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What problems am I seeing here? Spacing, tilt, collimation, something else? And how could I fix them? (APO frac)


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abb-inspector.jpg.51a1a82c5ea9dc6d96f0b1121a8c455a.jpgabb-inspector2.jpg.ea1a26d2be57f993834e865f55edc80c.jpg

Having been able to test my 3" focuser upgrade, and using my laser to adjust the focuser tilt to be as square as possible (as best as I can get it! maybe in a tolerance of 1-2mm at the objective lens), I have found I still have some issues with the images produced by my frac.

I think some of it might be sensor tilt, or possibly collimation (I had to cut the tube and re-install the flange, so some movement could be expected). I think in the images shown the spikes coming from the stars might be the heat packs I stuffed next to the lens cell to stop it dewing up (heater straps still WIP) but I can't tell until I next take the scope out 😕

I did notice that when adding and removing shims from my backfocus total, the astigmatism level didn't seem to change much. I'm finding the point at which it goes over from saggital to tangental tobe very hard to pin down. I am wondering if this corrector maybe isn't able to correct an APS-C sensor, as the previous owner used I believe a 1" format.

I can't see any obvious coma in the center, so maybe the issues I have left are purely to do with the corrector and a little bit of sensor tilt? I did try and check the camera for tilt when I got it and couldn't SEE any movement of the central laser dot, but that doesn't mean it isn't there... On the other hand, maybe the tilt could come from the various threads joining the camera to the focuser, such as the thread from corrector to filter wheel, to tilt plate, to male-to-male, to camera?w

I did notice that between the two images above, I had a meridian flip, and after the flip it seemed to make things worse, what do you think?

Here's some whole images to go with it:

whole-image1.thumb.jpg.a6ff3fa6e4705d44545647d4c7b310d3.jpgwhole-image2.thumb.jpg.d9d2aba87c9942215b224f16f1d6a2a9.jpg

Processes I should run through prior to and during the next session with the scope are greatly appreciated! I was hoping the world of fracs would be a bit easier to navigate but alas I am still struggling haha.

EDIT: The flattener is a TS-Optics 2” Photoline ED 0.79x specifically.

Edited by pipnina
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  • 2 weeks later...

Your last picture indicates your focuser isn't central.. your spacing could do with abit more, stars all seem to point towards the center.. but not really sure on what's happening with the stars in the first few images.. suggest collimation but they seem really weird.. what scope is it?

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1 hour ago, newbie alert said:

Your last picture indicates your focuser isn't central.. your spacing could do with abit more, stars all seem to point towards the center.. but not really sure on what's happening with the stars in the first few images.. suggest collimation but they seem really weird.. what scope is it?

Hi!

It's a "SET Optics 130mm f6.6 APO"

According to the previous owner the company went bust, I am the third owner of this scope so I am a bit distant from the original product page. I havent found anything about it online besides mentions of it being the same triplet lens cell as used in the now discontinued TS Photoline 130 f6.6. That said, without the corrector (unflattened/unreduced) platesolving claimed the scope only had about 800mm of focal length, which is closer to f6.1-6.2 than f6.6...

I've done a check on the focuser, by adjusting the tilt plate again. This time I used the laser and the crosshairs-paper on the front of the telescope, but instead of just aiming for the center of the paper, I noticed the laser got a reflection back from the lens cell. No chance to test this but I did adjust the focuser tilt a little to get that back reflection from just off center to bang on. I also tried to tighten the grubscrews on it a little. The weird thing between the image where the focuser looks fine, and the second one where tilt is evident, is it was on the same night's imaging! I think a meridian flip had occurred...

I realised why I was adding so many spacers trying to achieve backfocus too the other day when I was at work, it was because my previous newtonian scope also had a 55mm backfocus corrector, but had an m48 connection camera side. I hence used a 6.5mm thick M48-M42 step-down adapter, which is now missing as the corrector for this scope is already M42 like the rest of my camera end equipment. I found an M42 extension ring of 5mm on ebay, due to arrive in a week or so. Hopefully that lets me solve the issue as I have run out of threads to add spacer rings too for now!

 

I hope to test it again when the bits arrive, thanks for your response!

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6 minutes ago, pipnina said:

It's a "SET Optics 130mm f6.6 APO"

The one that was for sale on here?

I remember drooling over that scope as one day I will have a 130 triplet

Hope that the spacing sorts out some issues but got a feeling it may need a tweak on a optical bench 

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1 minute ago, newbie alert said:

The one that was for sale on here?

I remember drooling over that scope as one day I will have a 130 triplet

Hope that the spacing sorts out some issues but got a feeling it may need a tweak on a optical bench 

Indeed I bought it from a fellow SGL user recently!

I think you may have dodged a bullet however, I had to get a machinist involved to get the 3" focuser installed. The 2" crayford that came with it was absolutely not fit for purpose. It was wonky (more than 2mm deflection, produced poorly collimated stars and tilt as a result) and the grub screws that held it onto the circular dovetail flange were not sufficient either, and they also showed signs of a previous owner having frustrations, as the flat head cut in them were burred over and stretched as I tried to tighten them.

I ended up spending about £500 on top of the cost of the scope, to get a working focuser, plus weeks of stress worrying that I was out £2k before I worked things out with a machinist at work. I may as well have bought an equivalent scope new...

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On 08/12/2022 at 14:16, newbie alert said:

Your last picture indicates your focuser isn't central.. your spacing could do with abit more, stars all seem to point towards the center.. but not really sure on what's happening with the stars in the first few images.. suggest collimation but they seem really weird.. what scope is it?

The 5mm spacer arrived!

I have it out now. But despite best efforts things still aren't right!

I added two 1mm shims and there still seems to be astigmatism... and something else in the bottom left?double cluster test.fits

I include the fits since the jpeg edit I made might not be the best for seeing the problem...

double cluster test.jpg

Edited by pipnina
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I just took a picture with a bahtinov mask on. Looks normal in the middle, but in the top right and bottom left corners, the diffraction spikes seem to CURVE!?

In the top left and bottom right however, the diffraction spikes seem compressed on one side and stretched on the other? Is this indicative of a specific problem?

I've also noticed that the focuser is shifting again, when pointing east... I need to look at my cables and grub screw tightness again I suppose...

m39_test_bahtinov_mosaic.jpg

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