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Old 08-06-2017, 07:56 AM
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Bart
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Focusing my guide camera in an OAG

I have been trying to focus my guide camera in my OAG without success. Setup is QHY9, QHY CFW-2US, QHY OAG and QHY5L-IIM. I have focused the main camera on a star with a Bahtinov mask and then turned to the moon for the OAG as it is a big, bright, easy target. It seems the camera is too sensitive as the moon is washed out or I need more in-focus on the guide camera as when I draw the camera out, the moon appears to increase in size, so I'm assuming in-focus. I tried a star field with out success as well.
I did this once before with my QHY9 and Starlight Xpress wheel and OAG, and the moon trick worked well.
Any tips and advice greatly appreciated. I'm in an obs so not easy to try a terrestrial object.
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Old 08-06-2017, 09:49 AM
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billdan (Bill)
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Hi Bart,

This is what I did with a Lodestar guide cam and Orion thin OAG.

First up get the imaging camera (QHY12) set at perfect focus and measure the distance from the shoulder of the focuser to the end of the drawtube where the thumb screws are. I used a digital caliper but a short ruler would work.

I then did the same thing again but this time getting the lodestar at best focus and then measure that distance.

In my case there was a 2mm variance i.e the lodestar focal point was 2mm longer than the QHY12 focal point.

Now I could have added a 2mm spacer to the QHY12 but that would upset the 55mm distance required for the coma corrector to work. Instead I had to shorten an aluminium spacer on the Orion OAG by 2mm with a grinder. And then all good.

All the best and I hope you can get it sorted.

Bill
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Old 08-06-2017, 01:49 PM
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Thanks Bill, I was thinking that, and I think I may have done that last time. I'll give it a go.
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Old 13-06-2017, 05:50 PM
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I had the chance to do some day light testing, and definately do not have enough in-focus. The QHY5 has been a bit finnicky and now won't display an image at all either. Testing it with a friend tonight.

If I do not have in-focus, does that mean my imaging camera is too close to the prism? Do I need to space it out a bit further? Which of course will screw my flattener distance. I'm using the WO P-Flat 68-II which I'm not terribly happy with as there is a lot of slop in the barrel setup. Any thoughts on whether I can use my TOA-35 flattener instead? Would it work? It has a suggested distance to the metal back of 117.5mm as opposed to 51.5 of the WO.

Any suggestions and feedback wlecome, I'm in the dark here.
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Old 13-06-2017, 07:45 PM
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billdan (Bill)
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It won't make any difference which flattener you use, as the distance from the prism to the QHY9 sensor and the distance from the prism to the QHY5 sensor hasn't changed. This part has to be sorted out first, they both have to be the same distance.

It might be that the prism is located too deep into the light path and the prism stalk has to move up, closer to the QHY5.

You could try a small spacer in front of the QHY9 to make up the difference but as you say the WO flattener distance will then be wrong.

You could try getting it going without the flattener installed and then add a flattener later.

Clutching at straws here.

Cheers
Bill
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Old 14-06-2017, 08:17 AM
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LOL! Sounds like your in the dark as well.

I have some other attachments that can connect the camera to the filter wheel that will add about 12 mm to the length and I'll have a fiddle with the prism and move it further down. I might try playing with the prism and get it to just cast a shadow on the imaging chip then move it up enough to get rid of the shadow. I will take the suggestion on board and get it all working without the flattener and worry about that later.

See how it goes.
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Old 22-06-2017, 02:16 PM
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baileys2611 (Simon Bailey)
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I'm trying to use a non-rotating helical focuser for the same problem. Spacing is an issue as are adapters but it's about the closest solution I could suggest to getting fine focus on an OAG without shifting everything in the tiny FOV every time I make a manual adjustment.
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Old 22-06-2017, 07:34 PM
AstroApprentice (Jason)
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I use a Baader 1.25" Clicklock on my OAG. It has an adjustable micro-focusing collar with 6.5mm of travel. A more expensive option, but I had it spare after swapping to 2" EPs a while back - works well to fine focus my guider.
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Old 22-06-2017, 09:56 PM
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RobF (Rob)
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Hi Bart

I've been fiddling with my OAG config recently and feel your pain
Suggest you first of all pull out the QHY5, point the scope at something bright (e.g. a white wall) and look through the OAG port - hopefully you see a reasonably unobstructed FOV? If not, get the OAG adjusted so you're seeing out the front of the main scope ok.

Then, with the QHY5 back in, focus your main camera on a bright star, or a large bright cluster. Use a ruler to measure distance from OAG prism to face of your QHY9 sensor - that's the distance the chip in your 5 has to be at.

Adjust your QHY5, set guide program to 1sec images, move QHY5 in and out slowly and smoothly until you see stars.

Good luck (what could possibly go wrong!!)
If you're convinced your spacings are wrong, you should be able to figure things out during the day once you know the required focus distance for your QHY9 on a star at night.


P.S. If your 'puter isn't close to the rig while doing this, consider installing something like Teamviewer so you have a screen on your phone or tablet with you showing your guide software right next to you while adjusting the guide cam.
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