If you look at this thread you'll see that I have pmrid's F3.8 10" newt with a view to giving the beast a tune up while he's tramping around Europe. I've had quite a few newts and peter had a set of great collimation tools so I didn't think twice to help him out as he just got his MX mount and has his hands full with that. I've found that this thing is hard to get right, I'm getting it closer and closer but I still have problems and maybe you guys can help.
I'm sure the problem is the secondary alignment. It has a prostar 3" secondary which has the focuser offsets built in. However I found that the 2 short legs of the spider where 2 mm different as well. I spent a lot of time getting the center bolt into the right spot, I have also center spotted the focuser however I had to alter the focuser tilt to get the secondary in the middle of the focuser sight tube which indicates to me something is screwed.
Doing a star test shows it is concentric inside focus but shows coma outside focus. This is giving funny star shapes. When aligning the secondary it always seems to have an apparent flat spot on the top left of the mirror. Does anyone have any tips on aligning the secondary? Should I center the focuser and move the secondary to suit instead of moving tilt on the focuser?
I also have reservations about using an mpcc at this F ratio. The info on opt says it's for newts from F4 to F6 with visual users reporting good results down to F3.7. I have a qhy9 with wheel hanging off it.
Here's a set of lum on tuc 5sec subs and 253 120sec subs to see how close it is. Check out the big stars. and I had fast downloads on to avoid the clouds hence the vertical bars on tuc.
when i had my R200SS i found collimation a problem. it too is a fast f ratio. Looking at this link you can see things are not just centred but they also have to be orthagonal and aligned not sure if you can glean something from this? i do conceed that there is still an alignment problem rather than just the secondary and the focuser. you may have to shim the primary mirror as well as the spider to get it centered in the focuser?
This is what im seeing. yes the stars are on the pi55 but and they are consistent problem is that its not consistent with coma. why do i say this. well coma is radial from the center out your either under or over corrected your stars arn't radial they are a bit all over the show. I think if you really wanted to get good results you would have to move to the keller corrector not the reducer but the corrector version.
I could well be completely wrong but with the secondary i thought that its alignment was to do with illumination more than anything. As there are 4 different types of alignments of which if you can nail down 2 out of 4 your system is perfectly collimated. there is a really good read about it on the cloudy nights forum.
I still think something is pinched or warped as even though im nearly 1 f number slower than you my secondary isn't quite aligned and i don't have these issues.
As for centering the secondary properly a proper sight tube does it for you something like the cats eye sight tube.
Keep us in the loop of what happens because ide like to know whats the cause behind this annoyance
Cheers Dave, I have sort of ignored the primary so far as he has it in a hi end cell and it always aligns nicely. All the triangles line up well. I'll knock up a tool to ensure the primary is 100% centered in the tube.
When I last aligned it before these images, I did it without the primary installed as I was re spotting it with the red triangle that comes with the autocollmiator. Secondary alignment still wasn't exact in my opinion. I thought I might get a spring and washers to go between the spider and the secondary, that would probably help a lot.
Brendan, I thought I might remove the mpcc and wheel and just try it with a mono camera. If I can get the center right then the rest should follow with the right corrector. At the moment stars are all over the place
Yes I have peter's full catseye 2" kit. I wish I'd had one of these kits years ago, they are great tools.
It just all seems odd because even you saying that your mirror is slightly out of center mine is too and i can see it quite visibly and its not a issue. if i was around your area ide be saying to you lets catch up because i love trying to solve issues.
Have you watched the cats eye video on how they use their stuff? if not cruise on over and have a look. the other thing that you will need is a focused beam of light to shine on the hot spot because its too hard to decern P1, P2 and P3 in my 4.7 your 3.8 would be even worse.
I'm convinced it's a secondary issue. I'm sure it's not square. It's too easy to adjust everything else to make it look right.
Pete's probably reading this on his iPad but I'm sorry I respotted the primary now. It was working ok with the original white dot. Those stupid red triangles should be white not red, they are too dark. It has to pointed to bright sky for it to work now but I'll try it on the light box flat out tomorrow and see
I use the yellow one it works a treat. Don't be fooled by the not being square issue ive had some really bogus info before and i went around in circles trying to make sure everything was square. My Feather touch focuser does not have adjustments to "square" the focuser either so it cannot be that important.
I have found if you do a flat you can see where the center of the light cone is impacting your sensor.
if you have used the cats eye kit and have lined up the astrisims.... its right pretty simple
Hi Robin. all looks depressingly familiar. I have an f4 newt with an MPCC and have spent countless hours trying to track down exactly what causes odd star shapes - with limited success.
