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Backfocus question

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Hi I have uploaded my highest scoring Lum file from a couple of nights ago. There is some funny star shaped in it especially around the edges.

I am using A TS 107 Triplet with a 2 inch TS optics 0.79 focal reducer. EFW filterwheel and a Zwo asi1600mm pro Cmos camera.

By using the the adapters and adding them all up I get 56mm backfocus. According to this chart from TS Optics I need an extra 1mm.

Information from them here....

Back Focus - distances from the T2-thread to the camera sensor

depends on the focal length of the telescope:

♦ Focal Length to 350 mm .... 70 mm
♦ Focal Length to 420 mm .... 65 mm
♦ Focal Length 450-500 mm ... 61 mm
♦ Focal Length 510-600 mm ... 58 mm
♦ Focal Length 610-700 mm ... 57 mm
♦ Focal Length 710-790 mm ... 56 mm
♦ Focal Length from 800 mm .. 55 mm

My scope is 700mm So I am rigth on the end of one of those calculations.

Today I stripped everything down and checked everythintg. I was using a 1.2mm washer to get the extra 1mm distance but the washer was a hard plastic type and when I measured the gap it was more than 2mm. 

I managed to find some soft type washers in my old ZWO box remove the old one and put a thinner one in which is giving me around 0.9mm gap. Giving me a total of 56.9mm. Instead of 58.1mm

Do you think that should fix the problem? Can 1.2mm out make my stars look like this or have I got another problem. Not had chance to take the scope out to test the new configuration due to the usual clouds.

Any help would be great. Single frame is of M3.

Thank you for any help in advance.

 

 

 

 

2020-05-20_00-25-06_Lum_-20.00_60.00s_0018.fits

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Does look like back focus is just a fraction out. Sounds like you need to put the first washer back in.

 

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Yes I am hoping so. As soon as I get a few hours of clear skies I will restest with a short 20 second exposure to eliminate star trailing. Hopefully it will be better.

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