Tele extender+DSLR+scope for DSO?

[NorthernLight (Max)]

Morning everybody,

recently I tried shooting M74 from my backyard with a 6" F5 newton and a 450D. 10x10min shots + the usual.
After stacking and opening in PS I deleted the lot as there was not much more than white noise and a faint small hint of a galaxy drowned in the white glow.
Now I am thinking about getting a Canon 2x Telextender to increase magnification (for a bigger target) and reduce skyglow (by increasing fl).
A barlow is not really my favour as this extends the lever on my focuser and the coupling with only one little screw from the side appears not very trustworthy.
Has anyone used a Tele extender+DSLR+scope for DSO shooting? Or some theoretical input. I don´t want to buy one to find out that i.e. backfocus is insufficient etc.

Thanks for all advice!

Cheers

[koputai (Jason)]

Hi Max,

Check this thread:
http://www.iceinspace.com.au/forum/s...light=extender

It was the Moon, but similar equipment set up.

Cheers,
Jason.

[NorthernLight (Max)]

Hi Jason,

thanks for the link. So it can be done (with a little tape).
Very good. So I´ll see that I get a decent teleextender.

thanks again!

cheers

[wasyoungonce (Brendan)]

Usually for deep Sly Objects (DSOs) you need a wide field of view (FOV)...probably more so for Nebula than Galaxies. Galaxies are generally quite small in FOV, except for the closer ones, Like M31. But their surface brightness is generally quite low magnitude and are thus not good targets for Barlow's or extenders.

A Barlow or extender will increase the object size. But it will also spread the same number of photons you previously collected over this larger size. Thus the surface brightness of object will be "dimmer". Same goes for any object. You notice this less for objects like the moon.

Did you stack pics in DSS? Usually when I do this I see a reasonable image but when I import this to PS you loose some of the DSS contrast and brightness settings...so the stacked pic looks dimmer imported into PS.

Did you try some histogram stretches ...curves and levels in PS? You need to do this as the DSLR sensor a linear response to photons and your eye is not. So you need to gently stretch out the data.

Also..10 min exposures is a lot of ask for a STD DSLR. I get a lot of sensor noise (and sky light fog) when I exceed 5 mins so only go past this (up to 6 mins) on very cold nights.

You probably have data in your pics you have not tweaked out but maybe it would be best to start with 4-5 mins exposures ..lots of them.

Tell us how you go or post up a pic.

[NorthernLight (Max)]

hi wasyoungonce,

yes, stacking was done in dss. in ps2: convert 16bit with gamma correction, levels, curves, gaussian blur backround mask against noise, desaturation against chromatic noise.
But still, one has to guess what the image is showing. I now know that ten min from my backyard doesn´t really work.
The focal lenght of 1.5m shouldn´t be a drama on a 6". I reckon I could live with 6min subs due to decreased skyglow effects from longer fl.
Have you tried using a tele extender before?

[wasyoungonce (Brendan)]

Your scope at native focal length, F5, should be good enough to collect good data without an extender.

Your image is not too bad. Compare it with a google search of images... shows that "hey it ain't that bad". It does look as if the whites have been clipped a little. You have the centre portions of the galaxy but have probably clipped out the arms.

Maybe try something brighter like NGC253....ahh you have and did quite well on that!

I did use an SCT at F10 for DSOs. Guiding becomes problematic with very low image scales (arc sec's per pixel)..longer FL's and image exposure times go up quite a bit.

Pretty much what you have done or 5 mins should yield good results.

edit:
I meant to ask...what ISO was used for this PIC. Try at least ISO 800 even 1600..but at those ISO's you may need to lower the exposure time to 5mins and 4 mins respectively.

[NorthernLight (Max)]

Hi wasyoungonce,

it was taken at ISO800.

Question: when you take a single exposure, is your background sky really black or whiteish? I am still looking for a reliable indicator to determine exposure time.

Thanks for your advice!

Cheers

[wasyoungonce (Brendan)]

I get increasing sky fog as exposure time increases. Depends upon a lot of factors: If it is cold; If target is at Zenith; If target has high declination (ie:+20 degrees); and ISO settings & exposure time.

For example. I find I can image up to zenith and past for around 1.5hrs for for lets say Helix Nebula (given it's declination) around a month or so ago. But after this it is travelling west towards city lights...so I have to stop as the sky fog increases dramatically.

See attached raw Helix image subs (no processing), both 300s...which one has more fog? As you can see I reached my limit, IMHO.

Of course I can process a lot of this fog out but there is a limit to what is possible. Same with my camera, better to try to get the best data I can. My limit is 5 mins unless it is very cold or dark or ..conditions right. You may get more than me..depends on a lot as mentioned.

So all in all..take lots of subs...try different sub times but try to keep them as short as practical.

