (quite cheap at around $175 or less) of either brand may be a good and cheap starting point.

It is a common choice here on this site for extremely widefield DSLR astro images.
The results are usually superior.

Greg.

[troypiggo]

The 50 f/1.8 would be pretty disappointing for astro imaging.

[gregbradley]

Why do you say that?

http://www.pbase.com/gregbradley/image/66496862

Nikkor 50mm F1.8 on SBIG STL11. Its not the best image I have done but I thought the little lens performed quite well and its very very bright for a 30 second exposure.

Its cheap and it has the least amount of distortion, vignetting, pin cushion of virtually any lens. Same with the Canon version.
Virtually no other focal length sells a lens at F1.8 for under $150 that is distortion free and really low vignetting.

Greg.

[Phil Hart]

Martin.. it's 44mm from flange to sensor for everything EOS:

http://en.wikipedia.org/wiki/Flange_focal_distance

Greg.. i used the Canon 50mm f1.8.. it was OK aside from the tilted lens (construction issue?). but f1.4 is clearly the better lens.

Phil

[troypiggo]

Sorry my previous post was a bit abrupt and offered no reasons. I had the Canon 50 f/1.8. Bought it early on because it was recommended a lot as a fast, cheap prime. Good bang for buck and so on. I found that the cheap cost of it alone didn't outweigh it's shortcomings in terms of slow AF, cheap plastic construction, noisy focus, and average image quality. The link that bojan posted above comparing all 3 50's is excellent and shows the difference between the 3.

If I could be so presumptuous as to talk for Martin, it appears to me that if he was disappointed in those initial testings with the 50L, he's reeeeally going to be disappointed with the 50 f/1.8.

I think of it kind of like suggesting to Martin to sell his FSQ106 and getting an ED80 just because it's cheaper. And the fact that he already has the 50L, it just doesn't make sense to me to sell it to get an inferior and cheaper lens.

I don't think it's as sharp as the 50L, and for astro aren't we chasing the sharpest, pinpoint stars across the field?

[Martin Pugh]

Thanks for the info Phil - 44mm - Clear skies tomorrow night are forecast, so I am going to work the lens from f2 and up to see what happens.
The ST8300 has 17.5mm backfocus, and the FW8300 has 20mm - so thats 37.5mm. The Canon lens adapter easily adds probably 5mm or so, I have not measured it (but will), so that puts it very close, assuming a delta of perhaps 1.5-2mm is not critical - it probably is at f1.2.

cheers
Martin

[Phil Hart]

I'm not sure how critical the spacing is.. but would love to know!

With the correctors on fast newts, anything more than 2mm out is significant. Not sure what that means for lenses? Would think you would be ok by f3?

Phil

[bojan]

Guys,
with lenses at F2.. F4, microns are significant, not to mention millimetres.
You simply have to have some extra room to move focus in and out to be sure you are spot on.
Some lenses are too short for EOS (which is quite deep - that is why I went into all the trouble with modification of those FD lenses, as mentioned earlier).
Often, with some skill and determination, a missing 0.1 millimetre can be adjusted by moving the focus stops a bit (that depends on particular lens design).

The tilt of the lens (or sensor) can be fixed by gluing a layer or two of Al foil to a camera flange on the appropriate side.
http://www.astrosurf.com/buil/tilt/test.htm

[gregbradley]

This is an interesting thread. Thanks Martin for the explanation. I have only used Nikon and Canon 50mm F1.8 and they seemed much the same as each other. I haven't used the faster 50mm lenses.

Martin I only suggested the spacing as a possible issue. If you can come to focus maybe then that is not an issue. But as you know many optics are designed with a critical spacing but I suppose that is reducers and flatteners. But for lenses if you can get it to focus perhaps that means its OK. New territory there for me. But if you are experiencing massive aberrations which you are it is certainly worth checking out as perhaps the aberrations diminish with correct spacing. You can of course still focus a scope with a reducer set at the wrong spacing but you will see aberrations.

