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  #61  
Old 06-03-2012, 06:34 AM
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Quote:
Originally Posted by h0ughy View Post
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
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.
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  #62  
Old 06-03-2012, 07:08 AM
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multiweb (Marc)
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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.
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  #63  
Old 06-03-2012, 02:58 PM
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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.
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  #64  
Old 06-03-2012, 07:40 PM
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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
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  #65  
Old 06-03-2012, 07:54 PM
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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/s...ad.php?t=39293
The whole job cost me <~100$.

Last edited by bojan; 06-03-2012 at 08:05 PM.
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  #66  
Old 06-03-2012, 09:21 PM
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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
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  #67  
Old 07-03-2012, 09:59 AM
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philiphart (Phil Hart)
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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
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  #68  
Old 07-03-2012, 10:04 AM
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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
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  #69  
Old 07-03-2012, 02:37 PM
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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.
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  #70  
Old 07-03-2012, 06:51 PM
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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
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  #71  
Old 07-03-2012, 06:56 PM
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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.
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  #72  
Old 07-03-2012, 07:56 PM
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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.
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  #73  
Old 07-03-2012, 10:10 PM
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The 50 f/1.8 would be pretty disappointing for astro imaging.
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  #74  
Old 07-03-2012, 10:15 PM
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Quote:
Originally Posted by troypiggo View Post
The 50 f/1.8 would be pretty disappointing for astro imaging.
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.
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  #75  
Old 08-03-2012, 04:30 AM
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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
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  #76  
Old 08-03-2012, 07:23 AM
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Quote:
Originally Posted by gregbradley View Post
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.
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?
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  #77  
Old 08-03-2012, 08:16 PM
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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
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  #78  
Old 09-03-2012, 06:18 AM
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Quote:
Originally Posted by Martin Pugh View Post
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
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
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  #79  
Old 09-03-2012, 08:40 AM
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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

Last edited by bojan; 09-03-2012 at 09:06 AM.
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  #80  
Old 09-03-2012, 09:15 AM
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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.
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