Saturday, June 15, 2013

Weird reflection ... and tilt!

When I align my scope with bright stars, I often see a reflection:

Not sure if this is normal or if I should worry about it.

Check on the ccd-newastro mailing list and was referred to an interesting web page by Don Goldman that explains this. Very interesting article, but it doesn't seem to provide any ideas of how to fix this (short of buying more expensive optical elements). What it does point out though is that I have some tilt in my imaging train (the halo/reflection should be in the middle when the bright star is in the middle).

The donut has a diameter of 60 pixels, but the image was 8x8 binned, i.e. 480 pixels. The H694 camera has a pixel size of 4.54uM (=0.00454mm) which results in a diameter of 2.18mm. With my focal reducer, my scope has an f ratio of 6.2. That means that the reflection comes from a 6.2*2.18mm=13.5mm.

First, use Don's method to understand where in the image train these are. Here is a sketch of my imaging train:

Using Don's method of calculating at which distance the reflection is created yields to...

But first, check where the reflection comes from. I first checked if it's the same with all filters: yes! So, it's not the filters.

I checked my images with CCDInspector which confirmed this result. Here is the curvature map of a recent image:

The tilt direction is exactly what I could see in the image above.

The most obvious candidate is the adapter of the Lepus reducer as it is not screwed into the reducer, but fixed with 3 screws. I replace it with the celestron t-adapter.
... and no reflection...

Thinking about trying the Celestron reducer - which gets screwed in ...