Greg.

[Martin Pugh]

Hello all - thanks for the link on how to correct tilt....I clearly have it.

I have now operated the lens at f2, f2.5, and currently f3.5. I believe there is a significant improvement particularly in the lower left/right. The existence of tilt in the upper half of the image is clearly discernible now, so I will attempt a fix on that.

If I can null that out, it will be interesting to dial back the lens to f2 to see how things may have improved there.

I will keep you posted, but here is the result so far (full size FIT - 16Mb)- so, this is the 50mm L lens, stopped down to f3.5, 30 minutes exposure, ST8300, Baader 7nm Ha filter.

http://www.martinpughastrophotography.id.a u/Canon%2050mm%20Lens%20at%20f3_5%20a nd%20ST8300.fit

thanks for all the pointers.

Martin

[Martin Pugh]

I think I have come to the end of using this lens. I got very good results finally with the lens at f4.5 with very good stars across the field.

I put the lens back on to the Canon 550D, as a trial to see what sort of results I got there, and much to my surprise the excessive off axis aberrations were back - with the lens at f4.5. So what I achieved with the ST8300 did not replicate on the DSLR.

Anyway - one lesson - I think buying anything less than a full frame DSLR for astrophotography is a waste of money and time.

cheers
Martin

[Phil Hart]

16MB FITs?! Some JPG examples would be nice for those of us travelling without our full suit of astro-imaging software at hand! :lol:

I don't quite follow what you think the problem is with the 550D and why that would be any less of a problem on full-frame?

Have you checked that the tilt is in the 8300 sensor and not the lens? You can be sure that the 550D is as flat/square as any sensor can be.

Phil

[gregbradley]

I thought the same thing. I didn't follow how full frame is the conclusion from the lens not working with your APS sized 550D. The larger the sensor the more demanding it is for orthogonality by the square of the increase in size.

Greg

[bojan]

No surprises here.
The tilt is not in the lens it is at ST8300 sensor.

Full frame lenses will perform better on smaller sensors. though - because of obvious reasons (smaller sensor, aberrations stay outside the corners). However, they will also be much more expensive. Which means, DSLRs with smaller sensor (not full frame !! ) will not capture those aberrations.

[Martin Pugh]

Hi everyone
I posted the full FITS because I assumed that if people wanted to assess the image, an uncompressed image is the way to go, rather than a JPG.

Anyway - I guess the point I am making here, is that I dont believe I would get dissimilar results with the $380 85mm lens I first bought, and that I did not expect to go these lengths with a $1600 L lens. My next trial will be just that - I will take an image with the 85mm lens, stopped down appropriately, and compare those results. Really, at this stage, I think the differences will be negligble. We shall see.

The other point I was making is that I seem to have got fairly good stars across the field at f4.5 with the ST8300, so I expected similar results by using the lens at f4.5 on the 550D. This was not the case.

Tilt? Even if it is in the ST8300 - Cant do anything about that. I tried to measure with a caliper the depth of the SBIG Canon lens adapter, and was fairly satisfied that the measurements were very close. I am not ruling the adapter out though.

One question I have though - if I gently rock the lens, I can hear the lens elements moving about slightly - is this normal?

thanks
martin

[bojan]

If you hear something when shaking lens, more likely it's something else (focus mechanism or iris), not the glass.
However, if it is the glass, you have to tighten the relevant retaining rings that is supposed to hold the (movable) element in place

[Martin Pugh]

Thanks I will check that out.

Here is the best I could get out of the 50mm lens at f4.5 and the ST8300.

http://www.martinpughastrophotography.id.a u/Canon50mmf4_5_ST8300.jpg

Cheers
Martin

[bojan]

That doesn't look bad to me at all, mate :thumbsup:

[Phil Hart]

hi Martin

85mm is a little less demanding than 50mm, but broadly I would say you are right.. any high quality prime lens at f4.5 should be delivering pretty good results.

