Achieving smaller star sizes: focus, guiding, or atmospheric seeing?

Thanks for the suggestion, it’s actually a very interesting test and I’ll probably try something similar.

Unfortunately I don’t really have Polaris / the northern sky visible from my location, so reproducing the exact test may be difficult, but I could probably try the same idea on another high-altitude star using very short exposures.

One thing I was wondering about is whether, if the FWHM improves significantly in very short exposures, the limiting factor might not only be seeing but also some time-integrated effects such as:

• guiding/tracking,

• mount limitations,

• microdrift,

• vibrations,

• or seeing integrated over longer exposures.

So maybe:

• if short exposures are already bloated → likely seeing / optics / focus;

• if short exposures improve a lot → then tracking/guiding effects could also be contributing.

Still, I really like the idea of the test itself because it isolates several variables quite nicely.

It doesn't reduce the effects of seeing, but it does reduce the variables at play, since we've disabled tracking and autoguiding; Polaris is a suggestion because it's a good star and practically stable on long exposures, that's all... (at least that's how I interpreted it).

Thanks everyone for the suggestions and explanations, they’ve actually been very helpful.

I think I’m starting to realize that my issue is probably not caused by a single factor, but rather by a combination of:

• seeing / atmospheric turbulence,

• relatively aggressive sampling (~1.42"/px),

• GTi limitations at 420 mm,

• and possibly some DEC backlash/stiction behavior.

The comment about comparing guiding RMS to seeing also made me realize that one of the core problems is that… I don’t actually know my real seeing conditions.

With RMS around 0.9"-1.3", the interpretation changes completely depending on whether the seeing is:

• 2",

• 3",

• or 4".

And at the moment I have no reliable estimate for that.

The short exposure idea is very interesting because it removes guiding/tracking from the equation and could help isolate what is already present “instantaneously” versus what accumulates over time.

Unfortunately I don’t have Polaris / the northern celestial pole visible from my location, so I can’t reproduce the exact test properly. But I may still try a similar experiment with very short exposures just to compare FWHM evolution versus exposure length.

One thing I also noticed while reviewing my logs is that during guiding calibration, the last DEC reversal step does not fully return to the initial position. The mount seems to struggle a bit when reversing DEC direction, suggesting some backlash or delayed response there.

So now I’m wondering whether what I’m seeing is something like:

• seeing already limiting the minimum achievable FWHM,

• with guiding/tracking/DEC behavior adding a bit more blur on top over longer exposures.

What keeps confusing me is that:

• stars are generally still fairly round,

• 120s and 180s exposures produce very similar star sizes,

• but the overall star size still stays around ~3.5 px.

So at this point I’m starting to think this may simply be the realistic combined limit of:

• current atmospheric conditions,

• sampling,

• and a small portable mount working at 420 mm.

Astrophotography really is the art of discovering new ways the atmosphere can ruin photons.

The first thing to realize it you are working with a very small aperture. In general, the diffraction limit for your telescope is about 1.6 arc/sec. This is the best it can deliver even if it was in space.

Here are the things that in my experience will reduce star size in refractors:

Blue and IR bloat. Makes sure you are filtering the extremes of the spectrum out. If no other filters are used you’ll need at least a UV-IR cut filter and depending on your optics you my need something stronger like a fringe-killer. Instead if using something like the L-pro, you’ll get tighter results with a dual band Ha-Oiii filter, certainly you will get smaller stars.

Shorten exposure times. The longer you expose, the more your stars will bloat. I use a 585 and my exposure times are typically anything from 15 sec. to a maximum of 60 sec. Shooting the 585 in HCG mode brings read noise down to very low values. Going long with a 585 isn’t doing anything other than allowing more time for guiding and the atmosphere to blur your images.

Focus matters. Get an EAF. You are probably doing a good job if getting a decent focus to start with using the mask but refractors change focus pretty rapidly as the temp. changes. If you want sharp results, it’s critical that you maintain perfect focus and during a long session, the use of an EAF is the only practical way to do it.

Processing. with a small sensor like a 585, star reduction/sharpening is not an option, it’s a must. BlurX in Pi is the gold standard and will transform your images. If you want to stay on the free software side then Cosmic Clarity is the way to go.

Hope all that helps!

On guiding, make sure that your mount balance is spot on. The modern habit of loading everything on the OTA can sometimes lead to weird balance issues. The rig balances in one orientation but not another because the actual CG is offset. With dec unlocked make sure that the OTA doesn’t change position no matter what the orientation is.