Just got my first newt - it's about F4.3 and I'm learning how to collimate it, but given all the variables involved I'm not sure how to interpret my results.
I think this is about as close as I can get the primary collimation with the cheshire. I'm waiting on a threaded extension tube for the Moonlite so that I can get more outward travel and thus use the auto collimator.
Two images attached, right one is without the coma corrector, left one is with it. What do you think? Bad collimation, bad corrector spacing, tilt, or a combination of the above?
From what I understand, a f/4.3 Newtonian can be a very capable imaging instrument
My guess is that there is an orthogonalty issue resulting in uneven spacing and thus different correction of stars (over, good, under correction) in four corners.
very much FWIW - a few ideas that might possibly help.
1. get a laser collimator and align it precisely - as Alexander showed http://www.iceinspace.com.au/forum/s...ad.php?t=75601
2. carefully check the central spot ring on the primary and get it exactly right.
3. do a laser collimation through the coma corrector
4. check that collimation holds reasonably well by moving the scope around the sky with the laser running - if possible tighten anything that allows movement of the collimation spot. pay particular attention to the focuser and make sure that the shaft is tight enough that you do not get any rocking of the draw tube
5. if you still have corner-to-corner variability, then you have tilt, either in the camera or in the CC. make a cardboard spacer shim so that you can screw on the camera at a different rotation angle on the CC. If the tilt has rotated with the different camera position, it is in the CC, if it does not change, it is in the camera. if the corners all look the roughly the same, but are not fully corrected, check the CC spacing.
also FWIW, I had to add strengthening to the CF OTA around the focuser to stop flex and had to put new side shims on the primary to stop it from sliding too far sideways. I also added two nylon locking screws on the side of the focuser draw tube to ensure that the CC tube is locked solidly to the draw tube (it could wobble slightly when only locked at the top end). Also shimmed the camera to minimise very slight tilt in both the CC and the camera - found the rotation angle at which the two slight tilts came closest to cancelling each other. The optics still tends to drift a bit and recollimation is needed maybe every month or two - not a big price to pay for a fast scope.
Ok,It can be confusing so you need to take things one at a time dude.
1:center focuser (use a high quality laser)
2:center secondary to focuser
3:center main mirror to secondary
Ok basics over, now,looking at the second pic looks to me like the spacing of the coma correcter is good.....center right side of the image tells me this.
Top left and top right tell me you have tilt in the focuser.....this should be the first thing to take care of....center focuser before collimation of mirrors otherwise everything else is a compromise.
I see in the bottom right that you have field rotation.....PA dude otherwise It will confuse you.
From what I understand, a f/4.3 Newtonian can be a very capable imaging instrument
My guess is that there is an orthogonalty issue resulting in uneven spacing and thus different correction of stars (over, good, under correction) in four corners.
Hopefully you will get it resolved quickly.
Thanks Suavi :-) I thought there was some tilt, but I've never experienced a newt before, so wasn't sure I was interpreting the images correctly.
Quote:
Originally Posted by Shiraz
ahh- isn't it fun
very much FWIW - a few ideas that might possibly help.
1. get a laser collimator and align it precisely - as Alexander showed http://www.iceinspace.com.au/forum/s...ad.php?t=75601
2. carefully check the central spot ring on the primary and get it exactly right.
3. do a laser collimation through the coma corrector
4. check that collimation holds reasonably well by moving the scope around the sky with the laser running - if possible tighten anything that allows movement of the collimation spot. pay particular attention to the focuser and make sure that the shaft is tight enough that you do not get any rocking of the draw tube
5. if you still have corner-to-corner variability, then you have tilt, either in the camera or in the CC. make a cardboard spacer shim so that you can screw on the camera at a different rotation angle on the CC. If the tilt has rotated with the different camera position, it is in the CC, if it does not change, it is in the camera. if the corners all look the roughly the same, but are not fully corrected, check the CC spacing.
