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Need some advice on collimating a 72ed


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Posted (edited)

Hello fine folks, first post here but long time lurker as I learn the foibles of this hobby.

I've been struggling with curvature on this scope and so it eventually brought me to order a cheshire to check my scopes collimation and sure enough, things were a bit out of whack. I've been experimenting with an ovl reducer and an orion 0.8x reducer/flattener but was still getting some eggy stars towards my corners, my backspacing is currently ~57/58mm but I've read around here it could be anywhere up to 60mm.

My question is to anyone who's been down this path and attempted to collimate the scope.

When I look through the eyepiece, it's impossible to line up the "doughnuts" in any orientation without physically tilting the focuser. This leads me to believe that perhaps the silver housing on the scope that holds the focuser (and the 3 adjustment screws) might not be square with the focuser.  Would I be correct in thinking that? If I tilt approx 2mm, with some finesse I can get the scope aligned but I imagine this is probably not ideal? There is a concave lip machined around the focus tube where the screws go and it's difficult to get anything tightened up and aligned correctly.

I'm nearly certain I read someone post here that they had returned one of their scopes due to a similar issue?

Any help or nods in the right direction appreciated, thanks.

Edited by Sien
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Posted (edited)

Hello, nope you are correct in saying that. I did some research and people certainly seem to use them and I had one handy so.. I put the lens cap on, shine a bright led through the side of the cheshire and it gives me a view of the concentric circles or doughnuts in view of the eyepiece.

I got a bit further in my adventures here.. I was hesitant to mess with the optics but found another post where someone disassembled the 72ed and there are only 2 lenses held in place by the screw in front so figured it might not be too much of a problem to open it all up. Having removed the glue from the front screw and loosened the front element, it seems I can now get much closer to where I need to be (still fiddling away here!). I have a laser collimator as well but will need to make a proper mask to check the accuracy and alignment further.

Difficult to capture an accurate picture through the eyepiece holding a phone but this is where I started. I imagine collimation got out of whack due to shipping.

20220510_132942.jpg

Edited by Sien
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You can get it perfect by loosening the three grub screws on the silver collar. Wiggle until you have the donuts perfectly stacked. Re-tighten them whilst hand holding the position you found.

Cheers

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Thank you, that's what I was going for. I couldn't get to where I needed to be until I loosened up the main objective.

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Posted (edited)

Hi

I had to collimate my 72ed as I had a bit of slop in the focuser and having issues with back spacing too. I've altered the tension on my focuser and sorted the tilt on my 294mc pro sensor (hopefully) my back spacing with an ovl fiekd flattener is around 60.5mm need to test to see it this is better as was using 60mm before and still slight eggyness. 

Here's a link to my thread that states how to colimate the 72ed. I used a laser collimator and circular paper disc with a dot on it. And adjusting the 3 grub screws on the silver metal part of the scope at the focuser end. 

https://stargazerslounge.com/topic/389932-eggy-stars-my-sensor-too-close/

Cheers 

Lee 

Edited by AstroNebulee
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Cheers Lee, I will dive into that thread asap.

I have been trying to unweld two of the zwo extenders to reach ~60mm but I think I'll just have to order more at this point, I've tried every variation and trick in the book without success and I value my wrists!

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49 minutes ago, Sien said:

Cheers Lee, I will dive into that thread asap.

I have been trying to unweld two of the zwo extenders to reach ~60mm but I think I'll just have to order more at this point, I've tried every variation and trick in the book without success and I value my wrists!

No problem, hope you glean some info from it. Yep I know what you mean about trying to unweld extender rings, I've had to order more before, now I put a small dot of lithium grease on the thread hoping it helps. 

Cheers 

Lee 

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@Lee yep, I put a smidge of superlube on all my adapters now, been down that road one two many times now and it's not fun!

Appreciate the diagram fozzy, I wonder why so many are having more success at 57-60mm, I realise a mm or two either side is to be expected but 5 seems a bit excessive.

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7 hours ago, Sien said:

@Lee yep, I put a smidge of superlube on all my adapters now, been down that road one two many times now and it's not fun!

Appreciate the diagram fozzy, I wonder why so many are having more success at 57-60mm, I realise a mm or two either side is to be expected but 5 seems a bit excessive.

