Comparing Star Sizes with an Esprit 100ED and an Askar 130PHQ

To compare star sizes (well, the PSF anyway) correctly across instruments that images should be at the same pixel scale. So the question is: were they when you compared them across? Additionally, the instruments with the slower focal ratio produce larger Airy disk (directly in proportion to the focal ratio) so this is another effect you'd need to be aware of. Finally, seeing may also play a role there once the effect of different focal length, image scale and focal ratio are accounted for. For refractors there is also the additional encumbrance of accounting for refractive optics, viz. the wavelength of light and the different focal points (lengths) at different wavelengths.

interesting question Jerry. I've seen this play out on various scopes and always just expected it was Focal Length (EdgeHD8 @ 2000mm vs FRA400, etc). I hadn't expected that focal ratio would also play in. (still not sure why exactly)

Rafał Szwejkowski:
Askars always been known to have soft stars due to their optics but I am surprised that Esprit 100 outperforms a 130 scope.

Generally the bigger aperture the smaller your stars should be, relative to the pixel scale of course.

At this point I tend to believe the problem is more about my lack of experience and mediocre seeing conditions rather than the scope.  I've been studying the FWHM numbers of both the Esprit 100 and the Askar 130 and they compare very favorably, so I disagree that the Esprit is outperforming the Askar.  I think what is more likely is that I need to focus more often during a session because I've noticed that the FWHM numbers are creeping up larger as the night wears on.  That's either a focusing issue or seeing conditions are changing, or both.  I'm also finding that under Bortle 7 skies, the star diffraction spikes with the tri-Bahtinov mask are much harder to see clearly than when in a Bortle 2 or better site so I may have to up the exposure time and the binning when trying to focus under polluted skies and/or average or worse seeing conditions.

The other issue is my lack of knowledge in post-processing the stars.  I am using masks exclusively now to treat the background sky, the stars and the primary DSO as three separate components, that each require care and their own particular adjustments. 

Some of the images taken below with the Askar show very small, sharp stars while some don't look good but I still don't think it's the scope, I think it's the seeing and the photographer's technique or lack thereof. 

https://app.astrobin.com/equipment/explorer/telescope/7162/askar-130phq

Jerry Gerber:
When I compare star sizes on two unprocessed, unstretched subs directly from the camera (.fits)  I notice that the star sizes are quite larger with the 130mm refractor.  I know that my focus was highly accurate as I used a tri-Bahtinov mask and the 9 spikes were lined up exactly as they should be. 

Does a longer focal length produce larger stars, or is there something else going on here?  I'm assuming that seeing conditions were different on both nights so that may be a factor as well.

Thanks,
Jerry

Jerry,

Did you compare the FWHM (in arc seconds) of the images?  I didn't understand how you compared the sizes of the stars between the scopes.   Either way, differing conditions even on the same night would likely mask any optical difference between the scopes.  For example, I often experience subs at 1.5" and 3" FWHM on the same night and same scope.  Those are vastly different star sizes.

Kevin