View Full Version here: : OAG query - Pick off prism light path - COMA on half my FOV - Fixed moved Guide Cam!
22-07-2009, 11:49 PM
I have a Lumicon OAG in my imaging train on my C9.25 scope. Plugged into my pick off prism is an Meade DSI II Pro. I note in PHD that there is bad coma on the right hand side of my screen - it that a sign the prism isn't mounted correctly or that the OAG isn't mounted fully level to the centre of the image path?
I have posted a screen grab RA is left to right, Dec is Up and dow). Just wonder if anyone has ever seen this or has any ideas?
23-07-2009, 12:10 AM
It sounds like something isn't square. What happens if you turn the DSI around? Does the coma stay right?
23-07-2009, 01:48 AM
Will try tomorrow. From memory when I stick an eye-piece in the guider I see coma too - so I think its maybe the prism.
23-07-2009, 01:51 AM
From the orientation of the camera can you tell which direction the coma is facing. I mean is the coma radial to the optical axis? Do the images on the opposite side appear OK?
Which version of the Lumicon guider are you using?
Do your prime images show any coma at the edges??
I have the Lumicon Newtonian 2" version ( as I use the #1209 zero shift focuser) on the 12" Lx200 and never noticed excessive coma in the guider. Likewise I sometimes use the old Celestron Radial OAG and there may be some distortions of the star image but they are "constant" if you know what I mean, across the FOV.
Maybe as simple as something in the light train being out of sqaure???
23-07-2009, 11:25 AM
The coma runs along the RA, consistently top to bottom. I guess (to be confirmed, the further from the centre of the optical path the worse the coma).
On a DSLR imaging chip (Canon 400D) there is very slight coma at the edges of my shots.
The OAG version - its a one 1 year old 3" Lumicon OAG (about 8 inches long) specifically for the C9.25 SCT. I've removed its focal reducer removed - as with the Focal Reducer in the coma was horrible on the main DSLR imaging camera.
23-07-2009, 05:31 PM
Sounds like you've got a Lumicon Giant Easy Guider. From memory this has a large rectangular fixed prism built into the body and the "adjustment" of the guide pick off position is achieved by moving the guide camera tube forward and backwards along the body. Is this correct?
I'm also assuming that the unit is fitted to the rear cell with the guide camera at the 12 oclock position ie vertically upwards. That would mean that the long face of the prism is orientated along the RA axis and the "adjustment" is moving in Dec. Again, if the guide camera is central and square (?) to the prism, the the orientation must be at 90 degrees to the OAG body ( ie camera cable outlet to the side, not facing away from the rear cell) this would give the RA axis parallel to the RHS of the camera. If the camera cable is pointing to the LHS of the rear cell, then the extreme RHS of the CCD chip would be at the outer edge of the FOV. If all this makes sense, and is correct, then the RHS of the guide chip is see the extreme off axis image and as the OAG body is about 3" diameter, this is much further off axis than the Canon and may indeed have significant coma.
The coma should reduce as you move the pick-off closer to the centre of the FOV, this means moving the guide camera slightly further out the guide tube..... Worth a trial to see if this is what's happening, it then gives an option... If PHD is happy with the star image it sees and the guiding is OK, then all this is a bit academic; just keep using what you've got!!
23-07-2009, 06:34 PM
Its attached like this!
23-07-2009, 06:51 PM
With the guide camera sitting on the bottom, and the cable entry to the rear would mean the RHS of the chip is running up and down the dec axis. This I think is also the direction of the adjustment, so any edge coma would not be affected by re-adjusting the prism. ( It would show at the top or bottom)
23-07-2009, 07:41 PM
Using Ron Wodaski's CCD calculator I am seeing at 3*3 binning a pixel equates to 2.2 arc seconds - so average drift is +/- 1 pixel = +/- 2.22 arc seconds which isn't horrible.
25-07-2009, 08:51 PM
Interesting, I took a shot just after sunset when there was a strong light gradient across the PHD screen running West to East. Interestingly the brightest part of the screen is the region with the worse coma. So from that I interpret the closer the guide prism is to the main light cone - the worse the coma.
So I moved the adjustable glide for the pick off prism back about 6 mm - the screen is a bit darker but the coma is practically gone now!
Then of course clouds come in so I still have to tune the focus of the guide camera a bit better.
Now I just wish the guider had a micro adjuster - wish Lumicon had thought of that idea!
Just added a second shot - Jupiter broke through the clouds - so I centred it in my OAG and snapped it through the DSI II Pro (Mono) to see if I could get its moons into best focus (tricky). The attached shots show much improved coma and focus getting good enough to guide on!
It was quite a surprise watching the moons of Jupiter bounce around whilst I was capturing them in PHD - imaging at 0.2 sec shots on the DSI - PHD really showed the moons jumping around - making me think atomspherics could be playing havoc with my guiding?
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