also FWIW, I had to add strengthening to the CF OTA around the focuser to stop flex and had to put new side shims on the primary to stop it from sliding too far sideways. I also added two nylon locking screws on the side of the focuser draw tube to ensure that the CC tube is locked solidly to the draw tube (it could wobble slightly when only locked at the top end). Also shimmed the camera to minimise very slight tilt in both the CC and the camera - found the rotation angle at which the two slight tilts came closest to cancelling each other. The optics still tends to drift a bit and recollimation is needed maybe every month or two - not a big price to pay for a fast scope.
cheers Ray
Thanks for the detailed response, Ray :-) Of course I had a laser collimator that I used once to identify / correct some tilt on the Esprit...... and sold it.
The center spot is as close as I could get it - I removed the stock one and replaced it with a catseye hot spot.
The tube is a ~5mm foam core CF tube and it's not a cheap scope so hopefully will hold collimation across the sky--I'll be pretty unhappy if it doesn't, but I've also not verified it yet... needed to get it collimated first!
I'm not 100% sure on the spacing of the corrector. I tried a few different positions and this seemed to be the best, though it was about 3.5mm off the apparent spacing for this corrector on a scope of this focal length, so I dunno.
Quote:
Originally Posted by glend
Or you could buy a nice refractor. Sorry i could not resist. As Shiraz said, many things to check. Good luck.
You mean like the Esprit? ;-)
Quote:
Originally Posted by atalas
Hi Lee
Ok,It can be confusing so you need to take things one at a time dude.
1:center focuser (use a high quality laser)
2:center secondary to focuser
3:center main mirror to secondary
Ok basics over, now,looking at the second pic looks to me like the spacing of the coma correcter is good.....center right side of the image tells me this.
Top left and top right tell me you have tilt in the focuser.....this should be the first thing to take care of....center focuser before collimation of mirrors otherwise everything else is a compromise.
I see in the bottom right that you have field rotation.....PA dude otherwise It will confuse you.
Cheers mate. I did 2 & 3 to the best of my abilities with the tools I had available to me. Not so sure on #1.
When you say the second pic looks like the spacing of the coma corrector is good, did you mean the first pic? The second pic has no coma corrector in place.
As for field rotation... this was taken on a near new, pier mounted AP Mach 1, polar aligned to within a couple of minutes.... periodic correction was enabled and these are both 15 second unguided exposures -- that's not field rotation caused by PA.
Cheers mate. I did 2 & 3 to the best of my abilities with the tools I had available to me. Not so sure on #1.
When you say the second pic looks like the spacing of the coma corrector is good, did you mean the first pic? The second pic has no coma corrector in place.
As for field rotation... this was taken on a near new, pier mounted AP Mach 1, polar aligned to within a couple of minutes.... periodic correction was enabled and these are both 15 second unguided exposures -- that's not field rotation caused by PA.[/QUOTE]
(End Quote)
Yes sorry the first pic Lee....the right middle side looks well corrected so that would lead me to think the spacing is good. I should add to that you should see the coma well corrected right round the image when you aliminate the tilt I would say.
So,ok then your PA is good and this is a result of the tilt.... just goes to show how things can look confusing when indeed you can get the same result from field rotation(in ref to the bottom right of the corrected image)
Like I mentioned Lee,first off you need to get your focuser centered to the secondary...
Thanks Suavi--hopefully I can get this scope sorted out soon and then I can actually get a chance to really use the AP!
Quote:
Originally Posted by atalas
Yes sorry the first pic Lee....the right middle side looks well corrected so that would lead me to think the spacing is good. I should add to that you should see the coma well corrected right round the image when you aliminate the tilt I would say.
So,ok then your PA is good and this is a result of the tilt.... just goes to show how things can look confusing when indeed you can get the same result from field rotation(in ref to the bottom right of the corrected image)
Like I mentioned Lee,first off you need to get your focuser centered to the secondary...
All good, thanks Louie, appreciate your advice :-) Weather is looking like it should be good again tonight so I'll see if I can identify where the tilt is coming from tonight.
