Help on a Newtonian GSO 12" f/4

[nandopg]

Hi folks,
I got a Newt 12" f/4 from GSO for this Jupiter apparition. I already have the scope assembled on an EQ-6 mounting which surprisingly was able to deal with the scope's plus camera's load very well.

I run a quick star test on the scope noticing a total out of collimation situation. To collimate the beast I used the following tools:

Auto collimator
2" Laser collimator from Howie Glatter
2" TuBlug from Howie Glatter (Barlow collimator accessory)
Chishare

I proceed then with the following steps:

1- I measured the size of each spider's vanes to check on the symmetry of the secondary holder. It was OK, I didn't touch it.

2- I blocked the primary mirror with to check on the shape and symmetry of reflex of the secondary mirror using the auto-collimator. It was OK, I didn't touch it.

3- With the laser collimator I checked the alignment between the primary and secondary mirror. It was totally out, then I recenter the mirrors using the target on the primary mirror.

4- Then using the TuBlug, I checked the tilt of the primary. It was totally out and I adjusted it.

5- I repeated 3 and 4 interactively 5 times to get good adjustment in both alignments.

6- Re-run the star test: horrible !!!:mad2::mad2: ...but I noticed that the star shape is different when the defocus changes from inner to outter.

Obviously I am in that situation where everything is right but nothing works and now I am running out ideas to try to solve the problem.

Does anyone have some ideas to try ? I would appreciate any help.

Thank you,

Fernando

[astroboy]

Hi Fernando
It sounds like what you are seeing is astigmatism which could be one of the mirrors but may well be uneven cooling of the primary.
The 12" mirror is a large chuck of glass and will take a long time to cool maybe a couple of hours with the fan running .
Keep checking the mirror during the night and you may well find it gets better over time.
Failing this if its a truly astigmatic mirror time to see the people who sold it to you .
An F4 mirror wouldn't be my choice for planetary work , maybe a high quality 8" or 10" at F5 ( ie not Chinese ) better a good quality smaller scope than a bad big one.

Hope this helps
Zane

[Shiraz]

Had the same sorts of problems with an f4 scope and made up a small additional tool to see what was going on. This made it easy to see mis-alignments that were not picked up in collimation. Most users don't seem to need this sort of approach, but it can help if you're unlucky enough to get a scope with a fundamental offset in something.
http://www.iceinspace.com.au/forum/showthread.php?t=82230

I found that a 12inch GSO Newt needed some mods to work as a planetary scope. http://www.iceinspace.com.au/forum/showthread.php?t=85990 Agree with Zane re mirror cooling - even a degree difference between mirror and air can mess up the image at high mag. What you describe re star shape sounds like astigmatism - that will probably be due to the main mirror edge support.

Good luck, Ray

[brian nordstrom]

:) Hi Fernando , a scope at f/4 is very , very critical of any misalignment , so you probably need to do a star test alignment ( the most sensitive test there is ) , eg place a reasonabley bright star in the center of the field of view and dis-focus it until it gets big enough for you to see the secondary and spiders in the blob , :thumbsup: , this will instantly tell you if its a collumation issue .If it is remember tiny movements .;).
Astigmatisim is a problem if its built into the mirror , there are no fixes other than returning the OTA and trying another , sad but true :(.
Try loosening the 3 clips holding the primary mirror , it might have pinched optics? , they should only just hold the mirror , thats all .
Keep us informed as to how it goes and good luck.
Brian.

[barx1963]

Also check that the mirror clips are not too tight on the primary mirror. This can introduce astigmatism in the primary.

Malcolm

[nandopg]

Hi guys,
Thank you so much for the outstanding support.

Ray,
It looks like your directions were custom made for my problem with this scope. I did the test to iluminate the scope and see all the circles against a translucent material placed in front of the tube.
The image is attached. I am not going to say what I think about it and I kindly would ask you to do the analyses yourself.

Since yesterday the weather is close and it is raining down here, so I couldn't get an image of the star test result. I will do it and post it here as soon as I have some sky.

Once again, thank you all very very much.

Fernando

[Shiraz]

hi Fernando. the image attachment did not get here. regards ray

[nandopg]

Hi Ray,
Sorry about it, I forgot to attach the image. Now it is there, please see what you think.

Thank you

[Shiraz]

hi Fernando. This looks a lot fuzzier than I have seen - what light source and type of paper did you use? I found that a LED and baking paper worked best.

The physical alignment is not perfect, but its not too bad and I would think it should work OK for planetary as is if you collimate it with this setup. Mine was a lot worse than this when I started, but it still worked OK for planetary.

