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Getting elongated stars - even with correct Flattener spacing


dotmod

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Hi, hopefully someone can help me. I am getting elongated stars even with the required flattener spacing.

 

I am using the following set up, as seen on the picture:

- ASI 1600 MM Pro with EFW Filter wheel and zwo 1.25" filters (backfous of the 1600MM is 6.5mm and the filter wheel is 20mm plus the 1mm filter)

- Explore Scientific ED 102CF

- PrimaLuceLab 0.8 Reducer/Flattener (required back focus of 55mm)

- Baader 2" Cliclock

- the spacers are 1mm, 11mm an 16.5mm

 

With the spacers and the backfocus of the camera i am getting the required 55mm from the manual, but my stars are still elongated around my pictures.

 

Hopefully someone can help me or got the same problem and fixed it. Maybe I'm doing something completely wrong :/

 

- Nick

Setup.jpg

Screenshot1.jpg

Screenshot2.jpg

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That looks to be about as good as you can hope for. The flattener has done it's job as far as removing coma is concerned, but it cant do much for any spherical aberration which may be present in your setup.

Just a small crop and you are good, no?

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Yep, according to manufacturer's spec:

https://primalucelabusa.com/primaluce-lab-0-8x-reducer-flattener-for-refractors.html

(look at the table)

best you can hope to get fully flat is around 16-17mm diameter. ASI1600 diagonal is around 22mm, so outer 5mm is going to suffer somewhat.

Also, I would take 55mm with a grain of salt - for your particular scope, you might find that distance of +/- a few mm will improve things a bit (try 56 and 54mm and see if there is any improvement - simple 1mm T2 shim will do for added distance, and my bet is 56mm rather than 54mm :D if there is any improvement over 55mm).

What you are seeing in your image is astigmatism (cross shaped stars, with one component - sagittal, being dominant).

Here is diagram to help you recognize this in image:

seidel3.gif

(both types can be present, and then stars form a small cross rather than just being elongated).

Another thing to test with astigmatism is to check whether it is same in each corner - just find bright star - position in each corner, try to make it about the same distance to corner / edges and do a sub. Compare 4 subs. Level of aberration should be the same if each corner (although different orientation - see included graphs). If only one or two corners are affected - this means tilt or collimation issues. You can do this test with a single sub if there is plenty of stars in each corner - just look at average / predominant shape for each corner.

It might be that most of the sensor is at proper distance from flattener, while one corner is a bit further away or closer - due to tilt, and that corner will display most astigmatism. If this is issue - look into squaring focuser and / or getting threaded connection instead of clamping one.

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Well, it all makes sence now. Thanks a lot for your detailed anwers, that helped me alot.

It seems like I need to buy a new flattener that is fitting for my sensor size or just live with the 5mm that will get cropped on every image (rather not :D)

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