I have just been travelling down this same road. I have learnt that a good many things I took for granted in imaging are very different with an f4 Newtonian. Last night was the first night I had actually obtained round stars in every corner with nice tight focus.
This is what I did and it may help you with what you are going through.
I bought a Moonlite focuser and am using a GPU coma corrector (CC). I installed it thinking that a longer draw tube would be better. This is in fact a mistake. I found that having the draw tube extended too far created greater slop and made star shapes worse. I suggest using a 1.25" of 2" flange which goes on the bottom of the focusor. Doing this reduced star shape deformation remarkably.
Next I stiffened the secondary assembly. The secondary assembly on the GSO truss Newtonian is at present pretty average and not suitable for carry a good focuser and QSI683WSG-8. Spiezy was the first to put extra poles in the cage and I did likewise, though I think it needs diagonal bracing to reduce racking. Being a tube you are less likely to have this problem but you might have some hoop flex and stiffening via more support is a good thing as you have done.
Next I looked at every bolt and fixing on the scope and I mean every one of them. Tighten up everything, check your primary support, check your primary side slop (not an issue with a conical mirror on my scope but worth checking) and pack out if needed. Also check the secondary assembly itself. Make sure it does not have any movement in the stalk.
Now this I think is important and might pertain to your scope. Check the actual secondary fixing to the stalk. Mine was glued down with a very wide narrow smear of silastic. That caused astigmatism and I think I can see some in your image. The way to test this is to take an image intra and one extra from focus. If star shapes near focus on both side have an oval on both sides and each oval is 90 degrees to each other then you have some astigmatism. It's worth a check to eliminate this issue. I reglued mine down with 3 small blobs of aquarium grade silicone and supported the stalk with match sticks to keep them up high enough so as not to smear. This eliminated astigmatism.
As to collimation, I use the Glatter laser to check but essentially use the Cats eye collimation system for collimation. I cannot stress enough how finicky you have to be with this. The hotspot or triangle has to be perfectly inside the black cat white circle. Just a tiny bit out makes a huge difference. Then use the Infinity Cheshire very accurately to adjust the secondary. Then importantly repeat the process until you have both ends perfect. I finally use the laser to check both the centre spot alignment and use the Tublug to check the primary return. If every thing looks aligned you are now highly collimated. I found I had to be particularly fussy to get good collimation and again I think I can see a little collimation error in some of your star shapes.
Now check you CC back focus distance. CC's are particularly fussy about the correct back focus. Most on the market have a 55mm back focus but some vary a little to 50mm. The GPU allows some slack of about 1mm. The Baader one was a little more tolerant and allowed 5-6mm. Make sure you have that correct distance. This is probably one of the major problems you have here.
Screw thread every thing. Don't use compression rings and these can slightly miss align the CC and that can create tilt. Testing as suggested will tell you where the error lies. I bought adapters to screw thread and it made a huge difference.
Adjust your Moonlite for the correct lifting capacity. I did mine up too much and it caused flexing in the tube. Make sure it holds the weight under gravity but only just. That seems to work best. Though I still seem to have a tiny amount of slop in the focuser but I think that is part of how gravity is working against the focuser.
If your Moonlite has a rotator facility with the tilt adjuster, check that the tilt adjustment is actually adjusted correctly. Mine was way out and causing a focuser mis-alightment. To check this I put the laser into the focuser and very lightly clamped that down in the focuser and then rotated the entire focuser. The laser should not travel around in a circle; the laser should stay in the one spot.
Finally, and I think this is the main problem you have here; is tilt. Once you have the correct spacing of the CC and established you have tilt in the camera, then pack out with shims. I used the metal out of a coke can which is thin and it works well. This should go under the camera adapter behind the CC. It takes more than you might think.
I hope this has helped. It has taken me a while and I got some good advice from several people, which I have relayed to you. Best of luck.
Last edited by Paul Haese; 10-09-2017 at 11:54 AM.
Thanks very much for your response, Paul. That'll take a while to fully digest!
