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JamesAstro2002

What's causing these elongated stars?

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Posted (edited)

Hi,

Recently I've been able to get out under the stars and test my new Altair 102 triplet - however, I've noticed some odd star elongation and it's really got me stumped as to what's causing it.

https://drive.google.com/open?id=1rze_Lwnlm4LBiV2mer5gnLEQ2pKaRain  

This was the first test frame I was able to gather a couple nights ago. As you can see the stars are evenly stretched across the frame, going in the same direction.

https://drive.google.com/open?id=1BAE7u88qT0PBkm2jBS1atBiV8WVMftx9

This was the first frame I got from last night, and the stars looked a lot better. All I did was swap out the two 1mm baader derlin spacers I was using for just a single 1.6mm one, thinking the two stacked together might have been causing tilt or something. But later in the night have taken that initial sub I noticed the streaks came back again! I was looking at the same target, just a little later in the night and it occured again. Here's another frame of pinwheel to show it does it even towards the zenith.

https://drive.google.com/open?id=1kmJ9eN-d6PIeLPJ-G4pounnHZoCTjrdh

I'v ruled out tracking and guiding issues, as I can see the elongation even in a 1 second exposure, just as much as in a 3 minute exposure. Any ideas? It's got me stumped. Seeing that the stars were OK at the start of the night, could it be something to do with the scope getting cooler and the metal contracting? I'm not using any dew heaters.

Edit: Added stretched files:

NANeb.thumb.jpg.7352b2c2b18c7d5f25143fda113d2686.jpg180secSub.thumb.jpg.0f2c570ef014d5c2c5f64d6657fa6ac4.jpgWhirlpoolHa.thumb.jpg.31c70df2e1101bbd4ed5971ed823511a.jpg

Cheers,

James.

Edited by JamesAstro2002
Added Pictures

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Tilt would still be my guess, looking at those subs. If you're seeing it at short exposures then if you can introduce a tilt adjustment element into your optical train you could quickly adjust and verify this - they're not terribly expensive adaptors for T2, e.g. https://www.firstlightoptics.com/zwo-accessories/zwo_t2_tilter.html

If you've got any slop or slack at all in the optical train - and if you've got spacers in it, there will be some, then that may simply change by geometry of loading forces over the night.

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Hi James,

Any chance you can stretch and convert to jpegs and post directly to the forum?

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9 minutes ago, david_taurus83 said:

Hi James,

Any chance you can stretch and convert to jpegs and post directly to the forum?

Done :)

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Posted (edited)

Frankly not easy to see in those images, but if the elongation is even across the field, and trailing is to be excluded, then you may have a slight astigmatism issue. 

If this was the case, then reason for changing during the course of the night might be change in temperature causing the OTA to slightly contract or optics to adapt and hence focus shift. 

Moving away from focus could justify elongation increase. 

 

Edited by FaDG
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Run the images through CCD Inspector, It looks to me like colimation or focal plane alignment. I saw the same thing when I tested my RH200AT for the first time. CCD inspector made it super easy to fine tune it all in.

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