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Old 11-01-2019, 09:28 AM
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Better focusing with SGP

Hi guys,

For those using SGP, if I am getting tight stars with my biggest stars, but smaller stars appear out of focus, do I need to simply reduce the minimum star size setting? Or should I be adjusting the step sizes/no. of focus points for my V curves?

I am getting nice V curves with 9 points, BUT small stars appear out of focus/trailed. I can manually achieve better focus.

Suggestions welcomed!

Clear skies.

John K.

Last edited by John K; 11-01-2019 at 02:29 PM.
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Old 11-01-2019, 09:54 AM
garymck (Gary)
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Hi,
Focusing with SGP is problematic and in my experience is unreliable. So much so, that I have stopped using the software.

I have switched to using Starkeeper Voyager, which has fabulous focusing, the free CCDCiel also focuses well, APT is not quite as good in my experience (but being upgraded early this year, but is otherwise bulletproof.) Any of these 3 are better than SGP at focusing.

There is a 19 page thread on cloudynights regarding SGP issues. The best part of that thread is that I finally realised that the problems I was having with SGP were also being experienced by others !

FWIW
Gary
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Old 11-01-2019, 10:02 AM
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I still focus with a bahtinov mask. From refractor to SCT. The FSQ106N and the CN-212 are very sharp and demanding on exact initial focus. The mask is good enough to achieve very good near perfect focus visually. Once you are in focus you need to keep in focus and that's harder.

I used focusmax in the past and spent the time doing V curves or focusing by the numbers with a live view and a FWHM value. It's like polar alignment (provided you can see the SCP). You can get a level, a compass and good polar scope and be done in 30s or you can do drift align or use polemaster or use alignmaster, etc...

In the end, if you're not remote imaging what ever is easiest, usually visually, is the way to do it.
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Old 12-01-2019, 08:58 AM
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Ok, having to manage my expectations here. Severely light polluted skies in Brunswick, dark 4.5nm Ha Filter and F/7 system will always be tough on SGP or any software! So manual focusing does the trick. A 4 x 30 min stack without darks and just some noise reduction and stretching applied. Now just need another 40 of these!!

https://www.flickr.com/photos/johnka...blic/lightbox/
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Old 12-01-2019, 11:15 AM
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The data looks excellent. Just keep going.
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Old 12-01-2019, 12:29 PM
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The data looks excellent. Just keep going.
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Old 12-01-2019, 05:10 PM
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Even though they've worked on the SGP focusing over the years, it still seems to work best (most reliably?) with refractors. Works wonderfully for my 530mm FL - still giggle a little in juvenile satisfaction every time it picks optimal focus from the auto-focus run.
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Old 12-01-2019, 06:09 PM
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Hi John,

Not sure what camera you are using, however if you have a Lum filter in the filter wheel, you might find it easier and more reliable to focus with the Lum filter and use filter offsets to then set the focus position for the Ha filter. You can determine the offset using a Bahtinov mask to find best focus for each filter. You then tell SGP the difference between the focus steps for the Ha vs the Lum.

I think that's what the SGP boys recommend and that works for me.

Peter
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Old 12-01-2019, 11:08 PM
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When using anagrams "SGP" for example please remember we don't all know what they mean
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Old 12-01-2019, 11:21 PM
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SGP = Sequence Generator Pro, sorry Jeremy

SGP
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Old 13-01-2019, 12:25 AM
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SGP = Sequence Generator Pro, sorry Jeremy

SGP


Ok I still have NO idea what that is, must be a real thicky way too complicated for me
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Old 13-01-2019, 08:38 AM
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I have never had consistent results with SGP focusing. Focusing seems to almost reliable when the seeing is good and stable however outside this, not so good. (300mm f4 newt)

I do see there is a recent ""Feature Request" at SGP for an improved focusing process/routine.

Reply from Jared:
"Yes, Iíve actually been contemplating this a lot recently because of ďsome other tread on some other siteÖĒ. Iím not entirely sure what directly this is going to take. But initial thoughts are to improve the existing auto focus as well as provide some external mechanism for other parties to integrate with SGP. The latter piece may or may not happen. But minimally we probably need to provide a single star option as well as some improvements to the multi star option that currently exists."
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Old 20-01-2019, 04:51 PM
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Quote:
Originally Posted by DaveNZ View Post
I have never had consistent results with SGP focusing. Focusing seems to almost reliable when the seeing is good and stable however outside this, not so good. (300mm f4 newt)

I do see there is a recent ""Feature Request" at SGP for an improved focusing process/routine.

