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Can't find a guide star with my OAG?


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I have: C9.25 - FR6.3 - Brightstar OAG - QHY 5

I've focused the QHY 5 and checked focus on the moon but when I move to a target I just can't seem to find a star. I even tried the double cluster knowing there would be lots of stars but nothing, a 5 sec sub with the DSLR showed plenty! On M15 I think I had the glob iteslf on the PHD screen as there was a little group in one lump but this was obviously no good for guiding.

I have tried rotating the OAG, adjusting the angle of the prism, changing exposure settings and the brightness slider but no joy! When I tried last night there was a lot of moon and LP but would this really wash out a 3 sec exposure of the QHY5?! I've wasted the only 3 clear nights of the last month and I'm reaching the end of my patience! Any help gratefully received!

John

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Hmmm

I use a "modified" OAG every clear night to guide on my spectroscope...

I used a C9.25 at f10 with 1-2 sec PHD settings with a QHY guide camera and could always find my "target star"

I don't think the problem is the hardware....

When you check your guide alignment on something like the moon are you checking the off set and distance between the guide camera and the imagning camera?

Once you know how to slightly off set the field of view in the imaging camera to that in the guide camera - I found it very staright forward to find the "target star" for guiding...

Try again with the OAG prism set, say in Dec to the main image; that way you know which way to move the scope to bring a bright image star into the FOV of the OAG...I can assure you it does work, and works very well.. practise your focus and OAG alignment during the day on a distant object, so you waste little time at night under the stars.....

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Interesting, I've got the Brightstar and a QHY5. I spent the weekend getting the QHY5 and my imaging Atik 16ic to both meet focus together. What I did find was that over the 20m length of my garden the guide FOV was nearly 40cm offset from the imaging FOV and I couldn't get the two to overlap.

Does that sound right?

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Thanks Merlin, I am sure it's not hardware too, I did by chance find a star with a "fixed" Meade OAG and PHD happily managed 10 min subs. This was shear chance though stumbling across a star.

When you mention a "target" star is this a star you have somehow pre-selected or do you take a test image with the main camera and pick a star to find with the OAG?

Also I didn't think about moving the scope to find a guide star, what is your process? Do you frame what you what to image and take an exposure to pick your guide star and then move the scope to bring this into the OAG field of view without losing the frame of you imaging target?

I was thinking maybe the FL of the 9.25 was the problem but if you manage without the reducer then obviously not!

Thanks for your help.

John

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I think it was Blue Astra, in a thread last year showed his OAG set up. I've copied the images.

The field of view which shows the different parts of a distant pylon is very instructive...

You can see clearly the relative sizes of the FOV and the off-set of the imaging frame to the guide frame.

If the guide camera is positioned above the image frame, say in Dec, then you can confirm your focus and alignment by a small movement of the scope in Dec which would then move a bright star from the imaging field to the OAG field.

Under CdC you can prepare an overlay which replicates this view and allows you to check the framing and the position of a suitable guide star. I use the ASCOM arrow controls under CdC to fine position the guider.

(My situation is slightly different from astroimaging... I need to find a "target star" and position it onto the slit of the spectroscope...this somewhat limits the movements of the scope to subsequently find a guide star! I have basically replaced the small guide pick-off prism with a beamsplitter which allows me to see the whole of the FOV; it diverts 4% of the incoming light to the guide camera - the rest goes straight through to the spectroscope...)

HTH

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Interesting, I've got the Brightstar and a QHY5. I spent the weekend getting the QHY5 and my imaging Atik 16ic to both meet focus together. What I did find was that over the 20m length of my garden the guide FOV was nearly 40cm offset from the imaging FOV and I couldn't get the two to overlap.

Does that sound right?

You don't want the fields to overlap because if they do it means that your OAG prism is in the lightpath to the main CCD. What you use with a standrad OAG is the area in the FOV which is outside the area of the main CCD. If your CCD is rectangular then you've got more room to play with above the long side of the CCD, since the short edge is closer to the edge of the FOV.

Helen

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As allready mentioned the best thing to do when you are first setting up an OAG is to practice during the day on a distant terrestrial object - my neighbour's LNB on their satellite dish has a large barcode on it that makes a nice target. It is much easier to spend a leisurely hour or two getting the basics sorted out during the day. This will allow you to:

1. Get the correct spacings on the image/guide camera so that they can both achieve focus at the same time.

2. Familiarise yourself with the controls on the OAG and how they affect what you see, such as prism adjustments and radial positioning.

3. Familiarise yourself with the relative positions of the image/guide camera fields of view. In particular, what you need to do to move the scope so that an object that is in the image camera FOV subsequently appears in the guide camera FOV.

Then on the first night out centre a bright star in the image camera FOV and focus it, using if possible a Bahtinov Mask. If your OAG has a radial adjustment, ensure it is set so that the guide camera is vertically above the image camera. Then, whilst continuing to take 'frame and focus' type images with both the image and guide cameras and leaving the Bahtinov Mask in place, slowly command the mount to move so that the star moves from the image camera FOV into the guide camera FOV (this is where item 3 above comes into play). On my setup I can see the star move slowly up the frame on the image camera until it disappears, then after a short period it appears in the guide camera FOV, whereby you can then fine tune the guide camera focus. It doesn't need to be perfect but getting the guide camera reasonably well focussed will make finding a real guide star that much easier.

Theoretically you should then no longer have to make any adjustments, but typically I repeat the above procedure at the start of each session just to check - sometimes the guide camera's focus needs a slight adjustment.

In terms of finding a suitable guide star, so far I've not had a problem. When imaging objects in or near the galactic plain there's usually plenty to choose from. If imaging something like a galaxy where you're out of the galactic plain, there's definitely less choice and once or twice I've had to use the radial adjustment to move the guide camera FOV to locate a decent guide star. When this is the case, it is possible that the prism shadow may appear in a corner of the image camera FOV - I have a QHY8 and have had this happen just once, but it can be cropped out of the final image. When it is in the 'normal' vertical position the prism on my Orion OAG doesn't obstruct the FOV of the QHY8 chip. I am typically using a 3 second exposure length in PHD with my Meade DSI1 OSC.

Edited by r3i
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I have: C9.25 - FR6.3 - Brightstar OAG - QHY 5

That is also my exact setup and I have no problem finding a guide star in a busy part of the sky. I usually use the "tools->auto select star" option as the stars are not always visible on the display. I usually set the QHY5 time to 4 seconds. A few random thoughts:

Did you set the QHY5 gain to 100% from the default 50%?

What is the distance between the reducer and the camera chip? If this is wrong and you are getting lots of coma at the edge of the field where the QHY5 takes its light from then it is much harder for the QHY5 to see a star.

The same applies if the QHY5 is very out of focus.

Try switching the QHY5 length to 10s to really see if there is anything in the QHY5 FOV, just to diagnose the problem.

Cheers,

Chris

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Helen, "You don't want the fields to overlap because if they do it means that your OAG prism is in the lightpath to the main CCD", of course, obvious when you think about it, I was worrying about nothing.

cgarry, " I usually use the "tools->auto select star" option as the stars are not always visible on the display", this also makes me feel better.

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