I'm with bojan.. this result is pretty impressive though. I wouldn't be able to beat that image quality at 100% scale with any combination i've used either. I'd say my 50mm f1.4 lens might be a close match but i can't use it with my mono sensor (and i know there's tilt in mine which would kill it).

Not entirely unusual to feel a small amount of movement in the lens (but it shouldn't rattle when you shake it :lol:). Hard to say whether it is a problem or not from here.. i've got some lenses that have that slightly loose feeling but no issue as far as i can tell.

Can you send us a sample from the 550D.. f4.5 and maybe even something wider if you have it. Full res JPG more than sufficient for judging image/optical quality in this case. Perhaps a different story if you were trying to gauge noise peformance etc.

cheers
Phil

[gregbradley]

I disagree that image is nowhere near good enough. A large amount of distracting coma and tilt.

Martin has the highest standards and I can't see him posting an image with coma that bad. You'd never accept it in a scope.

Some of these short focal length lenses both Nikon and Canon but particularly Canon often get low scores for edge sharpness.It seems to be a difficulty with these super fast lenses. That is not always a big issue for terrestial photographers because they often shoot at F11 to F16 or so or are using for portraits and the edges are often out of focus blur of the background for bokeh effects.

You are using it F4 or F5.6 or so and the lens probably doesn't start to smarten up until F8 and by then you've lost your speed.

The simple 50mm F1.8 may be the better choice as its still fast but it lacks distortion and aberrations and often reviews of any 50mm lens seems to be very good. It must be the sweet spot for lens optics or they are relatively easy to make well compared to other focal lengths. I have both Nikkor and Canon 50mm F1.8 and in my limited use of the Canon 50mm F1.8 they both seem the same to me.
They are also very cheap and around $135.

Greg.

[Paul Haese]

Yep not real flat that image Martin. Surely there is going to be a lens that is flat all the way across.

[multiweb]

That FIT looks awesome for a lens. Different expectations. It's not a scope. I'd be stoked with that especially 50mm.

[gregbradley]

Admitttedly Martin has posted 100% and that will show up the warts but here is one I took at 2x2 binning with a Proline 16803 (about 16 times more demanding of squareness than an 8300 chip due to its size)
and a cheap ($250 or so) Pentax 67 55mm at F5.6:

http://www.pbase.com/gregbradley/image/142065586


There is slight tilt on the right side but minor. At 1x1 binning though the coma is gross. At 2x2 magically it is pretty precise and with the 16803 chip it still shows plenty of resolution at 2x2.


Here is one (sorry its a small image) STL11 and Nikkor 50mm F1.8 which as I recall was sharp corner to corner no coma:

http://www.pbase.com/gregbradley/image/66496862

I know you aren't interested in longer focal lengths but out of interest the Pentax 165mm F2.8 is sharp corner to corner at 1x1 binning wide open on a Proline 16803.

I conclude from this that lens makers have compromises enter in when they make short focal length faster lenses and longer focal length lenses are more likely to be useful than shorter focal lengths. And if you want shorter focal length lenses then there is no point in getting F1.2 type lenses.

There is also a Pentax 67 45mm lens. But per reviews it isn't as sharp as the longer focal length. The 75mm as I recall is considered very sharp. There is a list of Pentax 67 lenses and their ratings if you are interested.

Greg.

[Martin Pugh]

Hi folks.
thanks again for all of the input. I entirely agree - and hats off to you DSLR astrophotographers. I thought it was easy, and I have experienced nothing but disappointment thus far really. Greg - you are right - I would be hard pushed to produce images that had this kind of aberration - ask Mark Bolton who witnessed my AIC talk. He will testify to that. That said, I am very keen to image the Milky Way, and see no other way to do it other than huge mosaics. I will certainly persevere with the 50mm Lens at f4.5 and now go ahead and do RGB to see what turns out. Of course - I have to implement a focus solution now.

Anyway - just for kicks, I put on my 'mistake' purchase - the Canon 85mm regular lens i.e non-L.