Reply from Jared:
"Yes, I’ve actually been contemplating this a lot recently because of “some other tread on some other site…”. I’m not entirely sure what directly this is going to take. But initial thoughts are to improve the existing auto focus as well as provide some external mechanism for other parties to integrate with SGP. The latter piece may or may not happen. But minimally we probably need to provide a single star option as well as some improvements to the multi star option that currently exists."
That's great that this is happening - that would solve a lot of issues.

Attached is what I see is typically what can happen. SGP will not use the smaller stars to focus on which is what I want - and it will just average the results across the screen - but if you could select a small stat field or star(s) that visually are smaller, then you are closer to focus. In this case I simply used the star with the 3.16 HFR to manually focus on and bingo all was good!
Attached Thumbnails
Click for full-size image (SGPFocusing.jpg)
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Old 25-01-2019, 11:39 AM
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John, what are your settings, binning, exp length, filter, data points, star size, smart focus?


Have you used a digital vernier and physically measured the distance 1000 steps moves your focuser? This is very important.



What scope are you using?


The biggest problem I see straight up is that there are not enough stars in your screen shot and it is taken in twilight or the exposure is not long enough. The stars also do not look very round, are you using a flattener and if so, is the distance correct? This will affect focus very much.
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Old 25-01-2019, 12:41 PM
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Quote:
Originally Posted by Bart View Post
John, what are your settings, binning, exp length, filter, data points, star size, smart focus?


Have you used a digital vernier and physically measured the distance 1000 steps moves your focuser? This is very important.



What scope are you using?


The biggest problem I see straight up is that there are not enough stars in your screen shot and it is taken in twilight or the exposure is not long enough. The stars also do not look very round, are you using a flattener and if so, is the distance correct? This will affect focus very much.

Hi Bart,

The screen shot is zoomed in 100% or more, so there are in reality a lot more stars and the image is stretched as well, so just did that to illustrate the point about doing a live focus type scenario on a small star. There are many stars that SGP captures when I run auto focus, so this is not an issue.

The scope is a CDK 9.5" f/7 scope and I was using a short exposure affected by seeing taken with a narrow band filter before dark - it's essentially like a live focusing image/ scenario. When I get SGP to do a focus run, it will take 15 second exposures through the HA filter.

One of the things I have adjusted is the coarse step size - I have halved this from 100 to 50 - this is giving me a better result - this may have been part of the issue.

The focuser is a Baader Steel Drive that has about 13,000 steps and about 35mm of total travel.

John K.
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Old 25-01-2019, 03:20 PM
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Try smaller steps, say 35, try binning your exposures x2, reduce your point count to 9 and try 8 or 10 for star size. Really though, you should physically measure your 1000 step movement with high accuracy.


Also, my opinion only, get rid of the Crayford style focuser and use a rack and pinion with back lash compensation. Crayford's are notorious for slippage.
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Old 28-01-2019, 12:39 AM
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I've just started using autofocus and having a bit of a weird issue, The first two steps are seem to have no change in HFR so looks flat, but after that, it's produces an excellent V curve. but due to the first two flat samples, it skews the median value. Has anyone else come across this?
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Old 28-01-2019, 11:47 AM
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Quote:
Originally Posted by that_guy View Post
I've just started using autofocus and having a bit of a weird issue, The first two steps are seem to have no change in HFR so looks flat, but after that, it's produces an excellent V curve. but due to the first two flat samples, it skews the median value. Has anyone else come across this?
Usually thatís because the stars are too diffuse. Try reducing your steps a little, and it wonít jump out of focus quite so far.
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Old 28-01-2019, 03:15 PM
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Dunk is quite right, your steps are too large. What is your step size currently and how did you measure it?
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Old 28-01-2019, 04:04 PM
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I calculated mine following the instruction provided by SGP. Doing the whole

((abs.pos at 3~4x hfr - abs.pos at focus)*2)/(number of steps -1)

I found my stepsize to be 35. I have a nema17pg27 motor if that helps.
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