Help needed - changing targets when guiding with an OAG


#1

How do you do it?

I’m now guiding with an OAG on my SCT (1422mm FL) which has improved my stars immensely. Last night I had a sequence running that followed the first target until near my western limit. It then slewed back to start the second one which was east of meridian. Plate solving worked fine but no stars were visible to the guide camera so the sequence failed.

This is obviously not a bug with SGP. I’m looking for help with technique. How can I automate a sequence and be assured of a star being in the guide window? When the first target started, the second target was below the horizon so I couldn’t actually check to see that there were guide stars.

Is this just the limitation of using an OAG and I need to stick to one target, even if it finishes at 1 am? Right now I just let it go and hope that it finds a guide star for the second target. Usually it doesn’t and stops.

I am considering a new guide camera. Right now I’m using an ASI120MC at 640x480 resolution. Maybe a mono camera would be more likely to find stars. Something like a QHY5L-II, Lodestar X2, or SBIG ST-i? What do you think?

Hey, maybe that could be a feature request. Could SGP nudge the scope a bit if PHD2 can’t locate a guide star?

Chris


#2

I’m not familiar with the ASI camera, but I’ve never had any problems at
all finding a guide star using a Lodestar on my OAG. What specific OAG are
you using?


#3

Orion thin OAG

Chris


#4

Yes, that is the classic pitfall of OAG. I think your options are:

  • more sensitive guide camera
  • ONAG
  • camera rotator to use the same guide star on both sides of the meridian

Interesting idea to have sgp nudge to find a guide star, but I imagine it would be tricky to implement. How would SGP know which direction to nudge, and how far can it go before the framing is no longer acceptable? The later is a judgement call so I doubt SGP or any program could automatically decide on that.

Most planetarium programs can be setup to show the FOV of the imaging and guide cameras, so you may be able to use that as a tool when planning your framing to make sure you have a guide star.

Andy


#5

For “Meridian Flip Options”, you could check a check box to “Pause After Auto Center”. This way it will allow you to manually check if guide stars are available in your guide camera. If you cannot find a guide star, you could manually nudge the mount until you find a guide star and then resume from “Pause After Auto Center”. The only problem with this is you will have to wake up and go outside unless you already plan to stay up all night.

You said your guide camera is ASI120MC. I assume this is a color camera due to the suffix “C” at the end. Color cameras are far less sensitive than mono cameras. You could get a mono version of ASI120M which uses the same chip as QHY5L-II and should be quite sensitive but the pixels are pretty tiny for your longish focal length scope.

I probably would pick Lodestar X2 due to extremely high sensitivity and good size pixels (8.2uM x 8.4uM) for your focal length scope.

Peter


#6

Here’s how I do it without a rotator: I pre-plan my guide stars, using a planetarium program (I use Cartes du Ciel). I don’t do meridian flips since I have a fork, but I do use SGP to automatically acquire multiple targets during the night. Since I know exactly how my OAG FOV relates to the main cam FOV, I know where it will be pointing. Now, since I don’t use a rotator, I have to pick secondary/tertiary/etc targets that happen to have a guide star in the right FOV. But I have a long list of potential targets, so I’ve never run out of options. I just have to plan it all out in advance.

For folks who want to do meridian flips and stay on the same target, the options will be a bit more limited, because you’re limited to targets that happen to have a guide star in FOVs on opposite sides of the main cam FOV. But it’s the same principle.

Kevin


#7

I used a lodestar with a TOAG and it worked fine. Try and increase your exposure?


#8

I really think the issue here is with a guide camera that isn’t very sensitive. I had a similar camera as the ASI and I found it woefully inadequate. Since switching to a Lodestar I’ve never had a problem finding stars and I never have to switch orientation to find them either.


#9

A few months ago I purchased my first CCD, an STT-8300M with 8 station filter wheel. No OAG. I already had a Lodestar X2 attached to an iOptron Short Tube 80 mm. Large FOV, very sensitive sensor. Very light weight scope and camera. I have never, ever not found a plethora of good guide stars. Not even something I pay much attention to, other than to monitor the seeing and guiding. Does not matter what part of the sky you are pointing at. Finds guide stars. My longest scope is a RC8" at 1624mm. IMHO this is the way to go.


#10

+1 to using a lodestar (or similar sensitive ccd) with an OAG. I always
found a guidestar with it (though I only had an 8" SCT) - even during
"galaxy season" when you point your scope out of the milky way in less
star-dense areas.


#11

At 1400mm FL, similar to jmacon, you can likely use a ST80 or similar for guiding. Might not be as ideal as OAG, but would be more resilient to these situations. But with a decent guide cam, you shouldn’t have much trouble finding a guide star. Perhaps use the auto exposure in PHD?

Doing a spiral search for a guide star would certainly be cool.

The QHY5l-II is pretty decent, probably the best bang for the buck and would serve you well. I think the Loadstar X2 is likely superior, but whether it is worth the cost premium is a tough choice.


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