I'm also using a Moonlite and a GPU coma corrector (TS branded, but it's the GPU). This morning I played around with a few different adapters and now have the spacing at about 55.7mm, according to the calipers, which is hopefully close enough. I doubt I'll get any better unless I go down the precise parts path, which is always expensive... and even then there's measurement error so I couldn't be too sure it'd do more good than harm!
Everything in my imaging train (including the focuser -> coma corrector connection), is threaded, except for the TS OAG -> filter wheel connection, which uses a crappy system with three thumb screws--that may be causing tilt but I did my best to apply pressure and keep it square when tightening it.
When my camera is focused the focuser's 2 inch draw tube is only racked out about 5-10mm. In fact, I may need to move the primary mirror further up the tube as the GPU corrector is inside the focuser's draw tube and actually sticks into the OTA by about an inch when it's at focus.. when looking at out of focus star images you can see a nice flat side on all of the stars which I assume is the corrector.
I hope I don't have any astigmatism, but I did look at how the secondary is connected to the holder this morning and it seems to have a smear of some kind of glue / adherent so I may be in the same position as you there... hopefully not though.
I measured, using calipers, the distance between the coma corrector's adapter and the face of the camera's body and that seemed to be consistent, though I was restricted in where I could measure, which suggests either focuser or sensor induced tilt... I should be able to verify by rotating the OAG -> filter wheel interface and seeing if the tilt shifts with it.
Now this I think is important and might pertain to your scope. Check the actual secondary fixing to the stalk. Mine was glued down with a very wide narrow smear of silastic. That caused astigmatism and I think I can see some in your image. The way to test this is to take an image intra and one extra from focus. If star shapes near focus on both side have an oval on both sides and each oval is 90 degrees to each other then you have some astigmatism. It's worth a check to eliminate this issue. I reglued mine down with 3 small blobs of aquarium grade silicone and supported the stalk with match sticks to keep them up high enough so as not to smear. This eliminated astigmatism.
I got caught out by that one when I glued back my secondary after having it recoated. It is incredible the pull silicon can have on even the thickest piece of glass when it cures, even three small dabs. On my C11, too much of this stuff managed to stress the glass between the baffle tube and the bore in the primary and we're talking of about 100mm of thickness at that point in the mirror because it is bell shaped. I had to slice parts of it to release the pull.
Made some progress... after correcting the spacing of the coma corrector, the star shaped improved some... still definitely not close to good enough, but better. At that point I could see very obvious tilt -- stars in focus in the center, round donuts in one corner.
I rotated the camera in the OAG and the tilt moved relative to the camera sensor, so the tilt is before the OAG... I'm thinking the focuser now.
I've been waiting on a 2" drawtube extension so that I could get the autocollimator close to the focal plane and verify collimation. That arrived today and I think I have it collimated now. Unfortunately my secondary was spotted when I got it and I didn't remove it, so it's pretty hard to tell when looking through the central pupil because I have a fuzzy donut of the secondary spot overlaying the radioactive symbol that is the primary spot. In the offset pupil I have two circles though (comprised of the two sets of opposing radioactive symbols).
Interestingly, the Moonlite focuser can be rotated relative to its base... if I do that while looking through the offset pupil, the circles unstack, until I rotate it back to the position at which I collimated it. I'm guessing this confirms that the focuser is not square. I guess it means I'll have to recollimate the scope if I rotate the focuser... which I need to do to compose my images.
Fingers crossed the dark sky tonight shows me pinpoint stars, but I have the feeling I'll be seeing more donuts...
Made some progress... after correcting the spacing of the coma corrector, the star shaped improved some... still definitely not close to good enough, but better. At that point I could see very obvious tilt -- stars in focus in the center, round donuts in one corner.
I rotated the camera in the OAG and the tilt moved relative to the camera sensor, so the tilt is before the OAG... I'm thinking the focuser now.