Earlier posts are good advice (maybe JJJ's comment sums it up), but the following may also be useful - apologies if you have already been down these directions:
1. With fast Newts, if the secondary is not properly positioned and offset, the primary optical axis may not be aligned anywhere near the tube axis and you can get the end of the tube encroaching into the light column, causing flat spots on the out-of-focus images of stars and odd shaped in-focus star images. You have checked the secondary offset, but if you need to tilt the focuser to centre the secondary - and are still getting odd star shapes/vignetting - then either the secondary is actually offset away from the focuser by an incorrect amount or it is not in the right fore/aft position along the tube. try putting the focuser back to normal and move the secondary in/out to centre it. Also, it is always worth standing back and looking into the front of the OTA with nothing in the focuser - you can see if the secondary holder (or the edges of the secondary itself) looks tilted WRT the OTA tube and this will give some clues as to what way it might be misaligned.
2. A laser alignment tool can be very useful for initial alignment, since it is relatively easy to work out causes of gross misalignment - the passive tools are probably better for fine alignment, but interpreting what is going on is easier with a laser.
3. The MPCC is intolerant to misalignment. I would first try to get everything aligned without it - try to get a symmetrical coma pattern about the centre of the FofV. Then re-introduce the MPCC and see what it does. Even with the best alignment I could get, my f4 system cannot produce round stars out to the edge of the FofV. The MPCC probably meets the claim of "10 microns" when measured at half power width, but the PSF skirts are distorted and star shapes vary with brightness. The MPCC is definitely worth having, but not perfect at f4. And it is totally intolerant of misalignment - if you cannot get symmetrical star coma patterns without it, you will get bad results with it in place, so do your inital alignment without it.
4. I suspect that you may also have focal plane tilt in the camera, judging by the star patterns, but I would leave that as the last thing to check when you are convinced that everything else is properly aligned. A camera tilt that is fine at f6 may be disastrous at f3.8 and even a few microns of tilt will upset the star shapes. Make sure that the camera is perfectly seated in the focuser (without the MPCC) as a first step and then use any tilt adjustments that the camera may have to try to get a symmetrical pattern in the star coma patterns around the centre of the FofV. If the camera does not have tilt adjustment, check that tilt is the problem by rotating the camera in the focuser. The star pattern will rotate, but if the general nature of the star pattern distortions does not change (eg elongated in top left), then the chip is probably misaligned. Then you will need to adjust it - I spent many nervous hours with shims trying to get my QHY8 chip aligned with the camera body (at least to within about 10 microns) and it made a big difference. But only attempt it if you really need to.
Good luck - if you get it lined up, a fast optical system can be be great fun. Regards Ray
I am wondering whether it would impact on this problem if the primary was not absolutely central in the OTA.That Optical Suports primary cell does have the means to adjust the 3 'legs'. Maybe the OTA itself has become slightly distorted and has thrown the mirror cell into a non-central position.
Peter
Well i think that with everything thats been said my scope should be absolutely screwed but it isn't.
My Primary isn't centered in the tube its visibly out
My focuser does not give any tilt adjust (its a FT so it isn't a crap focuser)
I admit that it isn't a F3.8 but its classed as a fast scope at F4.7
What the cats eye gear does is align the optical axis with the focuser axis very effectively.
Be really careful when doing your alignments as at F4.7 i can barely see the P2 and from every thing i have ever read it gets worse the faster your scope gets so is it something that you may be using the wrong reflection. I know i was for a while untill i got a very bright led that focused down onto the hot spot. it was then that i realised that i had actually been using the wrong reflection.
Like i said before once you have those astrisims lined up .... you cannot be wrong unless your auto collimator is up the duff.
If you give me a few months ill be able to give you my thoughts and experiences as im just about to commence my wide field newt fast newt project.
Would be interesting to see some pics "down the barrel" of the tube and down through the focuser Robin. I would have thought with the Catseye gear you could get exceptional alignment of secondary and primary, even if they aren't perfectly positioned.
Sounds like optical abberation or pinching as Brendan has suggested, but I should really be listening rather than trying to help - no experience under F5.
If you look at this thread you'll see that I have pmrid's F3.8 10" newt with a view to giving the beast a tune up while he's tramping around Europe. I've had quite a few newts and peter had a set of great collimation tools so I didn't think twice to help him out as he just got his MX mount and has his hands full with that. I've found that this thing is hard to get right, I'm getting it closer and closer but I still have problems and maybe you guys can help.
