Finding Focus

[Buzzard75]

I'm currently using a Redcat with an ASI183MM Pro and a ZWO EFW. All my focus is done manually. I have used the included Bahtinov mask fairly successfully several times now. I've never had a problem with focus changing, at least not enough to notice over the course of only a couple hours when there isn't much temperature change. I've also never had to change focus between filters even though I'm sure there is some minor differences. I've never had an issue like I did last night though. I used the mask on a bright star and got it dialed in, or so I had thought. I used the focusing aid in APT and checked the focus on my object with both the FWHM and HFD. Everything looked good. I went on imaging for about two hours. This morning I looked at the data and when I stretched the first image I knew something was wrong. I had donuts on all the dimmer stars.

So here's the question, short of getting an autofocuser that does it all for me, what tips and tricks do you all have to prevent me from getting donuts again in a stretched image when the raw data looks good? Should I have performed focus on a stretched image instead of the raw image?

Having done this several times with no issues, I feel like I'm missing something obvious.

[wimvb]

The intermediate step between manual focus (even when using the APT metrics) and autofocus, would be a motorized focuser with hand set. I have found that on my SW Newtonian, focus can shift when I lock the manual focuser. But as long as a motorized focuser is powered, the stall torque will keep focus position, so there is no need to lock focus. A motorized focuser also causes less problems with vibrations during focusing.

Edited December 19, 2019 by wimvb

[bottletopburly]

This may help if you need to autofocus the redcat https://deepskydad.com/autofocuser/redcat#dovetail 

[Davey-T]

Not that fond of the RedCat focuser, mine practically seizes up at infinity but guess this stops it moving when focused, only using a DSLR so just zoom in on live view, on other setups with autofocus I still zoom in to a couple of hundred percent to check the stars.

Dave

[bottletopburly]

26 minutes ago, Davey-T said:

Not that fond of the RedCat focuser, mine practically seizes up at infinity but guess this stops it moving when focused, only using a DSLR so just zoom in on live view, on other setups with autofocus I still zoom in to a couple of hundred percent to check the stars.

Dave

Is that using AF2 unit or af3 unit ?

[Davey-T]

Got one system using Focus Max 4, one using Maxim and AF2 on guider.

Dave

[bottletopburly]

I know af3 uses a different motor with  finer 1/256 steps 

[Buzzard75]

I have a friend/club member who has the AF3. He hasn't had much time to use it. I wanted his thoughts on it before I pulled out my wallet, but I will probably end up with one at some point. Manually focusing a helical focuser is somewhat of a pain.

I'm going to try again tonight since we will have even better seeing conditions than last night. I'll just have to be more careful I guess and be sure to check the stretched data as it's coming in. I've just never had what I thought was good focus only to find holes in the stars when I start processing. The while situation is just very strange to me.

Thankful for any other tips or things to look out for and check.

[Buzzard75]

My donuts have been eaten! Lesson learned is don't trust the stretch in APT. I put the image in Pixinsight and it was much more obvious that something was wrong. The stretch in APT is so noisy it was hard to tell that anything was amiss.

[Lilu]

i have a problem with focusing when I have barlow lens on. i have meade 80/400 adventure scope and with out barlow I can focus every eyepiece that I have but with it I cannot find any focus at all and yes Im aiming at the same things far far away....

[Buzzard75]

22 minutes ago, Lilu said:

i have a problem with focusing when I have barlow lens on. i have meade 80/400 adventure scope and with out barlow I can focus every eyepiece that I have but with it I cannot find any focus at all and yes Im aiming at the same things far far away....

If you're looking for help for your specific issue, you might be better off creating your own topic in the Beginners or Observing sections. However, the only piece of advice I can give you is that your focuser may not have enough backfocus (outward travel) to reach focus using your barlow. If you have one, you may want to try an extension ring on your focuser, put the barlow in that, and then try to achieve focus again with your eyepieces.

Edited December 23, 2019 by Buzzard75

[Lilu]

Thank you I'll try it

[Starflyer]

I got fed up of losing subs due to poor focus from temperature drift and filter differences and moved to auto focussing a few years ago. I use Baader filters and none of them are parfocal in my scopes, there was enough of a difference to ruin images if I didn't nurse the focusing.

Auto focussing saves me imaging time; I now auto focus where I'm imaging using SGP, so no need to slew to a brighter starfield to focus.  SGP will automatically refocus every X degrees change in temperature, so no more subs ruined by temperature drift.  I believe SGP can focus more accurately than I can using a bhartinov mask on a bright star.

I can't recommend moving to auto focus strongly enough, it's the same sort of game changer as starting to auto guide.