Agreed, welded spacers aren't fun. The spacing issue must come from the 72ed older style as a tad longer than the 'newer' 72ed they brought out that's a few mm shorter as that's the only difference. Plus you Dan use the sw dedicated FF on the newer one and not on the older one as I've tried it on mine and couldn't achieve focus. 

Anyway I hope you get your scope collimated and sorted soon 👍

Cheers

Lee 

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19 hours ago, Sien said:

Hello, nope you are correct in saying that. I did some research and people certainly seem to use them and I had one handy so.. I put the lens cap on, shine a bright led through the side of the cheshire and it gives me a view of the concentric circles or doughnuts in view of the eyepiece.

I learn something new everyday 🙂

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  • 3 weeks later...
Posted (edited)

Don't we all :)

I took some pics tonight in variable weather, had gusts of wind up to +20kts but the sky was clear so I figured no time like the present. Stacked less than an hour of L data, tracking fluctuated between 0.5 and 1.5 arc". Stars look pretty damn good to me after collimating with the chesire. I recently installed a zwo eaf and it's driving me bonkers with the 72ed. Worked fine for a while but now i have slop and sometimes the tube slips altogether...if i happen to bump it while moving things it doesnt know where it is etc. Story for another day!

https://i.imgur.com/SmjZCRZ.jpg

elephants trunk last night with slightly more favourable conditions https://i.imgur.com/Eun72Kr.jpg

Edited by Sien
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That's a lovely image indeed 😊

With you focuser slop, I had the same issue on my 72ed (though I don't use a eaf) and tightened up mine using these instructions and now no slop or tube slipping. 

https://www.yumpu.com/en/document/read/22870696/adjusting-the-skywatcher-110-refractor-crayford-focuser-teleskop

And this is my thread where I fixed it. 

Cheers 

Lee 

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Posted (edited)

Thank you, I had set these previously when installing the electronic focuser but it seems things had worked themselves loose, things feel nice and solid again after re-adjusting. I also have an EFWx8 hanging off there, probably asking a lot of this little focuser!

Edited by Sien
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  • 2 weeks later...

Hi,

First time post here! Glad I found this, as this is exactly the problem I am having too, and been trying to fight it for the past month or so. I just wish I had found this sooner! I have added spacers, changed the 72ED connection to a screwed fix and even a faulty t-ring which had a lot of play and deviation.

My image train is M54-M48 Adaptor, Stellamira Flattener, spacers, DSLR, so it's Interesting you note 60mm backfocus...I have added 2.3mm so far and am running out of threads to screw the t-ring onto, so might need an extender in there too it appears.

Having thought all along my issues were backfocus and tilt using the command in Siril, I finally realised the other day having downloaded ASTAP that I have curvature as well, and as everything is now screwed together....that it is probably in the focuser.....and this thread seems to confirm it, so it looks like I need to order a Cheshire now and see if I can sort this out.

Just wanted to say thanks Lee for your threads linked here, very detailed and hopefully will allow me to sort mine out! How did you hold the Cheshire in the scope, as they typically seem to be 1.25" whereas the focuser is M54 / M48? Presume you have some sort of adaptor?

Tilt_00_Glob.png

Tilt_01_M51.png

Tilt_02_C38.png

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11 hours ago, WolfieGlos said:

Hi,

First time post here! Glad I found this, as this is exactly the problem I am having too, and been trying to fight it for the past month or so. I just wish I had found this sooner! I have added spacers, changed the 72ED connection to a screwed fix and even a faulty t-ring which had a lot of play and deviation.

My image train is M54-M48 Adaptor, Stellamira Flattener, spacers, DSLR, so it's Interesting you note 60mm backfocus...I have added 2.3mm so far and am running out of threads to screw the t-ring onto, so might need an extender in there too it appears.

Having thought all along my issues were backfocus and tilt using the command in Siril, I finally realised the other day having downloaded ASTAP that I have curvature as well, and as everything is now screwed together....that it is probably in the focuser.....and this thread seems to confirm it, so it looks like I need to order a Cheshire now and see if I can sort this out.

