Just a quick question - see the shot below, its 132 sces of Eta Carina last night, from a C9.25 auto guided using a Lumicon OAG with a F6 (I believe) focal reducer.
Is this coma, poor focus or lacking of colimination I am seeing? And based on this - if it is coma - how do I correct it?
Do I remove the focal reducer from the OAG, do I need some sort of field flattner or do I need to better focus and/or coliminate the SCT?
Hi Matthew,
while waiting for some more knowledgeable folk to answer this, a few questions.
Is this the full frame? What is the image like without the OAG?
The coma I get and have seen is normally like StarTreks, Warp Factor Nine, and receding from the centre towards the corners, all four corners. Yours is more in one corner than all of them.
Maybe have another crack at stars only I find something like Omega Cent, centred is a good test, as there are enough dimmer stars about the frame to show coma etc.
Gary
If you look at the medium bright stars or parts, you will notice a large drift. But within the same location the fainter stars dont suffer from the same aount of error, and in fact some stars are round. The brighter stars normally mask most of the error due to its blooming. So either there is wobble or a bump that is or caused your problem.
Thanks guys, it looks weird to me. Bottom left of frame the stars looked like they streaked or drifted in, Top left of frame the stars look like the drifted down and might be out of collimination / focus (but to only half the shift of the bottom left). You are correct in the middle of the frame there is some image shift slanting upwards.
So for some reason - there does appear to be a tracking issue with this shot (possibly with less than perfect collimination and focus too).
Will coma appear on even shorter shots - say 60 seconds snap of Omega Centauri - just to check I really have no coma and its all tracking. focus and/or collimination to address?
yes that was a full frame - reduced to 1024 * 768 to upload easily!
Hi Matthew,
yes, grab a shortish shot of something that fills the frame, if Omega Cent does, then do that. If it shows stars it will show coma.
Attached is a JPEG quick and dirty from the 8" I put together. It was a test to see if I needed the Baader MPCC. I think the answer is self-explanatory, LOL.
Depending on the scope you are using in your case maybe try this. I think it would have been perhaps a 30 second shot.
Gary
Thanks! Here a planetary nebula with alot of star s around it - I would guess its telling me the OAG's focal reducer is causing unintended consequences, as all the stars flare out from the cntre when you look at it under magnification. The fields was very flat before the OAG was added!.
I used CCD Inspector to check collimation - perfect in X, 0.2 arc seconds error in Y. And MaximDL check focus to get optimal focus (FWHM 2.28" with a pixel scale of 0.645), CCD Inspector says curvature is 60% - which seems extreme to me?
This is a stacked 20+ minute shot (55MB TIF reduced to a 185 KB jpg).
Well I e-mailed tech support at Lumicon - again - still awaiting reply and cc'd Steve at MyAstroshop who I bought all the gear off.
Steve suggested moving the focal reducer - I can't see that you can do that inside the OAG itself - anyone know different?
Then Steve suggested moving the DSLR - at about 2cm further out from the OAG coma is still very present but greatly reduced - almost bearable. Alot of effort to re-position camera, focus and then check for coma. Especial when Lumicon say pin point stars across the field of a 35mm DSLR...
Hoping Lumicon come back with something helpful soon!
Could you provide a Single frame image, do not stack them.
Then we can see what the shape really is like.
Just a single frame exposure and then lightly stretched.
Yes to the internal reflections - puzzled me at first - thought it was the camera - until I took out the OAG and cleaned off what looked like an oil drop on the lens - so that should be fixed. Will post a shot tonight showing coma on just one 4 - 16 second image.
Interestingly - when defocused on the 4 second shots - stars at the centre where like doughnuts - holes in the precise centre - stars at the edge of the frame where like squashed doughnuts - with the hole almost touching the inside edge of the perimeter in the worst cases. Moving the camera out from the focal reducer pushed the holes on stars at the edge closer to the centre of the doughnut - I presume indicating less coma.
A de defocused star or a dowble reflection focused star, will always show the shadow of the secondary in the midle when stars are in the middle frame. As stars move out to the edge, the shadow also moves, this is normal, as you are moving "Off axis". This is where you start paying dollars for the scope.. As properly corrected scopes will still show pin point stars when focused all the way to the edge, and the reflected double image or if out of focus will show the secondary shadow near the edge when the star is near the CCD edge.
Theo it does - the C9.25 was beautifully flat before the OAG (with its tricky little focal reducer was added). Lumicon advertise "pin point stars acorss the entire field of view of a 35mm DSLR".
I'm still awaiting their technical supports response...
The piccys - two 16 second shots - one of a faint star cluster near Hadar - the other Omega Nebulae.
Are you placing the reducer at the right distance from the CCD. Reducers need to be with +/- 1 mm in most cases, or you introduce coma.
Like the MPCC, it requires 55mm spacing between the reducer glass and the CCD surface.
You may need to check this out first.
Basically I think this is likely to be the crux of the issue. I removed the OAG and the field appears totally flat and CCD guider says collimation is perfect and curvature is great.
Lumicon said check the focal reducer isn't put around back to front (I reversed it an coma was horrible!)
So given my C9.25 has a Meade motor focuser in the image train, which is 3 inches long - maybe this changes the separation distance required between the focal reducer and the DSLR's CCD.
Unfortunately this means step by step move the camera in and out - major effort re-focusing then inspecting is coma better or worse... sigh!
Only that wasn't the way to go - moving the DSLR out is changing the focal plane, so the guide scope has to move a similar distance - which is really hard to do!
On a positive note - I took it all apart, cleaned it and re-seated the focal reducing lens and now - amazingly - its alot better. There seems to only be coma on the very far left middle of the CCD.
So I'll post a shot tomorrow. I'd say they simply didn't seat the focal reducer lens correctly in the OAG when they initially assembled things!
