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Trunk - Shame - Diagnosis?


Tim
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I grabbed 15 x 900 secs on the 20th April with my C9.25 @ f6.3, with the Elephants Trunk as the main target.

I focussed carefully as usual on a star near the centre of the screen, and left it to run.

When the subs were stacked, and on the individual subs too, the centre stars are slightly out of focus, as shown by the two sets of diffraction spikes (I have a crosshair that I fit to the SCT for some targets to produce spikes). However the bright stars at the edge of the image have pin sharp spikes.

The image was guided using the celestron OAG, and I am wondering if the spacing between reducer and camera, with the OAG in place may have been incorrect. Is this how it would manifest itself? I worked on 110mm from reducer to CCD, as I believe that is the right spacing? ? ?

As I had originally focussed on the central stars, it was maybe a temperature difference that caused the central stars to defocus, but the problem I am trying to diagnose is the variance in focus across the width of the image.

Anybody have any idea?? :icon_rolleyes:;)

Thanks

TJ

post-14037-133877371009_thumb.jpg

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I don't think its the CCD to Focal Reducer spacing Tim, unless its wildly out, as if you are a bit short, you just won't achieve the desired reduction in focal length.

Your 110mm sounds to be in the right 'ball park', as with the 6.3 and SXVF-H9C, my spacing is 105mm.

Dave

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The field flattener doesn't do a whole lot of flattening and I think that is likely to be the issue. The temperature shift might have taken the centre out of focus but brought the edges into focus.

I use this reducer with my LX200 ACF which has a naturally flat field, In theory the reducer should really mess up the field flatness but it only has a small effect.

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Tim,

I may be entirely wrong, but from the looks of things, you are in focus throughout the frame, but there is one star with two diffraction spikes. i think that is in fact two stars very close together producing two spikes.

In any case, I don't believe out of focus stars would produce two spikes, it would not produce them at all, or they would be "wider".

Andrew

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Thanks guys. The double star was my initial reaction, as there are several of them in IC1396 it seems.

However, sadly it isn't as easy an answer as that. All the central stars exhibit the pairs of spikes, showing the focus to be just a touch off.

As Martin says, I think the temperature change would be enough to lose sharp focus, I just cant understand why over such a narrow FOV the focus is different at the edges. More testing required methinks. It'll be easier once I have made a bahtinov mask for the C9.

Thanks for the suggestions!

(I should perhaps point out that I have tried my best to 'deal' with the issue in the picture above, but it should show a bit more clearly on this one, which is a single unstacked image. As yuo can see, the double has double spikes as well as the others.)

Tim

post-14037-133877371142_thumb.jpg

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No Martin, I never bothered to upgrade the trial. The stars at the edges aren't stretched though, can it happen that the stars are round but also at a different focussing distance between centre and edge?

George, thanks. I am just disappointed because a lot of work and preparation and a night without sleep went into the image, only for it to be out of focus. What a shame to waste a night on it.

TJ

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