I've been waiting on a 2" drawtube extension so that I could get the autocollimator close to the focal plane and verify collimation. That arrived today and I think I have it collimated now. Unfortunately my secondary was spotted when I got it and I didn't remove it, so it's pretty hard to tell when looking through the central pupil because I have a fuzzy donut of the secondary spot overlaying the radioactive symbol that is the primary spot. In the offset pupil I have two circles though (comprised of the two sets of opposing radioactive symbols).
Interestingly, the Moonlite focuser can be rotated relative to its base... if I do that while looking through the offset pupil, the circles unstack, until I rotate it back to the position at which I collimated it. I'm guessing this confirms that the focuser is not square. I guess it means I'll have to recollimate the scope if I rotate the focuser... which I need to do to compose my images.
Fingers crossed the dark sky tonight shows me pinpoint stars, but I have the feeling I'll be seeing more donuts...
Lee, I think you might need to check the tilt adjustment on the Moonlite. I found that mine was way out and when I rotated it the laser I was using would form a rather large circle on the primary. Sorting that will ensure you focuser is square to the optical axis. Then you can collimate from there.
Paul,
With all the adjustments you have to make , what did the Ronchi screen show.
I found the Ronchi very helpful with the 12" f5 that and the final star test.
Lee, I think you might need to check the tilt adjustment on the Moonlite. I found that mine was way out and when I rotated it the laser I was using would form a rather large circle on the primary. Sorting that will ensure you focuser is square to the optical axis. Then you can collimate from there.
Cheers Paul. I think you're right. I tried adjusting it using trial and error tonight and got nowhere. I've just ordered a laser, sounds like that will make it easier if all I have to do is adjust the focuser until rotating it causes the laser to remain in place as opposed to tracing a circle.
Paul,
With all the adjustments you have to make , what did the Ronchi screen show.
I found the Ronchi very helpful with the 12" f5 that and the final star test.
Ken, I have not used a Ronchi eyepiece on this system. Might be worth taking a look, since GSO don't provide test results themselves.
Cheers Paul. I think you're right. I tried adjusting it using trial and error tonight and got nowhere. I've just ordered a laser, sounds like that will make it easier if all I have to do is adjust the focuser until rotating it causes the laser to remain in place as opposed to tracing a circle.
I am going to get adapters made up so I can use my Atlas focuser on the scope and that will narrow down where movement can take place.
I am going to get adapters made up so I can use my Atlas focuser on the scope and that will narrow down where movement can take place.
Interesting... good luck with the Atlas, hope it works out!
I think I might have narrowed down the problem. It may not be *the* problem, but it's certainly a problem.
If I put the autocollimator in the focuser drawtube and have the drawtube parallel to the ground, then apply some downward pressure to the drawtube (simulating load from the camera and other gear), the collimation clearly shifts.... looking through the offset pupil I can see the reflections "unstack" as I apply pressure. Looks like there's significant slop in the drawtube.
I've also noticed that I can "unstack" the images in the offset pupil of the autocollimator by:
Wiggling the AC in the focuser; or
Rotating the AC in the focuser; or
Rotating the focuser itself
It's hard to know if it's collimated given the inconsistencies.
Anyway, last night before I discovered the above (which I noticed this morning), I got everything well collimated according to the offset pupil of the AC and the cheshire. I then attached the camera and captured the first image. You can see very bad tilt, which I think now is due to the weight of the imaging train screwing up the collimation.
Taking 15 second exposures and then analysing them in PixInsight's aberration inspector I was able to achieve the next image by making small adjustments to the focuser's collimation. Clearly still out of focus in one corner and poorly corrected in some areas, but much better than it had been. I assume I could have managed this by adjusting the secondary as well. The field illumination improved during this adjustment also. Sadly, this is still not usable and if I'm correct, it's going to shift out of collimation again as I point to different targets and gravity has varying impact on the drawtube. Not sure how I can resolve this... might send Moonlite an email and see if there's an adjustment I can make to reduce the slop in the drawtube.
This is the very reason I am going to get the Atlas working. I think the problem essentially is that the focusor cannot hold the load and hence introduces tilt and astigmatism. Pity really because Moonlite promises so much with some of its features and finish.