I'm sure the problem is the secondary alignment. It has a prostar 3" secondary which has the focuser offsets built in. However I found that the 2 short legs of the spider where 2 mm different as well. I spent a lot of time getting the center bolt into the right spot, I have also center spotted the focuser however
Doing a star test shows it is concentric inside focus but shows coma outside focus. This is giving funny star shapes. When aligning the secondary it always seems to have an apparent flat spot on the top left of the mirror. Does anyone have any tips on aligning the secondary? Should I center the focuser and move the secondary to suit instead of moving tilt on the focuser?
I also have reservations about using an mpcc at this F ratio. The info on opt says it's for newts from F4 to F6 with visual users reporting good results down to F3.7. I have a qhy9 with wheel hanging off it.
Here's a set of lum on tuc 5sec subs and 253 120sec subs to see how close it is. Check out the big stars. and I had fast downloads on to avoid the clouds hence the vertical bars on tuc.
Hi Robin,
The F-ratio doesn't change things too much. A lot of people way overplay the difficulty in collimating fast newts. They are no harder to collimate than slow newts. The optical effects of poor collimation are greater, that is all.
Quote:
I had to alter the focuser tilt to get the secondary in the middle of the focuser sight tube which indicates to me something is screwed.
There are 101 ways to skin a cat but I would have done this the same way. ie. Get the spider in the correct position with regard to the mechanical axis of the tube. Remove the secondary holder from the spider and square the focuser to the axis of the spider and the mechanical axis of the tube. I normally use some thin plastic shims to do this under the respective corners of the focuser, as needed.
Quote:
Doing a star test shows it is concentric inside focus but shows coma outside focus.
I am pretty sure this is caused by the secondary mirror sitting too high in the tube. You should be able to pick this up in the sight tube.
Any misalignment of the primary "should" be minor as most of the telescope components are made on CNC machines and should be able to be fully compensated by adjusting the primary tilt only.
Quote:
My focuser does not give any tilt adjust (its a FT so it isn't a crap focuser)
Brendan,
You adjust this by using thin plastic shims under the focuser baseplate adjacent to the mounting screws.
I've got a f/4.7 10", MPCC, and the Cat's Eye kit as well. I'm getting similar not-quite-right stars and am trying to pin down the cause of that as well. They look very similar to what you're showing in those shots.
Pretty sure it's not the collimation, as the Cat's Eye kit indicates it's all good, and I've had Peter (peter_4059) check it a couple of times to reassure myself I'm on the right track. He has a very similar scope to mine and his stars a something to aspire to.
Am convinced now it's a combination of the spacing between the MPCC and the camera, and a bit of the MPCC not sitting perfectly square in the focuser.
For the spacing, I had calculated the theoretical distance that should be between the camera and MPCC and it rounded off to 20mm. But given tolerances it could be plus or minus a couple of mm from there. Have ordered a set of rings that should allow me to now get spacings from 17mm to 23mm in mm increments so I can fine tune it.
For the squareness of the MPCC in focuser, still playing with ideas but either filling in that indent around the MPCC barrel so it doesn't cause a problem with my focuser compression ring, or also thinking of putting a ring on the focuser side of the MPCC's flange stop. My thread on this here.
The spacing rings should turn up in the next day or 2 and I'll be doing some testing (fingers crossed) this week. Will let you know how I go.
Thanks for all the tips guys. I pulled it out of the dome yesterday arvo for another go and recentered the focuser again and got the secondary done before that storm rolled in. This time the secondary looks exactly as it should do in the sight tube. No flat spots and perfectly centered.
I'll make up a cardboard gauge today to measure the gap around the primary but a quick knuckle test in the gap shows it's pretty close. Rolling the tube on the workbench reveals no warping in the tube. My baader laser died earlier this week which is a shame, as barlowing it and adjusting the primary that way last week gave the best results in ccdinspector.
Looks like it will be clear enough tonight to get another look.
[edit]
So much for the knuckle test, I found the primary mirror was easily about 4mm off center axis.
Starting to look the goods now. I think the axis offset I found on the primary was causing most of the grief. I had this scope textbook aligned on the bench, as soon as I put it back on the mount and double checked, it had moved.
I believe that is another issue to be looked at later as my back was also complaining today so I didn't want to spend much time adjusting alignment on the mount. Just did a quicky so it was close. The light cone is off to the left in these images so even though alignment is not perfect the stars are way better than they were in earlier images. FWHM is down as well.
I notice that inside focus a star test shows a lump on one side while outside focus the lump is on the other side. I'm hoping that this is normal and correcting the alignment will have them both concentric.
Here's 253 and tuc, without and with the mpcc. There is obviously some camera tilt going on in there as well so concentrate on the center
Also added the 253 center star close up, last week and today. I got some 5 minute colour 253 coming for a look see.
lets catch up because i love trying to solve issues.