Just wanted to say thanks Lee for your threads linked here, very detailed and hopefully will allow me to sort mine out! How did you hold the Cheshire in the scope, as they typically seem to be 1.25" whereas the focuser is M54 / M48? Presume you have some sort of adaptor?

Tilt_00_Glob.png

Tilt_01_M51.png

Tilt_02_C38.png

Hi

I hope you can sort your issue too. I do find that the focuser on the 72ed needs a tweak once you start hanging more equipment other than a diagonal and ep. 

I'm up to 64mm of backspacing now and could still do with a little tweak.

I didn't use a Cheshire in the end but I used my laser collimator (I did check that it was collimated before use) and that came with a 1.25-2 inch adapter and that allowed me to for it easily and no play in the connection. The collimation of the 72ed is quite an easy fix once you get the hang of it. Check that your focus tube doesn't have any slippage first. If so fix that then collimate the scope. Anything else just ask. 👍

Cheers 

Lee 

Edited by AstroNebulee
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Thanks Lee.

Out of interest, which laser collimator did you use? If it worked for you I’ll order the same one 😂

Elsewhere I was informed that both a Cheshire and a laser will not work for a refractor and I should instead line up concentric rings on a bright star going in and out of focus. 

I don’t appear to have any slippage, I’ve never noticed any when focusing, and I have just checked out the mechanism and all is really tightly screwed in so this seems to be ok.

I believe my issue lies in the focuser attachment to the tube, ever since the scope arrived some slight OCD on my part never liked how the tube ring wasn’t square to the focuser…I checked this out the other day, and the end of the focuser doesn’t appear to be square to the tube ! I thought it was in the rings, but you can just see a part of the silver thread in the picture. I’ve tried removing it from the tube, but it’s rock solid and I don’t want to force it off.

Doing my best to measure , it’s between 1-1.5mm out between each end of the tube. So hopefully I can adjust the 3 grub screws on the focuser to sort this out. Don’t Skywatcher check these before they get shipped 🤦‍♂️

Thanks

Chris 

95CADA08-844B-4313-9783-3D962118DE6D.jpeg

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16 minutes ago, WolfieGlos said:

Thanks Lee.

Out of interest, which laser collimator did you use? If it worked for you I’ll order the same one 😂

Elsewhere I was informed that both a Cheshire and a laser will not work for a refractor and I should instead line up concentric rings on a bright star going in and out of focus. 

I don’t appear to have any slippage, I’ve never noticed any when focusing, and I have just checked out the mechanism and all is really tightly screwed in so this seems to be ok.

I believe my issue lies in the focuser attachment to the tube, ever since the scope arrived some slight OCD on my part never liked how the tube ring wasn’t square to the focuser…I checked this out the other day, and the end of the focuser doesn’t appear to be square to the tube ! I thought it was in the rings, but you can just see a part of the silver thread in the picture. I’ve tried removing it from the tube, but it’s rock solid and I don’t want to force it off.

Doing my best to measure , it’s between 1-1.5mm out between each end of the tube. So hopefully I can adjust the 3 grub screws on the focuser to sort this out. Don’t Skywatcher check these before they get shipped 🤦‍♂️

Thanks

Chris 

95CADA08-844B-4313-9783-3D962118DE6D.jpeg

That's great to hear the focus tube is nice and solid with no slippage. 

The laser collimator is a starguider one I bought back in 2020 for use on an old 150p I used to have and works perfectly fine. I think I got it from the amazon jungle or the bay of dreams and both available there. It would be great to use stars to test the concentric rings but with our weather in the UK whenever are there stars about haha. The lase collimator and paper mask worked brilliantly for myself. 

I never removed the silver ring on my 72ed and as you say is stuck fast (I can see what you mean about the thread showing). You can probably get around that offset when adjusting the grubscrews anyway. So, I just placed the laser collimator (checking that it was already collimated) into the adapter with it in the focus tube and undid the three grub screws on the silver ring and gently moved the white section arrowed until the red dot lined up in the paper mask on the objective lens and carefully tightened the 3 grub screws back checking the dot stayed central on each turn of the grub screw. 

Cheers 

Lee 

 

IMG_20220611_153159.jpg

Screenshot_20220611_153609.jpg

Edited by AstroNebulee
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Cheers Lee,

I’ve got one in order so will give it a go. Agreed on our weather! I had clear skies here all day and evening…within an hour of setting up the scope at 11 last night…clouds everywhere! Even the Moon disappeared!! 