Below is a link to that image - 30 minutes, Ha, ST8300, at f4.5.
Look past the noise and I accept that there is something not quite right - take a look at the top left and compare it to the top right. HOWEVER! If I was on a budget, I would be buying the $385 85mm lens, and not the $1600 50mm L lens when you look at this result.

http://www.martinpughastrophotography.id.a u/Canon85mmf4_5_ST8300.jpg

Here is the direct comparison to the same field with the 50mm lens.
http://www.martinpughastrophotography.id.a u/Canon50mmf4_5_ST8300.jpg


Phil - you asked for a sample of the 550D. Link below, taken with the 50mm lens also at f4.5. Shockingly bad.

http://www.martinpughastrophotography.id.a u/IMG_0008.JPG

This was a single 30 second image at ISO1600 I think.

cheers
Martin

[Phil Hart]

hmm.. well it certainly gets complicated.

the 550D image is poor for f4.5.. i wouldn't be surprised by this at f2 but not right given how stopped down that is. definitely tilt somewhere given different shape aberrations left vs right side of frame. if you can prove it is the lens, then i think you would be able to ask to swap it for a new one. i think my 50mm f1.4 can do a little better than that.

the 50mm version on your 8300 does not look quite so bad though? and the 85mm only looks bad in one corner. are you always focussing on the centre of the frame? obviously if you focus away from centre and there is tilt then aberrations will be worse on opposite corner. looks like that in 85mm image to me?

keep at it..

Phil

[bojan]

Greg, I did a lot of testing with various lenses, and came to exactly the opposite conclusion (the same follows from Christian Buil's test results).

For example, F1.2 lens, stopped down to f4.5 will perform better than F1.8, stopped down to F4.5.

Also, lens design for full frame (24x36mm) will perform better with smaller sensor, than equivalent lens, designed to be used on that smaller sensor (1.6x smaller, used in cheaper Canon camera models).
The reasons are, lower F-ratio and bigger sensor require much more careful design of the lens system.

That is why the older, manual and today way cheaper lenses will very often perform better than new, k$-range lenses (with IS, AF and what'snot, all that mostly useless in astrophotography anyway).
As for tilt, sometimes it is a matter of lens production and/or assembly quality - I found couple of Canon lenses with un-symmetrical distortions in corners - but all this is visible on star images only. For terrestrial photography, they are (almost) perfect from corner to corner.

Astrophotography is VERY demanding on lens performance, especially in corners.
From my experience, instead of searching for perfect wide field lens, it is better to concentrate on what we have and how to squeeze maximum from that
Mosaics done with more narrow field lenses take longer time to complete, but the results may be closer to perfection, compared to single frame wide field photo (Avandonks work is a very good example of this philosophy - his (relatively) ultra-wide fields with 300mm lens are really worth looking at)

[Martin Pugh]

Hi Phil
I think I am now happy with the performance of the 50mm lens at f4.5 on the ST8300. I have posted a couple of examples in the Deep Space forum.

I think I will sell of the 85mm now - it is, as you can see, perfectly usable for someone on a budget or getting started.

cheers
Martin

[gregbradley]

[QUOTE=bojan;833958]Greg, I did a lot of testing with various lenses, and came to exactly the opposite conclusion (the same follows from Christian Buil's test results).

For example, F1.2 lens, stopped down to f4.5 will perform better than F1.8, stopped down to F4.5.


Thanks for that.

I would have thought what you say is the case originally as those lenses should be better made and with exotic glass types (they often have ED elements).

Cheers,

Greg.

[bojan]

All designs are a compromise.
It is VERY hard to design a F1.2 lens to perform in similar fashion like F1.8 at full aperture and in corners (the centre of the frame is relatively easy).
Stopping the lens down engages only the central parts of the glass, where the aberrations are negligible.
Because the F1.2 lens is much more carefully designed (hence more expensive) - to give acceptable performance in corners at full aperture, it will perform better than it's F1.8 counterpart at the same stop-down (because both are designed to have the same MTF at full aperture)