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I went back to the 350d for this shot, just cannot get the hang of the CCD.

Canon 350d 18X5min shots ISO1600 taken through the OO 8" Newt. One problem is the star shapes, I only have a 2 vane spider (Which annoys me as i like the four diffraction spikes of other scopes) but a few of the stars appear to have another spike at a wierd angle. Could this be collimination problems?

post-13107-133877372178_thumb.jpg

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Looks more like your tracking was slightly off to me, stars seem slightly oval....but then again it could be my tired eyes :)

Nice image all the same.

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Not too bad though Craig. Nothing getting into the light path from the focuser is there? The spurious spike does not have any spectral colours, the diff spikes do. I don't suppose your secondary cell is misaligned by some chance?

Yo could Make yourself a curved spider mate. If you want spikes, insert them artificially.

The glob itself looks excellent.

Ron.:)

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Ron,

What do you mean about the secondary being misalinged? THing is I did remove it to clean it last weekend so it could be. How do I check?

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Hi. Craig,

Alignment of the secondary is just a part of the collimation procedure as a whole. It's all a matter of 90 degree angles.

I used to do a master collimation, at least that was my name for it.

I always had a central spot on my mirror, and I know the physical centre of a mirror, is not necessarily the optical centre, but it's near enough. Any further adjustment, I did with a star.

I used to take the secondary out of the tube altogether. If the spider had central boss with a hole in it, I left that in place.

I had a disc of stiff card cut to the same diameter of the mouth of the tube. At the exact physical centre of the cardboard disc, I placed a pinhole, a diameter of a millimetre was best for me. I bisected this hole with a pen at 90 degrees, forming a cross with the pinhole at the centre. Carefully placed the disc into the tube, ensuring it was level al around. I shone a pen torch down the focuser to illuminate the disc inside the tube. It was easy to see the reflected cross in the main mirror. I would then adjust the mirror adjustment screws until the cross lined up exactly with the pinhole.

That ensured that the optical axis of the mrror, was running centrally up the tube. I replace the secondary, and positioned it centrally in the bottom of the focuser. Only at 90 degrees would it line up properly.

Unless the focuser was misaligned on the tube, and if it was, only the secondarry required adjusting. Of course in a fast instrument, an offset of the secondary is required, to ensure the flat captures all the reflected light from the objective. If the flat is an oversized one, then the offset can be ignored.

When you look through the focuser of the collimated reflector, the reflection of the secondary is a circle, along with the main mirror and the bottom of the drawtube of the focuser. No part of the secondary mirror holder should be seen. If the secondary has no cell, and is just cemented to a post, then no part of the chamfer should be seen. The elliptical mirrors reflection should be a perfect circle.

This may seem a long drawn out procedure, but It never bothered me to do it. Any subsequent adjustment it might have required, always involved only the secondary, and only because I often jarred the scope in the dark, fumbling about in the darkened observatory. The primary, although never tight in its cell, only had a minimum of movement, and not enough to warrant touching its adjustment screws.

I think it is Astro Baby who did an excellent collimation tutorial on the forum, and is highly recommended.

Anyway, I hope you get the problem sorted Craig, it can't be much, just a tweak here and there is sometimes all it takes mate.

Good Luck.

Ron.:)

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I you really want 4 spikes you could always tape a piece of cotton to the OTA at right angles to the 2 vanes already there.

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Forget about the diffraction spikes for a bit and congratulate yourself for capturing the dark lanes of the 'propeller' - excellent!

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Yes, don't get so bogged down that you can't see the picture. As Steve says. the prop is nicely visible and the overall it is a pretty M13

On the other hand you have given it a good amount of exposure so i think there is more that could be pulled out from the data. I agree with George, that star shape looks like a tracking/guiding error rather than miscollimation especially if they aren't quite identical from sub to sub.

What processing have you done on the image?

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I think it looks like star trails due to me only having 2 diffraction spikes. It makes stars look elongated! Or maybe I am just telling myself that!

I need to get somebody to take a look at the collimination of the newt as I think that must still be slightly off.

I am pretty pleased with the image, just wondered what the extra spike was.

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