For what it’s worth I did get some form of rings (within the star shape) and the blue halo  around it appeared uniform in the centre of the frame, but not anywhere else. But that may be curvature due to backspacing, so will await the laser and try from there. Thanks for the very quick responses, will report back hopefully with a good result! 

Chris

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On 11/06/2022 at 16:32, WolfieGlos said:

Cheers Lee,

I’ve got one in order so will give it a go. Agreed on our weather! I had clear skies here all day and evening…within an hour of setting up the scope at 11 last night…clouds everywhere! Even the Moon disappeared!! 

For what it’s worth I did get some form of rings (within the star shape) and the blue halo  around it appeared uniform in the centre of the frame, but not anywhere else. But that may be curvature due to backspacing, so will await the laser and try from there. Thanks for the very quick responses, will report back hopefully with a good result! 

Chris

No problems, at all, glad to hopefully help here.

Yep agreed with the weather, I did manage 3 quick sessions a few weeks ago but probably won't try now until late July. I'll look forward to your report when you manage to get out and test it. 

It's does sound like the backspacing with the oval stars in the corners and ok in the centre depending on it being to close or far away. You can check that via the attached screenshot on your previous images whether you need to less or more spacing 👍

Cheers 

Lee 

Screenshot_20220602_183718.jpg

Edited by AstroNebulee
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Posted (edited)

Sorry Wolfie was away on holiday and just came across these posts, I had a 1,25 adapter handy from my 10" dob so just slotted into that. The glue on your tube looks a bit suspect alright, have you measured from every side from the silver ring to the end of the tube? It's possible they applied the glue and then moved and set it after, perhaps leaving some residual that makes it look like it's offset? In my case I could not for the life of me collimate or get the focuser set correctly until I opened the ring on the front of the scope and moved the lens objective itself.  I relied solely on the cheshire for my adjustments, I felt that was enough and am a lot happier with resulting images from where I started but I'll tinker a bit more with spacing. I had to set the lens and fiddle and twist the retaining ring until I was happy with what I could see through the cheshire.

PS, plenty of people collimate their refractors with cheshires and lasers, I watched a vid where it was done professionally and they literally suggested it was better to learn how to do it yourself because they can fix your scope but unless they hand it to you, they can't guarantee it'll still be in shape after shipping.

Edited by Sien
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Thanks @Sien and @AstroNebulee

@Sien - I measured along the length of the tube on the side with the thread showing and the opposite side from this, and it was between 1-1.5mm difference. Assuming a perfectly accurate measurement along a tube I'd guess that that's pretty reasonable (?) and probably within tolerance at this price point.

Interested though in what you say about moving the lens, given that it is screwed in I had assumed it would be square and the problem couldn't lie here, but will take a closer look at mine now.

 

Well, just an update on an "interesting" week learning about collimating...my word, what have I got myself into 😬

I ordered a collimator (not a starguider like you have Lee....but it arrived in the same "Next Generation" box and the same instructions....must be an Amazon special !), and it was fractionally out, so I made a v-block and dug out the rubber blocking access to the screws. Just digging the rubber out, the screws got altered and this sent it MILES out of collimation. After several hours trying to fix it, and getting it to about a 50mm diameter circle, I dropped it and it broke !! So I ordered another one, and this seemed like it was collimated out of the box when I put it into the scope (I didn't test it as it arrived late in the evening, and I had clear skies...), and I also got a set of longer spacers, so did a test with 5mm spacing, and then 5mm spacing after collimating with the new laser, and below are the results...

image.thumb.png.491809f4bf4d9da8a81e37c8ed4c9ef4.pngimage.thumb.png.a6ea9fb74764f2933af077d350c077f8.png

......in other words, broadly the same result even if the tilt has flipped from one side to the other, and slightly less percentage of tilt reported by ASTAP.

