How does focus and collimation affect your star size?

[Doversole]

Hi folks!

I pointed my RC8 scope yesterday on a nice Spiral Galaxy NGC2903 in Leo.

My equipment: StellaLyra RC8, EQ6-R mount, ASI533MC color camera.

I selected 44 subframes of 300s each, Gain 100, Bin2. PixInsight gives me a FWHM close to 4" which I found a bit disappointing.

My seeing conditions in West Sussex yesterday were around 1.88" (according to MeteoBlue), my guiding RMS was 0.85.

Giving these conditions & guiding, I believe I could get a better FWHM. So my question is: Can this be due to collimation? Or average focus (I use a bahtinov mask)? Would an EAF improve this dramatically? Or am I missing something?

Thanks!

[vlaiv]

1 hour ago, Doversole said:

I selected 44 subframes of 300s each, Gain 100, Bin2. PixInsight gives me a FWHM close to 4" which I found a bit disappointing.

Something is a bit off here.

Stars in your image don't really look like they are 4" FWHM.

Could you post raw linear stack in fits format for inspection. I want to see what sort of FWHM figure AstroImageJ will give.

By the way, I expect FWHM to be around 2.8"-2.9" for conditions that you mention - 1.88" FWHM seeing and 0.85" RMS guiding with your scope

[Doversole]

Thanks @vlaiv. Here you are!

I did use deconvolution to reduce the star size in the processed image above.

Light_NGC2903_300.0s_Bin2_533_gain100_20220322-221854_0022.fit

[vlaiv]

Well, yes - it appears that star FWHM is indeed very high.

One explanation could be - local seeing. That one won't be picked up by forecast but can impact things greatly.

This means - heating from local houses, or large concrete / asphalt structures that absorbed heat over the day if day was sunny. Anything that will affect high power planetary viewing will also increase FWHM.

Another thing could be focus. I personally find Bahtinov masks to be very imprecise at focusing. Best thing to do is to monitor FWHM / HFR values in frame and focus exposures and choose focus that minimizes those. Better and more expensive way is to let the computer do that to you by using auto focusing with focus motor (something I plan to DIY add to my imaging scopes).

 

[Doversole]

Thanks for the advice. I will try the focus tool included with the ASIAIR. I agree it may also be the local seeing! Fortunately we got a few clear nights in front of us!

[ONIKKINEN]

+1 vote for ditching the bahtinov mask.

I started comparing bahtinov mask focus to just HFR reading focus with NINA a while back and found that star HFR can often be improved by as much as a whole pixel even when focus seemed perfect with the mask.

Now that im used to it i get consistently up to 30% better focus by just looking at the reported star HFR values compared to a mask.

[Doversole]

I tried last night ditching the bahtinov mask and using instead the focus tool on the ASIAIR.

The curve was a bit all over the place, not really sure of what I was doing but I was surprised this morning to discover that I decreased the FWHM by about 30%.

So thanks for the advice!

[Shibby]

Wait - what?? I've always trusted my Bahtinov mask but now I'm gonna have to try without. Is there an easy way to manually focus in NINA with HFR?

[powerlord]

Im shooting this tonight, and have much the same - high 3.x.

forecast said seeing should be good too.  🤨