QHY8 - Stretching, and the next steps

[toryglen-boy (Duncan)]

Howdy

Ok, so it the eejit again, asking for more advice, and a huge thanks to those that have PM'd me, offering help.

So, here is a capture of M45, its 10*10min subs, and i *think* i am getting there with flats, as the field looks kinda flat to me, although its all guesswork still, although a flat of around 0.035 secs, with an ADU count of around 9000 seems to work best.

So i am having issues, of each bright star having a doughnut on it, although i have checked, and the scope is collimated, so maybe its stray reflections, although my guess is the grub screws knocking it off when tightening, and it looks like a reflection of the primary collimation marker, but thats another matter, my problem is:

I need to know the best way of stacking, and with what? i have tried, and i cant stack images taken with both Maxim or Nebulosity ,its either one or other, but maybe more importantly, and i touched on this before with another thread, the best way of stretch data, because i should have ALOT more details than this, but when i stretch its almost like the image is 8bit colour, and it looks all clipped, and digitized, but at least i am making some progress

A huge thanks to the imaging comunity who have gotten me this far.



PS>. i know the image has some dust bunnies, and a massive killer cosmic worm, but its a learning curve

[troypiggo (Troy)]

Firstly, I'm no expert so please take more notice of others more than me. But I thought a couple of things when I read your post.

Your comments on stretching and getting something that looks like an 8 bit image. Assume you are using at least 16 bits? Are you using Photoshop? Took me a while to realise that when stretching in PS, the first few big stretches do look a little posterised as you describe - until you flatten the image or if you don't want to lose the history/layers that you've done so far, hit shift-ctrl-alt-e. That sort of flattens the image, but leaves all the previous layers below. Pretty cool. Does that help?

Re: stacking with Nebulosity - I'm sure there are many others here more experienced, but I can walk you through stacking using that if you'd like to persevere with it. I was using DSS, but now exclusively using Neb and really like the control it gives.

[multiweb (Marc)]

Hi Duncan, looking good You can get rid of the dust donuts with flat fielding. That's what it's for besides taking care of vignetting. The halos on the bright stars are the reflections of the glass at the back of your MPCC onto the glass of the QHY8 nose piece. Check here for more halos. Donuts look familiar? I'm still working on getting rid of these but replacing the nose piece by a NULL adaptor (nose piece without a glass) seems to help a great deal.

[toryglen-boy (Duncan)]

Originally Posted by multiweb

Hi Duncan, looking good You can get rid of the dust donuts with flat fielding. THat's what it's for besides taking care of vignetting. THe halos o the bright stars are the reflections of the glass at the back of your MPCC on to the glass of the QHY8 nose piece. Check here for more halos. Looks familiar I'm still working on getting rid of these but replacing the nose piece by a NULL adaptor (nose piece without a glass) seems to help a great deal.

Thanks again Marc, ok, so the flat fielding is looking better, but is still not right, i had taken the today, after storing the OTA overnight indoors, although the orientation of the camera, and focuser where not changed, so this should be fine, right? or maybe i should take them at teh exact same time as the lights, i can see that the "cosmic string" in the image has a dark and light part, so maybe this moved, when i shifted the OTA

As for the the reflection from the MPCC, i thought that, but the only thing that lets the theory down, was that i dont use any nosepiece filter on the QHY8, i use the spacing rings for the MPCC, the MPCC itself, and thats it!

Troy, thats some good advice, i will check that out to

[multiweb (Marc)]

Originally Posted by toryglen-boy

Thanks again Marc, ok, so the flat fielding is looking better, but is still not right, i had taken the today, after storing the OTA overnight indoors, although the orientation of the camera, and focuser where not changed, so this should be fine, right? or maybe i should take them at teh exact same time as the lights, i can see that the "cosmic string" in the image has a dark and light part, so maybe this moved, when i shifted the OTA

The dust donust weren't corrected and are slightly offset so I reckon your focus wasn't the same when you shot the light frames and your flats. Interesting point about the halos still without the nose piece. Maybe it's an internal reflection into the MPCC reflected from the CCD glass.

[toryglen-boy (Duncan)]

ok, so i had another play, maybe i need to take the flats at the exact same time, so i have a better chance with it.

[scopemankit (Chris)]

I don't think those are dust donuts. If you push the image you can see that the donuts are in a certain position radiating out from the central area. this could indicate the source is reflections the bright stars on one of the optical surfaces. What is the order of your setup, could you change it and try again?

[Tandum (Robin)]

I agree, that's something else. If it where me, I'd pull all coma correctors, reducers and all the other crap you have plugged in out of the train and just stick the camera down the tube and take some pictures to see what they look like.

Then add reducers/flateners/correctors etc if you need to down the track somewhere.