Hi all, I wonder if someone could point me at a good process for autofocus using a celestron C11. The reason I’m asking is that where I live, the opportuities for night time viewing because of clouds/ rain have become so few that I don’t really have time to experiment anymore. I have been an SGP user for a few years and it is great software, but one area I have stuggled with is autofocus with the SCT. Currently autofocus fails having made a few attempts, so I look at the curve lowest point and set ny focuser to that, which gives a good visual focus. The problem is as you know, you can’t always be there…
I use a starizona feather touch focuser.
my autofocus settings are pasted below:
Many thanks for help,
Paul
Here are the settings that I have been using for years including when I owned a C11. I was using a Micro touch focuser for that scope. I’ve never cropped frames.
Sean
HI Sean,
I have a HD SCT-11 and autofocus works very well with my setup. I use the N-step focuser so I think the setup/setting might be similar. With that being said, every setup is different and what might work for my setup might not work for yours. but I think the following might help.
- Depending on your camera, I always bin at 4x4 with 5 seconds exposures with a minimum star diameter of 6. I also use a one shot color camera, ASI071
- My step size is 325 with 9 focus points. I’m thinking your step size is way to small. I suspect your focus curve is somewhat flat. Not like a “V” or “U” curve?
- If your step size is too small you’ll end up with a flat curve. if the step size is too large you’ll end up with “wings” out at the extreme end points.
I feel your pain, living in Ohio we don’t get very many clear nights as well.
Maybe if you can post screen shot of the focus curve that might help. I am not an expert of the subject but been using SGP for a few years now and learned a lot by trial and error.
Hope this helps,
Mark M.
Hey Mark,
I was actually trying to help Paul with his C11. I’m not having any real problems with my autofocus currently. I am always interested in finding out what other people are doing though.
I usually get a decent curve on my RC8 and Willams Optics 102. I’m using Moonlight for my focus control. I will however tell you that I don’t totally trust the focus routine. I get a decent curve but in the end I can almost always get a better focus after it runs by tweaking it on my own.
I’ll post a curve for you and let you see what is going on.
As for Paul up above. I think he is getting frustrated. I’d like to give him a hand if I could, but you might be the better person to assist him.
Thanks for taking the time to write me.
Sean
Funny, I didn’t realize it was two different people. I wonder why the screen shots were different but didn’t think much else about it. I’ll be glad to help Paul if I can.
Mark M.
Thanks Sean and Mahaffm, Did you have a good method for determining the backlash setting to be used for an SCT with feathertouch focuser?
Also having just read the SGP help file on backlash, how does one determine whether the backlash direction should be IN or OUT? The way I’m thinking about this is that for an SCT with a feathertouch focuser acting on the main mirror, the mirror weight will generally mean that as the focuser moves in the gears are meshing and when the focuser then moves out the backlash has to be taken up. So I think I should specify backlash as an IN value. Is that right?
thanks for helping out.
Paul
To add to the discussion, I use a C14 and have spent a lot of time refining the autofocus rouine. At a fl of 2.7 metres, I opted for a bigger step size to get a nice deep V curve - in my case that’s around 100 steps. I use 11 points and six sec exposures binned 2 x 2. I like the new focusing routine offered by SGP - seems to produce better results, at least with my set up. Well done all those who contributed to this improvement. I also dial in about 400 steps of backlash compensation (IN), plus I use a motorized focuser directly on the primary, so no Crayford.
Hi Paul,
I’ll explain how I have my setup working and maybe it might help you with yours.
A. Yes, my focuser, N-Step is setup so the primary mirror moves in and out to obtain focus.
B. Exposure Time = 5 seconds
C. Binning = 4 x 4
D. My Backlash is set to “IN”.
E. Backlash is set to 500 steps. I would recommend something higher to start with. I actually started out with my Backlash set to 1,500 but over time I found 500 steps work well for my setup.
F. Data points set to 9
G. Step Size = 325. Be careful I think this is were folks get a little confused. the “Step Size” is the number of steps SGP tells the focuser to move for each data point.
H. I have my crop set to 20%. It seems the stars on the outer edge of my scope don’t play well. So I found if I tell SGP to ignore those outer 20% it helps with my focus.
I hope I can explain this next part correctly.
For my setup of 9 data points Ideally we want the 5th data point to be the lowest point in “V” curve, “best focus” point. Therefore the purpose of the step size and Data points is so to get far enough out of focus to start with so the focuser can “Walk” back into focus stopping at each data point, in my case every 325 steps and takes an image and determines the focus value. So when the focus routine starts it moves out (4 data points * 325 steps) + my 500 Backlash steps. Lets say my focuser number when starting out is 1,000. It will move out (4 * 325 + 500) steps for a total of 1,800 steps. the focuser will be reading 2,800 since I actually started at position 1,000 plus the 1,800 steps it moved out. Once it walks out 1,800 steps from where it stated it then walks back in, “Backlash” 500 steps, stopping at the 2,300 step position and takes the image and determines the focus value. It now moves in another 325 step, another data point, and repeats. If you started out close to focus your 5th data point will be close to the best focus value. At this point the line should be dropping down and to the left. Higher step numbers on the right and lower step numbers to the left. and the HFR values will be lowering as the focuser walks in. once it reaches the 5th data point, again in this example if you started out in somewhat good focus this will be your lowest HFR. It then continues moving another 325 steps, step position numbers are going lower, for another 4 data points but the HFR values are going back up. Thus you should see something that looks like a “V” or depending on the seeing maybe something that looks like a “U”. Anyway the bottom of the “V” is the best focus step position. After all of that, using the the 9 data points SGP determines the step position that will give you best focus. Lets say it determines the “Best Focus” position is not the 1,000 that it started out at but 1,150. In my case it will move out to position 1,650. (1,150 best focus position + my 500 Backlash) then back in 500 step to land at the best focus position of 1,150.
One added note, determining Step Size is trial and error. but from best focus position to poor focus position you want the HFR value to be something around 4. So if at best focus position the HFR value is 1 you will want enough data points and step size so when the focuser moves out the HFR value is around 4 times bigger then the best focus HFR. Nothing really scientific about it just a rule of thumb to shot for. some folks use 7 data points with a higher step size. what matters is you move far enough out of focus so you can get a steep looking line when moving back toward best focus.
I know this was a long explanation and I hope it helps to make sense.
Mark M.
Thanks Mark for taking the time to do this. It all makes sense having read the SGP helpfile again. I haven’t used my kit for a couple of years whilst building an observatory and now getting back to it, but as mentioned above this last few winters where I live have been hopeless from an imager’s perspective, so trying to optimise the kit prior to a clear night has become more important.
Thanks to all for contributions to this, it all helps.
best wishes
Paul
I also have a C11 and Feathertough focuser. I use an Optec controller.
I have had good luck getting very nice focus curves.
I always do a rough focus using Focus & Frame tool. Usually set it to 2 sec and 2x2. I manually adjust until I get decent stars
For autofocus I use:
2x2 binning
8 sec exposure
600 steps
7 iterations.
I have backlash set to 1500.
I also turn off Smart Focus
I tried using 9 interations, but if the focuser gets too far out-of-focus, the focus curve gets distorted.
Roger
thanks Roger, much appreciate the help.
Paul
Thanks Everyone,
I knew people with more knowledge would prevail. Sorry I could not have been more help myself Paul but I saw you sitting there and wanted to help. I knew the folks here would chime in. What a great site.
Sean
