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Old 26-06-2011, 01:51 PM
Ross G
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Wide Field Platform - Potential Problem

Hello,

I had an unused WO vixen long dovetail bar sitting in my garage. I mounted it length wise on my EQ mount and was able to mount a finder-guidescope and 2 cameras side by side on it.

Great I thought.
I can double my imaging time (..great for cloud infested suburban Sydney...).
I can use 2x 50mm lenses and create a panoramic mosaic.
I can even add an extra bar and attempt 3D images.

Last night was clear so I set it up for testing. I had a 135mm lens with a Ha filter on one camera and a 24mm lens with no filter on the other.

Then as I looked at it I had a thought.
Would having the guidescope on the far right and not in the middle over the DEC axis affect the guiding?
If I put the guidescope in the middle and had the cameras on each end, would this still affect the images?

I imaged the Scorpius region for about 4 hours. I guided with PHD and all went well.

This morning when I looked at the images, I noticed that the stars to the left of the frame were not as round as the stars to the right of the frame.This was visible on all the shots from both the 135mm lens and the 24mm lens.

Could this be a result of having the guidescope on the far right of the platform?

What are your thoughts?


Thank you.


Ross.
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  #2  
Old 26-06-2011, 03:09 PM
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rogerg (Roger)
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Would having the guidescope on the far right and not in the middle over the DEC axis affect the guiding?
Only if balance, regidity or flexure are adversley affected. Providing the guide scope is moving with RA and Dec just as the cameras are, and that there is no flexure or balance issue, you can put the guide scope where ever you wish.

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This morning when I looked at the images, I noticed that the stars to the left of the frame were not as round as the stars to the right of the frame.This was visible on all the shots from both the 135mm lens and the 24mm lens.
This can be caused by many things and often is something you just have to live with and might be able to rectify in post processing. Some possible causes:

- Atmospheric refraction. You are working with wide fields so it is easily possible the difference in altitude of some stars in your FOV will mean they appear to move at a suffciently different rate other than sidereal due to atmospheric refraction. You would find that even though a star guided on in one part of the FOV is circular stars in another part of the FOV might distort.

- Lens or mount of lens to camera that is not perfect. If something is not perfectly flat to the focal path then you might find stars are not perfect across the whole FOV

- Field curvature of the lens combined with atmospheric refraction. Often at wide fields of view these two can combine to make different corners or sides of the FOV have circular stars and other areas do not.

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Could this be a result of having the guidescope on the far right of the platform?
I think it's unlikely. It's possible flexure and balance combined with field curvature could produce non-uniform stars, and balance and flexure could be affected by your chosen positioning of the cameras and guide scope. But specifically having the guide scope off-centre, that in its self shouldn't produce any adverse results.

Hope that helps.

Roger.
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Old 26-06-2011, 06:57 PM
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multiweb (Marc)
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Quote:
Originally Posted by Ross G View Post
Would having the guidescope on the far right and not in the middle over the DEC axis affect the guiding?
If I put the guidescope in the middle and had the cameras on each end, would this still affect the images?This morning when I looked at the images, I noticed that the stars to the left of the frame were not as round as the stars to the right of the frame.This was visible on all the shots from both the 135mm lens and the 24mm lens.
Could this be a result of having the guidescope on the far right of the platform?
What are your thoughts?
The position of the guider on your plate doesn't matter. What matters is which star you guide on (point the guider to) in relation to the center of the FOV of the imaging cameras. Ideally it should be fairly central to the FOV so field rotation won't show up as much if you PA is not spot on on long exposures.
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Old 26-06-2011, 09:19 PM
Ross G
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Thanks Marc, thanks Roger.

Your explanations have put my mind at ease and saved my wide field platform from the scrap heap.

Everything was ok until I set it up and started looking at it. I imagined how each item would be moving relative to each other and that's when the doubts set in.

I can now get on with taking some good wide field photos.

Marc, how were last nights images from Crago?


Thanks again.


Ross.
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Old 27-06-2011, 07:59 AM
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g__day (Matthew)
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If I understand how you've set your guide camera up - its a short telescope / guide scope - held in a finder scope bracket with small 3 pins to lock it in place and maybe a web cam or guider camera at the back?

I personally wouldn't trust the finderscope bracket and its three small screws to be secure enough to hold all of this without differential flexure. Maybe try experimenting - carve a crade out of plywood or something firm - attach the guide scope to your mounting bar firmly with that and see if you guide better or worse. It may look crude - but for a few hours effort and a few dollars cost you can check where your bracket and its three screws are too flimsy to hold the guide scope securely enough.

Just guess work on my part, but long exposures demand absolutely no flexure meaning total rigidity and solid engineering of brackets and clamps in my experience.

Hope that helps, Matt
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Old 27-06-2011, 08:51 AM
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I personally wouldn't trust the finderscope bracket and its three small screws to be secure enough to hold all of this without differential flexure.
At that FL you could boot the mount and still have a workable sub. Got the same setup and I can image even in high wind, trust me. The image scale is very forgiving.

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Marc, how were last nights images from Crago?
Ran out of power at 10:00pm... well not really power but voltage too low to run the mount. After doing a musical chair exercise on all the remaining batteries and power tanks to suck up any remaining amp I ended up packing up. Shame because the night was crystal clear with great seeing. I got about 2h worth of stuff. The lens is not ideal for RGB. Bit of star bloating especially in the blues so I might have to get a filter in front or work something out. Will post some stuff shortly.
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Old 27-06-2011, 02:10 PM
Ross G
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Hi Matt,

Thanks for the advice. The finderscope I have is from a Vixen reflector. It has the 3 adjustable screws at the back, but also has a further 3 small screws at the front of the bracket. I will check it for possible movement though.

Marc, pity you only got 2 hours as it was such a clear night. However, I am looking forward to see the images you shot. I am sure they will be excellent.
Which lens were you using, the 100 or 200? It might have some chromatic abberation. Stopping down might fix it.


Thanks.


Ross.
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Old 27-06-2011, 05:17 PM
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multiweb (Marc)
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Marc, pity you only got 2 hours as it was such a clear night. However, I am looking forward to see the images you shot. I am sure they will be excellent.
Which lens were you using, the 100 or 200? It might have some chromatic abberation. Stopping down might fix it.
Apparently a stepup converter DC to DC 12V to 16V would work (from JayCar). Matt uses one for his Titan.
I used the 200mm. I'll have to use a UV/IR cut in front I reckon.
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