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How to improve clarity of narrow-band images (better filters or more integration)

12 Nov, 2019 23:45
Hi all. I am using an Astrotech AT65Quad with a ASI183 mono camera. Image scale is about 1.17" and dawes limit is about 1.78". Guiding is not the best, pixel peeping reveals egg shape but ti is the best I can do with my gear.
Median eccentricity is .59 and FWHM mean is 4.xx for an average sub.
I have had the mono camera about a year and have some older Schuler 9nm HA and Sii filters (no Oiii yet).
My seeing is typically average in New England.
I think my focus is OK, I admit it could always be better.
I see such amazing clarity in images with much nicer gear than mine so my question is what is the best upgrade for more clarity., or is it just a matter of much more integration time? ATM, My best NB image is about 3 hours of integration, typically 300 seconds at gain 111. Oh and my camera is not cooled but I fell that my darks are within a degree or two and I take 30 of them, I could take more now that I have my SHE shed built.
More narrowband filter for better contrast? Larger objective for better resolution? Better mount for better guiding?
Here is my gear list.
Scope: AT65EDQ
Mount: iOptron iEQ45
Camera: ZWO ASI183M non cooled
Guide camera: QHY5Lii
Guide Scope: Stellarvue 60mm
Orion 5 position manual filter wheel
Schuler HA 9nm, Schuler 9nm Sii
MyFocuer Pro v2 (Robert Brown)
Bahtinov mask
Software: APT, PHD2, Sharpcap, CdC, Pixinsight, Photoshop, Nic Dfine 2, Astronomy Tools plug in, Team Viewer, Google Chrome Remote, autostakert!3, Registax
14 Nov, 2019 07:33
Do you refocus when you swap filters?  H-alpha is far out in the red end of the spectrum.
14 Nov, 2019 08:34
I'm not familiar with the CMOS cameras (I use CCD) but at the very least a cooled camera should improve your images.  More data probably and get yourself an Oiii filter.  A 7 position filterwheel will make life much easier for you so you don't have to keep changing filters.

01 Dec, 2019 03:42
Hi, I agree with Carole that a cooled camera will put you on the fast track to better images. I've read that noise is cut in half for each 6-7 degrees C you can cool the chip and that the first 15-20 degrees of cooling are the most important.
Best of luck !
01 Dec, 2019 11:34
There are two things that stick out to me about your post:

1A The first is your statement that "… pixel peeping reveals egg shape but ti is the best I can do with my gear".  I am unfamiliar with your mount, but a quick Bing search revealed that it is not a cheap mount.  You should be able to get round stars with a mount of that expense/quality, especially with subs of only 5 mins duration.  So what might the problem be?  Unfortunately there are many possibilities.  How is your guide scope attached to your telescope?  If it's in a finder-shoe then that might be something to consider.  Are you confident about your polar alignment?  Are you sure everything is tight (but not too tight, of course).  Any cable snags?  What is your Total RMS guiding error?  I would certainly check all of these things before spending any more money.

1B) It could be that eggy stars have nothing to do with your guiding.  I had an (expensive Japanese) scope that was incapable of giving round stars and which (after more wrangling than should have been necessary) had to go back to the supplier.  Could you have an issue with coma?  Before doing any of what I suggest in 1A), I'd check the scope.  Fortunately this is relatively easy to do.  Move your mount so that the telescope is pointing straight up in the air (at the zenith).  Take a handful of short exposures with the Lum filter (5 or 10 seconds, or maybe even less with your CMOS camera).  This will eliminate any guiding errors.  Are the stars round?  If not, then you will have to consider what might be amiss with your scope.

2)  The second thing that stuck out for me was your comment about focus.  Focussing with NB can be difficult due to the limited amount of light getting in.  I'd double check with a Bahtinov mask (on a very bright star) that your focusser was doing a good job of getting focus for you.  Because of the difficulty in NB focussing, many people like to focus with a Lum filter and then apply a 'filter offset' (programs like SGP make this easy to do).  You might have to devote a night or two to calculating your offsets, but then you will save time every night because focussing with an L filter requires much shorter exposures than focussing with an NB.

The other thing to consider is the frequency that you check focus.  I would suggest that it needs to be checked at least every hour.  I refocus depending on the ambient temperature.  At the start of the evening the temperature can be falling quite rapidly, and it is not unusual for my autofocus routine (triggered by a temperature drop) to kick in after each (20 minute) sub during the first hour of an imaging run.

A cooled camera would probably help with noise, but I'm not sure that that is what you meant by 'clarity'.

I hope some of that was helpful.

Good luck.

Edited 01 Dec, 2019 15:58
01 Dec, 2019 21:54
Steve Milne
You should be able to get round stars with a mount of that expense/quality, especially with subs of only 5 mins duration.

I second this comment.  With an image scale of 1.17, a median eccentricity of .59 suggests a pretty big guide error.  This is likely the first place to start.  A few questions about this:

1) Have you done a Periodic Error Correction (PEC)?  If not this is really low hanging fruit.
2) What sort of guide error do you see in PHD2 when guiding?
3)Are the corrections much larger in RA or DEC?
4)Are successive corrections often in the opposite direction?
5) How are you achieving the Polar Alignment?
02 Dec, 2019 07:54
I looked at your images at full resolution, and the issue is now more clear to me.

First of all: the images are great.

Yes, the stars are egg-shaped, and stretched along the same direction across the frame.

Poor guiding is the obvious explanation.

A less likely explanation is bad optics. I guess a flatfield design can produce equaly bad stars across the frame, if one element is astigmatic. (in a more simple design, there would be a sweet spot off center.)

The way to find out is to take short exposures of a bright star, and see if it is egg shaped.
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