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Still getting some rotation - What could it be?

swag72

By swag72
in Imaging - Discussion

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Whippy

Whippy

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Is the flattener a screw-in unit or one that fits in a 2" eyepiece hole? It could be that.. It could be the drawtube isn't sitting straight to the optics of the scope.... TBH it could be anything from the optics to the sesnor or anything inbetween unfortunately. It's a shame that you haven't got a second scope as you could test with that and eliminate either the camera/reducer or scope.

First thing I'd check is make sure everything is tightened up on and around the focuser, drawtube tension screws, adapters and everything else and take it from there.

Tony..

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swag72

swag72

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The flattener is a screw in one, so I am happy that must be straight. I think that the issue may be the focuer, I have had to tighten it a number of times as it struggled with the 5D2 and would just move out of focus on it's own.

I will check tightness all around - In fact we had to tighten the focuser again the other day - It seems to work itself lose somehow. That is what I am lacking confidence in.

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steppenwolf

steppenwolf

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My own analysis of the data (slightly different figures but the trend is the same) confirms to me that the camera (sensor) is not square with the light cone. This could be caused by 'sag' in the focus tube. I'd concentrate on checking this out - I'm seeing a 33% tilt at 89 degrees though.

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steppenwolf

steppenwolf

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It sounds worse than it is but the proof is in the image (especially the full sized version I worked on) - a clear degradation from bottom of the image to top. I tend to use these figures for comparison purposes after making a changes to the optic path only rather than using them as empirical data.

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Psychobilly

Psychobilly

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Thanks guys for takignthe tiem to look at the data I only looked at it by eye as I still havent got round to buying CCD Inspector as the bad winter meant that all the tinkering things i had planned didnt occur...

It might be worth rotating the scope in the rings in 90 degree steps - dont move the camera leave it in the same orientation as it is for the first image ... if the problem is drawtube related I would expect the "distortion" to move around the edges of the image ...

I think its largely due to focuser Sag.. which screws did you adjust under the focuser?

Peter...

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swag72

swag72

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We tightened all of the screws, except for one at the upper end as the allen key thread was knackered. Whether that one is lose, I don't know. There's a thread on here that someone pointed me to about the focuser and taking it apart and filing it and putting it back again. The screws on mine are different - Perhaps this is an excuse to replace the focuser all together. Hubby has just asked what I might like for my birthday and expects it to be astronomy related!

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steppenwolf

steppenwolf

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Whoaa, hang on to that money a little longer, right now, I would want to track that error down further and then see what if any adjustment can be made. What time and day did you take this image? I have plate solved it and the angle of rotation on the Rosette is 131.88 degrees East of North so depending on time and date, that will indicate how much gravity is playing a part here.

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swag72

swag72

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Hi Folks and thanks for your help so far.

The details of the image Steve is as follows - time 20:03 date 20.02.11

I'm not sure why I think another focuser would be better. I don't think that the focuser itself isn't sitting square in the OTA, more likely the play on the tube itself. That is the part that has been tightened on numerous occassions and seems to sometimes have a mind of it's own.

I do appreciate your support and do always feel like a bit of a numpty asking such dumb questions!

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steppenwolf

steppenwolf

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No problem with asking us all questions, that's what forums are for.

From what I can see, at that time/date and with that orientation, the camera would have been tilted at roughly 45 degrees to horizontal (east side high) so it is probable that there is more than just gravity at play here as I would have expected any 'sag' to be perpendicular to the ground and that would seem to not be the case here.

Can anyone else confirm my calculations?

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swag72

swag72

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Good point about the time Peter, the time I gave in the previous post would be +1 GMT - So for your calculations in UK Steve, unless you took that into account it would be 1903 your time.

Next time I am setting up I can take some pics of the setup if people feel that would be helpful? Next time I am out and setup I'll take some images as suggested, moving the camera round by 90 degrees. Can anyone suggest anything else to do to try to sort this problem. We have tightened up all the screws today, if that will help.

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steppenwolf

steppenwolf

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If your focuser were a rack and pinion type, you could have tried a Mylar shim but I am not sure how successful this would be on a Crayford.

I am a little concerned about constant screw tightening that you mention. I think we could really do with some annotated photographs of which screws you have been adjusting - could you post some?

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