Next up, I thought to actually check the new collimator, and found that whilst the laser dot was small in the scope, when projected over a longer distance the laser became a huge line instead of a dot....I then discovered a manufacturing fault had placed the washer inside partly over the laser!!! Unbelievable. So that's going to be returned, but for some reason, I tried the one that I broke previously.....and it was working again! :) After FAR TOO MANY HOURS, I managed to reduce the original laser to about a 12mm diameter circle, which is what it was to start with (!). By this time, it was pushing 11pm and clear skies again, so using my printed target, I managed to set the laser to rotate on the inner most ring (theory being, the centre of the laser should be in the middle of said circle shown below). Even if its slightly off centre from where I think it should be....it should get a better result in ASTAP. Right?

image.thumb.jpeg.110e391afe40c0abba98c84c6f20f8a7.jpeg

 

And the result? With a slight change of backfocus to 4mm.....

image.thumb.png.40869be3afacbff83e018fc2023e702b.png

 

........exactly the same. What a time consuming and frustrating week, other than knowledge gained I have literally got nothing better to show for it. Other than the dark circles under the eyes from the late nights 🤣

Still a bit unsure on the backfocus. I tried adding a 10mm macro extension tube prior to this, and it resulted in the star pattern for being too far away, and now by reducing it I have gone the wrong way so will try increasing slightly on my next attempt which may be midweek now looking at the forecast. At least I know it's between 5-10mm spacing, but I think the backfocus is also being thrown off by the curvature/tilt, since the same RAW file shows eggy stars in the bottom left+right, slightly in the top right but they are spot on in the top left.

I'm also going to have to try collimating the laser to be even more accurate, but it's requiring so much time to reduce the circle and requiring the absolute tiniest of turns on the screws now. I find it amazing, that a device sold for accurate collimating.....needs collimating itself. Surely they should be sold pre-collimated....🤷‍♂️

Clear skies!

Chris

 

Edited by WolfieGlos
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1 hour ago, WolfieGlos said:

Thanks @Sien and @AstroNebulee

@Sien - I measured along the length of the tube on the side with the thread showing and the opposite side from this, and it was between 1-1.5mm difference. Assuming a perfectly accurate measurement along a tube I'd guess that that's pretty reasonable (?) and probably within tolerance at this price point.

Interested though in what you say about moving the lens, given that it is screwed in I had assumed it would be square and the problem couldn't lie here, but will take a closer look at mine now.

 

Well, just an update on an "interesting" week learning about collimating...my word, what have I got myself into 😬

I ordered a collimator (not a starguider like you have Lee....but it arrived in the same "Next Generation" box and the same instructions....must be an Amazon special !), and it was fractionally out, so I made a v-block and dug out the rubber blocking access to the screws. Just digging the rubber out, the screws got altered and this sent it MILES out of collimation. After several hours trying to fix it, and getting it to about a 50mm diameter circle, I dropped it and it broke !! So I ordered another one, and this seemed like it was collimated out of the box when I put it into the scope (I didn't test it as it arrived late in the evening, and I had clear skies...), and I also got a set of longer spacers, so did a test with 5mm spacing, and then 5mm spacing after collimating with the new laser, and below are the results...

image.thumb.png.491809f4bf4d9da8a81e37c8ed4c9ef4.pngimage.thumb.png.a6ea9fb74764f2933af077d350c077f8.png

......in other words, broadly the same result even if the tilt has flipped from one side to the other, and slightly less percentage of tilt reported by ASTAP.

Next up, I thought to actually check the new collimator, and found that whilst the laser dot was small in the scope, when projected over a longer distance the laser became a huge line instead of a dot....I then discovered a manufacturing fault had placed the washer inside partly over the laser!!! Unbelievable. So that's going to be returned, but for some reason, I tried the one that I broke previously.....and it was working again! :) After FAR TOO MANY HOURS, I managed to reduce the original laser to about a 12mm diameter circle, which is what it was to start with (!). By this time, it was pushing 11pm and clear skies again, so using my printed target, I managed to set the laser to rotate on the inner most ring (theory being, the centre of the laser should be in the middle of said circle shown below). Even if its slightly off centre from where I think it should be....it should get a better result in ASTAP. Right?

image.thumb.jpeg.110e391afe40c0abba98c84c6f20f8a7.jpeg

 

And the result? With a slight change of backfocus to 4mm.....

image.thumb.png.40869be3afacbff83e018fc2023e702b.png

 

........exactly the same. What a time consuming and frustrating week, other than knowledge gained I have literally got nothing better to show for it. Other than the dark circles under the eyes from the late nights 🤣

Still a bit unsure on the backfocus. I tried adding a 10mm macro extension tube prior to this, and it resulted in the star pattern for being too far away, and now by reducing it I have gone the wrong way so will try increasing slightly on my next attempt which may be midweek now looking at the forecast. At least I know it's between 5-10mm spacing, but I think the backfocus is also being thrown off by the curvature/tilt, since the same RAW file shows eggy stars in the bottom left+right, slightly in the top right but they are spot on in the top left.

