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Stretched stars and the case of the poorly made OAG


GalaxyGael

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A recent set of images showed stretched stars that were, to me, awful. Given the poor weather, I could not test tilt options but I knew that my camera sensor was dead square from my laser alignment rig. This was previously tested on the Tak e-130D and gave good stars across APS-C so tilt was not the case.

I also rotated the imaging train after having calibrated guiding, but tested that again and it was not the issue. Polar alignment, guiding all good (no excursions, DEC and RA both with similar rms and the guiding spread graph was a neat gathering of tight dots forming a circle).

I decided to examine the imaging train and always in the back of my mind I was waiting for the OAG to cause an issue, since hot-cold working of the screws last summer loosened it during a session. It bugged me that a screwed imaging train had a three pressure screw attachment from the OAG in the train, similar to having a screwed imaging trains and then attached it to the 2 in visual back like an eyepiece. But there is plenty of evidence that such as approach is fine once everything is set square and the screws are tightened symmetrically and gently etc.

Getting the the crux: the OAG was machined with a 1 mm difference in thickness around it! Thankfully I didnt try to vary the camera tilt to compensate for the non-square OAG, and just removed it. Result: perfect round stars across the frame.

I nearly ended up on a wild goose chase with tilt, guiding except that they checked out. I checked thickness of all adapters and they were good to about 0.1 mm in all cases, but the ZWO OAG was way out by 1 mm.

Maybe a useful tip or cautionary sanity check for someone who has odd stars - clearly some product QC in machining is not up to scratch and my first time being affected by it.

Anyone know of a full screwed OAG option by the way?

 

Edited by GalaxyGael
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We had this with a Starlight  Xpress OAG which could rock considerably once tightened up. Our solution was to run an alloy strap from the top of the guide camera to the scope. I dare say this would also work for you.

Olly

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5 minutes ago, windjammer said:

HI Colm

I bought one of these rings

https://www.firstlightoptics.com/zwo-accessories/zwo-m54-to-m48-adapter-short.html

and drilled and tapped m3 holes into the face of the OAG.  Screwed the adapter plate onto the front and put away all those internal bits into the junk box.

Simon

 

 

that could be an option. What I must do is check whether the 1 mm thickness difference is on the main OAG block, or the adapter that clamps into it. Together, the unit is off by 1 mm. If its the adapter, and the OAG is sound, I might go your route or similar

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18 minutes ago, ollypenrice said:

We had this with a Starlight  Xpress OAG which could rock considerably once tightened up. Our solution was to run an alloy strap from the top of the guide camera to the scope. I dare say this would also work for you.

Olly

If it works, it works!

although it poked me to daydream a kind of wallace and grommit mechanism of an imaging setup with twine, clamps, gizmos and cogs compensating for all sorts of issues together.. .!

12841D46-D9FC-3804-1947C172A5B7401F.gif.3e993d1cef7e9b74f672457870e1869c.gif

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22 minutes ago, GalaxyGael said:

If it works, it works!

although it poked me to daydream a kind of wallace and grommit mechanism of an imaging setup with twine, clamps, gizmos and cogs compensating for all sorts of issues together.. .!

12841D46-D9FC-3804-1947C172A5B7401F.gif.3e993d1cef7e9b74f672457870e1869c.gif

Wallace and Grommet are regulars at my place. They were here over the weekend, in fact, making new Samyang hardware...

NewSam1.thumb.jpg.2c0c958ea30f4d036f850ed48ddebe38.jpg

:Dlly

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Lovely, and that brings another topic up, maybe for the Lounge forum through.

Astro gear patina.

I wont sell my lovely figures 90 mm f/6.67 triplet since its still lovely to use but has black alu hardwear  (dew shield rim and back, rings, etc.) that in the sun, wind and the odd drizzle has oxidized to that lovely browny/gold color. Your rings and dovetail look like they owe you nothing, well -loved and I hope not used in anger...

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