How can I make my images better? Galaxy Example

[TanjaC (Tanja)]

Hi everyone. It's been lots of trial and error for me, I've learnt a lot from the forum but could do with some advice on the pics I'm posting.
In general, I just want to take better pics.

I've attached 2 pics, NGC 5128 the Hamburger galaxy. It was my 1st session imaging a gx. It's 24min (16x90sec) Stacked in Nebulosity, 7 dark frames averaged - subtracted. I didn't do flats as the stacking process had an error with them (I shot cr2 and fits and they didn't work nicely together) ISO 400, no light pollution filter (and shooting from suburbia)
http://www.flickr.com/photos/astro-t...in/photostream

My problems/issues:
I don't have a guide scope. My polar alignment is okay - thats why only go up to 90sec. Mostly I shoot 60sec and just try get as many subs as possible. Can I get better shots with not going guidescope route?

Is my focus okay? I fine focus and view on my Mac while I'm doing it - and it all looks fine while I image.

I've got a coma effect, and not sure why it;s happening as I've got a SCT scope. Is it cos my Canon sensor is also a bigger size? The distance between scope and sensor - if I put a field flattener on the vignetting is worse - i.e the size of image on the sensor gets smaller.

Attached is another image of NGC 4755 The Jewel Box, prime focus VS Focal reducer/Field Flattener.

Just looking for some suggestions on what I can try to improve the quality of my pics... I have tons more questions - will deal with them as I go.

Equipment: C11 CGEM DX mount, Canon 5Dm2, Meade 6,3 focal reducer.field flattener

[jjjnettie (Jeanette)]

Tanja, your pictures are fantastic considering the amount of data you've used.
If you don't wish to guide, you can still improve your images.
You'll just need to keep adding more data. Try stacking 60 or 70 images instead of 16. The more photos you add, the more you can stretch the image to reveal more detail.
One bonus of taking short subs is that you will retain your star colour.
Flats are really important, as you already know.

[Garbz (Chris)]

I've got your problem as well. Just got a D800. Shooting with a C8 and the same f/6.3 reducer you have I have exactly the same vignetting problems. The 35mm sensor is simply a bit too big. The vignetting is completely uncorrectable.

I've taken the view of more shots is greater and these are my results a few nights ago for the Hamburger galaxy: https://secure.flickr.com/photos/100...in/photostream

That was something around the order of 80 shots at only 8 seconds each with 20 flats and 20 bias frames. However I was limited by really crap polar alignment at that point. (Guide scope is on order)

The suggestion I got was that the benefits of doing many frames start to drop off above 30-50. Haven't looked into it further yet but I guess that would mean the idea of stacking an infinite amount of frames will not necessarily be the best option and is certainly not preferred to getting better exposure times.

But I'm new at this so take it all with a grain of salt

[TanjaC (Tanja)]

Thanks guys - I'll re-shoot & stack more subs. Also will fix my flats problem...

Originally Posted by Garbz

I've taken the view of more shots is greater and these are my results a few nights ago for the Hamburger galaxy: https://secure.flickr.com/photos/100...in/photostream

That was something around the order of 80 shots at only 8 seconds each with 20 flats and 20 bias frames. However I was limited by really crap polar alignment at that point. (Guide scope is on order)

Your image looks awesome! I think I'm going to lessen my exposure time a little and obviously up the number of subs as a start and see if that yields better results. At least then I'll cut risk of oval stars.


Thanks guys

[Helo (Peter)]

Hi Tanja, great shots. Really work on your polar alignment (drift align) to get it as spot on as possible even with a guide scope. Took me ages on my C11 with a wedge but certainly made a difference. Flats should help with vignetting which I have a similar problem with using a f6.3 focal reducer. I also bought a microfocuser (still have to add the hands free part to it) but probably the biggest initial gain I had with focusing was a Bahtinov mask as it lets you get it spot on even if you are wobbling the scope around - take a photo and examine and adjust until it is correct. Hope this helps and look forward to seeing more from you.

[TanjaC (Tanja)]

Originally Posted by Helo

work on your polar alignment (drift align) to get it as spot on as possible even with a guide scope.

thanks for the comments Peter. Regarding my polar alignment, what can I do to better it. Doing 2 star alignment, adding lots of calib stars and then my polar alignment, for every time I image. I do it through my camera now, if I do it with an eyepiece I have to rebalance the counterweight later which throws things out again if I shift the mount.
If its just drift alignment - I'll go read up and give it a try.
I'm just waiting for the clouds to clear and then I'll do loads more subs again.

Bathinov mask, yip will do one... On my Mac screen it looks focused - but I always doubt myself.

I see in your signature that you have a Hyperstar, I've Got one on order- very excited any starter hints welcome!

Thanks for the help

[Garbz (Chris)]

Why do you need to shift the mount to adjust the counterweights? Polar alignment for me so far hasn't changed when I've done anything. In fact I do polar alignments before I actually attach telescopes / counterweights as the mount is easier to move...

[TanjaC (Tanja)]

I meant if I adjust the counterweight after alignment to accommodate for my imaging equipment, I run the risk of moving the mount out of alignment slightly.

I have a computerized mount so I have to do my polar alignment with either telescope or camera to centre stars to successfully align.
Which means if I do it through telescope and eyepiece, once I'm aligned and I attach a heavier camera the balancing is out, which will then affect the tracking on long exposures...

That's why before I start anything the counterweight is balanced perfectly with my imaging equipment.

Am I missing something here? How could I do accurate alignment without my scope?

[peter_4059 (Peter)]

Regarding number of subs and sub exposure length this is quite a useful read:

http://deepskystacker.free.fr/english/theory.htm

[Helo (Peter)]

For your polar alignment, the Polar Align feature in your C11 will help you get it roughly right but drift alignment is the best (takes practice to get faster at it - I have the luxury of a fixed setup in a dome). This will control declination drift so a guidescope mainly works on the RA and hence longer exposures become more practical. Your balance shouldn't change your alignment but does affect tracking so it is important (changes again when you have a hyperstar). Flats will be very important with the hyperstar as a lot of vignetting but you can image so many objects in one night you will be very excited. If you have any problems getting the 2ry mirror out to place your hyperstar, let me know and I can give you some tips or Dean at Starizona can. Bahtinov mask I find is best used by taking images and examining them zoomed in or in live view zoomed in to really nail it (remember to take it off when you start imaging, not that I have ever done this ). Good luck