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Old 18-09-2016, 07:05 PM
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barx1963 (Malcolm)
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A Question about Off Axis Guiding

Hi all!
I have an Atik One camera with the OAG kit. I have not used it yet as I have preferred to learn other things. I am now ready to give it a try!
It has a Prism that can be adjusted. I assume the idea is to set it so that the edge of the prism is just on the edge of the image? I have had a play with it and moved so when I take a flat frame with my light box, then stretch the heck out of the image there is just a hint of a dark line at the edge. Procedure was to set it low so it was very obvious and then keep taking images while edging it up.
I assume a thin line can be dealt with via flats or should I keep going until it is completely clear?

Cheers

Malcolm
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Old 19-09-2016, 01:07 AM
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billdan (Bill)
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Gooday Malcolm,

I would leave the dark line there. When we stack images there are usually stacking artifacts along the edges that need cropping out, this way gradient removal is a lot easier. So if we are going to crop the edges you may as well leave that dark line there.
As you say, good flats should remove it anyway.


Cheers
Bill
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Old 19-09-2016, 07:16 AM
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Slawomir (Suavi)
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Hi Malcolm,

I would personally move the prism a bit further away from the optical axis, so it wouldn't affect/obstruct light falling onto the CCD at all, as in addition to casting shadows, light can be diffracted on the edge of the prism and thus introduce more variations to the data. Yes, it can be corrected with good flats but all in all I feel that not allowing the prism to affect the photons hitting the main CCD could in some cases be superior to later correcting data with flats and hoping it will entirely remove all introduced artefacts.
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Old 19-09-2016, 11:24 AM
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alistairsam
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hi Malcolm,

Not sure if it has a filter wheel or not but try and look through the opening so you can see the window for the sensor and position the prism to be just above the long edge of the sensor window when looking straight (assuming its rectangular)

or can you pls post a pic of the view through the opening of the OAG

CHeers
Alistair
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Old 19-09-2016, 04:04 PM
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barx1963 (Malcolm)
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Thanks guys.

Alistair. The prism is in front of the sensor and I have an empty filter slot anyway. I have lined it up by eye but as I cannot guarantee that In am looking at it square, I thought I would test it's position with a flat.
Bill and Suavi. Thanks for the ideas. At this stage I am planning on leaving it where it is on balance. The shadow only shows up if I stretch the absolute crap out of a flat and I almost always crop anyway.
I am also trying to minimise the handling of the innards of the camera. Hopefully may get to do a proper test tonight and see what effects it has.

Cheers and thanks again

Malcolm
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Old 19-09-2016, 10:11 PM
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barx1963 (Malcolm)
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OK, so here is my next (possibly silly!) question about OAG
Does the position of the prism affect the focus distance to the Camera being used for guiding?
I have the Atik kit, and I simply cannot reach focus with the guide camera. It has a fairly simple focus arrangement, but when I focus the main camera the guide cam will not be focussed when wound fully in, and the focus get worse as I wind out.
I did place the prism in a bit to try to maximise the amount of light it picks up. Could that be the reason I cannot get focus?

Cheers

Malcolm
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Old 20-09-2016, 06:02 AM
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Hi Malcolm,

Yes, prism's position will affect the relative distance from the OAG unit where your guide camera will reach focus. If you push the prism in towards the optical axis, your guide camera will also need to follow that direction to reach focus. After all you may have to move the prism further away from the optical axis
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Old 20-09-2016, 07:59 AM
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Merlin66 (Ken)
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Malcolm,
in your OAG is it possible to tilt the prism?
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Old 20-09-2016, 12:02 PM
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barx1963 (Malcolm)
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Quote:
Originally Posted by Merlin66 View Post
Malcolm,
in your OAG is it possible to tilt the prism?
Ken
No, only move it up and down

Malcolm
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Old 20-09-2016, 02:48 PM
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Moving the prism up just a few mm should be enough to reach focus with guide camera.
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Old 20-09-2016, 03:16 PM
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barx1963 (Malcolm)
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I suppose if you think about it, it makes sense that moving it would affect the focus. The prism will be picking up light from the same distance in front of the image plane but the distance from that point to the guide camera sensor will change.
My thinking with moving it in was that this system is fairly fast at f5.6 and I am thinking that you are more likely to pick up a suitable star the closer to the optical axis you are.

Thanks for the advice guys!!

Malcolm
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Old 20-09-2016, 04:12 PM
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Slawomir (Suavi)
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You shouldn't have any problems with finding suitable stars at f/5.6; my telescope with a reducer works exactly at the same f ratio and I never had any problems with finding a good star.

I think keeping the prism as close as possible to the optical axis is particularly important with SCTs and telescopes that cause star shapes to deteriorate quite signicantly further away from the optical axis.
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Old 21-09-2016, 09:46 AM
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Yeah with a refractor you shouldn't have any trouble, you should have plenty of image circle diameter to play with.

Not had any probs with my Edge 8 @ f/7 either, but an uncorrected SCT or newt could get messy as Suavi says...
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Old 21-09-2016, 10:07 AM
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Merlin66 (Ken)
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Hmmm
I use OAG on SCT's (well sort of..) and the ability to tilt the prism to better align the guider optical axis can improve the guide image....
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Last edited by Merlin66; 21-09-2016 at 10:31 AM.
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Old 21-09-2016, 08:12 PM
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barx1963 (Malcolm)
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Managed a quick test last night. Dodged some clouds and after moving the prism closer to the guidecam a couple of mm, I was able to get focus. Probably needs a little tweaking as I was chasing sucker holes for the 2 hours I was out but I know it can be done.

Cheers

Malcolm
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Old 22-09-2016, 05:12 AM
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Glad to hear it worked for you Malcolm. With OAG longer exposures should be a tad easier
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