I did have once see a web site that had users publish their sub exposure data and times on quite a few a few DSOs...but I cannot find that page...Damn!

Your image appeared as if you clipped out some data? Just a thought.

edit:
the images are slightly not exactly the same frame as I was manually dithering...what a pain that is. But Dithering is well worth it.

[NorthernLight (Max)]

we are all shooting the same at the same time, aren´t we? Your Helix subs are much darker than the ones I took and leads me to conclude that I am defenitely overexposing. Thanks for the practical example.
RE: clipping whites- I believe the subtle whites got lost by applying the blur mask to smooth out the background noise. The subs came in so badly, almost completely white, that I have deleted them already.
I was led to believe that 10min is the way to go, programmed it, left the scope for an hour on autopilot and came back for darks - clearly mistake.
Is the exposure in your attached images your threshold for exposure or are you suggesting to stop earlier? and are you aware of method tutorials for manual dithering in PS2?

I really appreciate to be able to tap into your knowledge.

Cheers

[wasyoungonce (Brendan)]

Hi Max.

Trial and error for astro snaps..I'm afraid. Obviously the lower the magnitude of the target the longer you want your subs....to a point that longer subs only give you too much noise and sky glow. If you can get away with longer subs..great but I suspect you will see lots of sensor noise and sky glow..a law of diminishing returns from 300s onwards.

Generally from what I have found for many DSOs try: 180s, 240s and 300s snaps. For lets say M42...45s to 120s snaps. A lot Depends upon your scope as well. An F4 Next...30s subs for M42 and an F10 SCT around 180s and more are probably needed.

If you can get away with longer..great but I suspect you will see lots of sensor noise and sky glow when going longer than 300s.

We mentioned factors that effect sub exposures...but by taking lots of subs you are effectively taking a longer exposure. So the more subs the better the signal to noise of the final image.

Dithering is moving the scope/camera/mount a pixel or so (or more) in the pause between subs. What it does is allow DSS (your stacking software) to recognise hot pixels/noise/banding/dust (artifacts) more easily and remove them. You do not have to do it every sub..lets say every few. Of course there is software that can "auto dither" for you.

I probably shouldn't haven't mentioned dithering just yet..as it's probably something that you may want to try with a little more experience. Probably best to just get the basics at this stage.

That said..there is a discussion on CN right now about DitherMaster if you wish to read up on it. Ivo, who has written a camera control piece of software, APT, is incorporating auto dithering in his next release and Guyroch is also doing this in his camera control software. You may be interested in this..or at lest file it away to try in the future.

You are doing very well and should be congratulated. Keep our chin up..it is a one step fwd 2 steps back learning curve...and only those that make mistakes will learn.

[NorthernLight (Max)]

from all what I have got from this forum so far, you have been the one that inspired me the most to push further.
lot´s of "down to earth" advise, plenty of "read further tips" and no notion of superiority.
a big thanks for that!
I consider this question (thread) as perfectly answered. Thank you for your time, wasyoungonce.

Cheers and may your skies be clear!

[wasyoungonce (Brendan)]

ha...your too nice.

What I know about "all this" you could write on the back of a postage stamp.

[midnight (Darrin)]

Greetings Max,

I found the Canon Tele extender a good solid alternative to a barlow for the reasons you originally stated. The price is 1 F stop for daytime stuff but I assume this also applies for prime photography.

I have a 1.4x converter and use it as a good tool for getting just a bit extra so if you go down that line I would love to see your results.

Just remember to cover the terminals on the tele extender on the camera side to fool the EOS into thinking there is nothing there otherwise you'll get the error.

Stacking is the way to go and then if there is a little too much background light, a fellow member Atalas http://www.iceinspace.com.au/forum/s...ad.php?t=68095 has some very good photoshop tutorials one of which shows how to reduce this effect by shifting your dark offset in PS.

Good luck and would love to see how you go.

Cheers,
Darrin...

[NorthernLight (Max)]

Hi Darrin,

thanks for the link - bookmarked it and will "stay tuned".
I´ll let you know when I got one and tried it out.
Cheers

[dannat (Daniel)]

borg make a 1.4x tele-extender which has native scope couplings (no need to worry about the eos terminals -though i think it may just be suited to refractors

[wasyoungonce (Brendan)]

Probably something I should have mentioned before is ....try to get your subs so that the Histogram peak is around 1/3 the way from the left (dark point)...when using DPP or PS (or any program that shows the histogram).

If you get less than this the subs are tending to be underexposed. You can go longer and get more data.

If you go more...then the subs are tending to be overexposed...blooming of bright stars, data and skyglow.

Thus irrespective of how long or short your subs are...you should be aiming for this type of histogram (yep I know its about slides and scanner histograms but it's pretty much the same for astro pics). Of course trial and error and experience will give you an idea on how to achieve this.

Hope this adds some clarity.