Difficulty focusing on a target - Coma cluster/Abell1656

[ONIKKINEN]

Im having difficulty reaching ideal focus with this target, i have some ideas how to improve but tips welcome!

Now why is it difficult: I focus manually looking at the HFR values reported by NINA, and normally its very straight forward. 2s or 4s exposures depending on seeing guarantees that the measurement is more or less independent from high frequency seeing wobbles and is still reasonably short so it doesn't take forever. But with the coma cluster there are hundreds of galaxies, some of which are picked up as stars by NINA and they obviously are not point sources, so will impact HFR calculations drastically. They are also so faint that it varies quite a lot between subs, so HFR based focusing is difficult. I also think that as focus gets better, more and more of these faint galaxies are picked up as stars and HFR actually increases as focus improves. Judging from the reported HFR values my frames are junk (HFR of more than 4 pixels, in other targets this is go back home territory of bad) so i really cant use this (i should note that the frames can look ok or good with poor HFR though, so just the measurement is suspect). Having a newtonian i have handy diffraction spikes that can act as focus indicators quite easily, but here i run into another issue: my coma corrector, which produces uneven star images across my APS-C sized camera so in order to have a middle ground i need to use a star not in the center, but not in the edges either (not that i would want to use the edge ones, since they are not good).

Solutions i have:

Spend longer on focusing, with longer focus exposures to have better star images to visually gauge diffraction patterns from brighter stars. I used 15s subs and i was able to determine decent focus in a few minutes. But since my scope takes at least an hour to cool down i have to do this a couple times a night. These are painful minutes as the night is not even 5 hours long at this time of the year, but i must have good focus or the subs are a waste.

Focus somewhere else and then slew back to the target. Problem is: even though my diamond steeltrack is a very good focuser, i find it can slip after slewing when the weather is cold and the critical focus area to get good diffraction spikes and star images across the screen is ridiculously small with my Maxfield 0.95x coma reducer. Once all of the scope is equalized in temperature to the ambient temperatures, this problem goes away with tensioning the focus drawtube using the tensioning screw. Before that, this is an issue.

Would having an autofocuser fix this issue? Can the algorithm figure out ideal focus when the target is inherently throwing a spanner in the works? So far i have not considered autofocus because i am always scopeside when imaging and the setup is not left unattended.

 

Edited April 2, 2022 by ONIKKINEN

[scotty38]

I think an autofocuser is a definite quality of life thing even if you ignore all its other benefits 🙂 

Just off to have a look but can you do anything manually with the Hocus Focus plugin??????

[ONIKKINEN]

2 minutes ago, scotty38 said:

I think an autofocuser is a definite quality of life thing even if you ignore all its other benefits 🙂 

Just off to have a look but can you do anything manually with the Hocus Focus plugin??????

Just looking at the values, im not using the plugin.

Check the value reported from a few subs to get a rough estimate then move the focuser slightly one way and check if it improves or not. Once it doesn't improve its set. Good seeing and another target and this takes no more than a minute.

[michael8554]

Before moving to the target I focus my DSLR on a nearby bright star, using magnified LiveView, a Bahtinov Mask, and Bahtinov_Grabber software.

17 hours ago, ONIKKINEN said:

my coma corrector, which produces uneven star images

Shouldn't you fix that first ?

Michael

[ONIKKINEN]

39 minutes ago, michael8554 said:

Before moving to the target I focus my DSLR on a nearby bright star, using magnified LiveView, a Bahtinov Mask, and Bahtinov_Grabber software.

Shouldn't you fix that first ?

Michael

Ideally yes, of course. Bank account not in agreement on that though 😆.

Over summer break, which starts in 2 weeks ill do a number of upgrades and a new corrector is on the top of the list for sure.

[michael8554]

21 hours ago, ONIKKINEN said:

Ideally yes, of course. Bank account not in agreement on that though 😆.

Over summer break, which starts in 2 weeks ill do a number of upgrades and a new corrector is on the top of the list for sure.

Uneven star images may be due to tilt rather than a faulty CC.

Michael