I'm also going to have to try collimating the laser to be even more accurate, but it's requiring so much time to reduce the circle and requiring the absolute tiniest of turns on the screws now. I find it amazing, that a device sold for accurate collimating.....needs collimating itself. Surely they should be sold pre-collimated....🤷‍♂️

Clear skies!

Chris

 

Keep persevering at it. You will get there, it is a time consuming job indeed but once completed so worth it. Keep checking your images as you go, you may still have tilt but the images coukd look ok to thd eye. I still have a slight bit of tilt in mine but looks ok when processed. 

Regarding your dslr and spacing issue, when I used my 600D with an ovl FF and my scope I just used a T ring direct in the FF to dslr. I thought the Stella mira FF is much the same as as the ovl and was 55mm. What dslr are you using? 

Cheers 

Lee 

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24 minutes ago, AstroNebulee said:

Keep persevering at it. You will get there, it is a time consuming job indeed but once completed so worth it. Keep checking your images as you go, you may still have tilt but the images coukd look ok to thd eye. I still have a slight bit of tilt in mine but looks ok when processed. 

Regarding your dslr and spacing issue, when I used my 600D with an ovl FF and my scope I just used a T ring direct in the FF to dslr. I thought the Stella mira FF is much the same as as the ovl and was 55mm. What dslr are you using? 

Cheers 

Lee 

Hi Lee,

I hope so! I feel like it's been a long time, a lot of testing and tweaking. Not helped by the late nights at the moment either haha.

I have a stock Canon EOS 77D. Correct, it is supposed to be 55mm for the Stellamira too and I started out with no spacers albeit with the standard 72ED push fit and a faulty t-ring which wobbled. These have since been replaced with a screwed fit and a Takahashi T-ring, which has no give in it at all.

On my first target (using the original push fit and faulty t-ring) with the 72ED, which was M51, it reports a 63% tilt in ASTAP and actually looking at the individual subs it is quite clear that the stars are very eggy, and to me ASTAP is reporting it correctly. As M51 was cropped quite a lot it's hard to see it in the final image (which I'm quite happy with!), but it is there, and without cropping it is very evident and from the star patterns suggests the backfocus needs increasing. That's why I have been adding spacers.

image.png.d9f4b76ab337c21bf250ee241247ae75.pngimage.thumb.jpeg.cdc584039da64bca72440e12f7d3aabe.jpeg

 

Since I have been trying to collimate, I could actually try removing all spacers as well and see what that does.....thanks for that thought, will give it a shot and see what happens. But at the moment, my subs definitely show eggy stars, not so much of a problem if I'm cropping the image down like above, but a full framed target (i.e. Andromeda, NA, Veil nebulae, etc) will cause me big problems.

The below test image from Markarian's Chain shows it quite well with only the very extreme edges cropped off. Not a great image as it doesn't have many frames but taken as an example to see how bad it would be on a full framed shot. The individual subs report anywhere between 45-58% tilt, so similar to M51 above. When Andromeda rises into view again I intend to frame it diagonally from corner to corner to make it fit, but based on the below it won't be great. The bottom right corner is so bad the stars appear out of focus.... I would almost be forced to do a Mosaic.

Markarians Chain was the last one I took with no spacers, push fit and faulty T-ring, I got into contact with FLO afterwards who have been VERY helpful, and since those items were replaced (and spacers added) the tilt values and eggy stars have reduced but they are still evident.

I'm convinced it's in the focuser so will continue with it, I guess you're right, perseverance will get me there in the end.

Cheers,

Chris

image.thumb.jpeg.9c42d3ab1a1a0103aced63d6fc3a2055.jpeg 

 

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