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SCT imaging: Is this Coma or poor collimation?


lukebl

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Hi folks.

I'm a complete newcomer to imaging with an SCT. After years of imaging with a Newtonian (with some excellent results), I thought I'd have a go with my 12" SCT. Now, I know that many would say that attempting DSO imaging with such a long focal length on a humble NEQ6 is madness, but I thought I'd give it a go. I am using a f/6.3 reducer.

Now, not knowing much about SCTs, can anyone tell me what these teardrop-shaped star shapes are caused by? (It's a quick and dirty capture taken in strong moonlight and with no flats, so please excuse the dust bunnies and other imperfections). Is it coma or bad collimation? Or something else. The distortion seems to occur evenly across the frame, and I thought that if it it was coma, then it would just be at the edges radiating from the centre?

28944364291_2d633e6f52_b.jpg

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Hi Luke,

What are the exposure details for this frame? If it is a guided exposure, how are you guiding this long scope?

I think that you can rightly eliminate coma as that would present itself more as elngated stars out towards the edges, as far as I know.

To me it looks more like poor tracking.

Once it's working it's going to be a lovely scale though - the Dumbbell is just wanting to burst through there!

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It's a stacked image using Starlight Live. A stack of about 15 x 30s images, as far as I recall. In my experience, though, poor tracking/guiding would just produce eggy stars, not tear-shaped ones.

I was using just a 50mm finder-guider with QHY5 cam, and was amazed that it managed to guide as well as it did on such a long scope.

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Poor guiding can indeed produce tear shaped stars and the fact that they are so uniform across the image would make me think that is indeed guiding. 

To get a 50mm finder guider to accurately guide a scope in excess of 2.5m is a massive ask and I would say impossible. The general rule of thumb is that your focal length of the guider scope should be a minimum of 1/3rd of the focal length of your imaging scope. The guiding may well have looked great as far as the PHD graph went, but this in no way will reflect upon your final image at that focal length. 

An Off Axis Guider is generally recommended at these focal lengths...

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I completely agree, Sara. A finder-guider seems complete madness for such a big scope!

I do have an OAG, but I simply couldn't focus on any stars with it when fitted to the 12". The finder-guider was a bit of an experiment, and I was amazed that it worked as well as it did.

I guess I will need to persevere with the OAG.

EDIT: Unfortunately, I don't have any individual frames as I was just tinkering with Starlight Live. I have some images with fewer stacks, and the teardrops are the same, so I don't think it's a guiding problem.

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Update: a further test with short exposures (1 sec) shows that the teardrop stars still occur, so it can't be the guiding.

A quick query regarding reducer/flatteners. I have read that the optimum distance between the back of the reducer and the sensor is 85mm. But is that true for all SCTs? It's apparently suitable for a wide range of SCTs, but they obviously have varying  focal lengths. Does this change the distance?

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There seem to  have been two identical looking versions of Meade Reducers over the years, many are 105mm spacing:

http://astro.ecuadors.net/celestron-f6-3-reducercorrector-with-dslr-on-c9-25/

Adding a reducer will, erm, reduce your focal length obviously, which reduces the guiding problem, and will reduce your exposure times too.

I would try the OAG again. Get your reducer+OAG+camera spacing right, then get the pickoff mirror to guidecam chip distance the same as the pickoff mirror to imaging chip distance, can be roughly done during the day on a distant object with the guidecam in planetary avi mode - is the qhy sensitive enough for OAG guiding folks?

Michael

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