When using the SGP auto focus feature I am trying to reach best focus with an SCT and am finding quite difficult to do so when compared to my refractor.
I use the Rigel n-step system connected to the main focus knob.
The focus points determined by SGP are not always the best with the SCT.
Would anyone have any pointer for me?
Thanks
Use the hysteresis feature to be sure that the mirror only adjusts in one direction (i.e. “in”) to minimize flop, and take long exposures.
Beo
Thank you.
I did not notice a hysteriris fonction in SGP…
I will take longer exposures as part of the focusing routine.
Yeah, it’s kind of hidden. I forget exactly where now. Let me know if you don’t find it.
Beo
When I use the Rigel I set it at 150 steps and 1500 (IN) backlash compensation. So just make sure that the compensation winds the knob clockwise for 1500 steps and then reverses itself 1500 steps counterclockwise. With that said, I think SGP could do with a better routine for SCT’s. It just isn’t very accurate and isn’t very repeatable in my experience. Seems I always have to do a bit of manual focusing to make up for the inadequacies of the focus routine,
Thank you for this. I will give that a try. So turning the focus knob counterclockwise raises the mirror back to the focused position. Is that correct?
That’s it Steven. You got it.
I tried that and it helped. I don’t quite get the backlash compensation in SGP… it went thru the 1500 steps out and then back in BUT before the focusing routine. It DID NOT do it after the focusing routine to make sure backlash was gone before reinitiating the imaging.
That’s odd. I’m pretty sure that mine finishes up with the 1500 out and back in on that final focus and analyze. I’ll double check that tonight though. But if by chance it isn’t then that kind of makes BC worthless.
Honestly on an SCT with a mirror that can flop around it is very difficult to get it dialed in. This is why most people give up and go with an external focuser and just lock the primary mirror.
The main issue is the way V curves work. You have a premium focus point of lets say for example 30000 steps. So the focuser has to rack out as in move the mirror towards the camera, lets say 5000 steps before it moves through 30000 and on to 35000. With all the back lash you are never quite sure where you will end up.
Then there is going to be some backlash before the motor actually starts moving the mirror up again. This is very difficult to measure because the mirror will actually tilt on the focuser side before the opposite side actually starts to move forward. So this is going to be trial and error. The good news is you can do it with an eyepiece. Get the moon in sharp focus by focusing so the mirror travels back towards the eyepiece. Write down the step position.
Then focus up away from the eyepiece until you can see in the eyepiece the focus change the slightest bit.
Note the new step position. The difference is your backlash.
Now reverse the procedure. Find perfect focus by focusing away from the eyepiece. Naturally you need to start 5000 steps (just a random number) behind focus and slowing go up util it is super sharp. If you go too far you start over.
Make note of the step position then focus back until the focus starts to change in the eyepiece. Note new focuser step position. The difference is the backlash on that side.
You should be able to use those numbers to compensate within SGP.
Under Equipment Profile/focuser/focuser name there is a box to the right called “other” I believe. Within that “other” you can input which direction you want to focus and how much backlash you have.
The only thing I have never experimented with regarding SCT’s with floppy mirrors is temperature compensation. I am not sure how that works because you can’t focus during an exposure like you can with a refractor. But I guess you could focus between exposures using it. I am not sure how SGP handles that.
Oh also if you checked the box “Use Auto Focus” and I assume you have, note that “smart focus” should be disabled if you have a SCT as the central obstruction can cause you issues.