If you want to tidy up the alignment further, I would check that the centre spot is actually central before you do anything else. and of course re-collimate if you change anything.

Issues that will increase stray diffraction and that could be a problem in long exposure imaging are:
1. the spider is not straight and you could adjust it to pull the secondary up towards the focuser a bit. That will help centre the beam a bit as well.
2. the focuser is intruding into the beam a bit - was it wound right in or something?
3. the secondary shadow is not circular - the secondary is twisted which probably means that the hub attaching to the spider is not quite pointing straight down the tube. the collimation process has required that the secondary be twisted to bring it back to a roughly central position.
4. the light column is not quite central, so you will have the OTA intruding into the light column in some parts of the field - not a major issue for planetary, but you could tweak it up by shimming the focuser and/or moving the secondary in/out of the tube.

I have no idea why there are two shadows from parts of the spider. If you use a single led source, do you still see the double shadow?

did this agree with your interpretation? regards ray

[nandopg]

Ray,
First, thanks again for your reply.

The execution of the test was far from the perfection. I didn't have backing paper nor a tripod to hold the camera, so I used the cover of the telescope that is made by a translucent white cloth. To illuminate the tube I used the laser attached to the focuser.

" The physical alignment is not perfect, but its not too bad and I would think it should work OK for planetary as is if you collimate it with this setup. Mine was a lot worse than this when I started, but it still worked OK for planetary."

With this alignment the shape of the unfocused star was owesome bad, being totally fat on one end and very skinny on the other end, as if the scope was totally out of collimation, what doesn't agree with the light column image. Unfortunatelly I did not have time to do an image of the unfocused star because the weather closed and quickly start raining.

"the focuser is intruding into the beam a bit - was it wound right in or something?"

I am not sure about it, I am going to repeat the rest in a better situation and will be testing on this.

"I have no idea why there are two shadows from the spider nor what the dark bulge at about 1 o'clock from the focuser shadow is. If you use a single led source, do you still see the double shadow?"

I don't know either. As I said I will be redoing the test over the weekend.

On the other matters, basically your diagnosis match with the mine, however looks to me that the primary mirror is somehow out of center respect to its cradle. It looks to be displaced to the left. What do you think ? Don't you think this could be the primary reason of all the other issues ?

It is amazing how everything looks different when I check the collimation using a Cheshire.

But what really is puzzling me is how bad, bad, bad the result of the star test is.
:confused2::confused2::confused2:

Regards,

Fernando

[Moon]

Fernando
I suggest to find a good guide to collimation on the internet, print it out and go through step by step. There are plenty of good ones available, for example: LINK (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2193204/page/0/view/collapsed/sb/5/o/all/fpart/1)
Don't assume anything and don't skip any steps. A bit of practice and it will work out.

On top of that, I think I have the same scope as you - so I might have some helpful advice.
I noticed from robofocus numbers that when the scope is moved from horizontal to vertical, the mirror rocks back into the cell, sometimes this takes over 1 hour to complete. So if you collimate with the scope on a bench (horizontal) and then do a star test (vertical) you will get some movement that can throw it out of focus and collimation. The solution in my case was to attach the mirror to the cell with silicone instead of the sticky pads, put some silicone inside the ball joints in the primary mirror cell and add some tensioning bolts to the cell. In the end I think it was the ball joint caused the issue.

James

Edit:
Here's another one:
http://www.cloudynights.com/ubbthreads/download.php?Number=5053116

[Shiraz]

hi again Fernando

agree with James - the GSO primary mirror mounts definitely need tightening up. Even with a stiffened up mounting, things move around so much that you will always need to touch up the collimation using a star near to the target object if you want to get the best possible resolution for planetary imaging.

Re the offset of the shadow pattern, if you have any small offsets in the (focuser/centre spot/secondary in-out), the action of doing a collimation will push the input light column off the tube axis as you adjust the primary to compensate - the mirror does not move sideways, it just points away from the tube axis a little and you get the sideways offset that you see in the shadow picture. A little bit of offset doesn't matter for planetary imaging and the result you posted does not show any really bad mechanical misalignment, so suggest you put that on the back burner for now and refine the collimation as James indicates. You should be able to get good planetary images with it as it is set up and if it produces really bad star tests, it is not due to the mechanical alignment that you have shown. Maybe fine tune the alignment later on if you feel the need, but it really does look good enough to work OK.

"It is amazing how everything looks different when I check the collimation using a Cheshire."
then rely on the Cheshire - which shows the optical alignment. The shadow projection technique is primarily designed to show mechanical alignment after collimation and it only indirectly indicates some aspects of optical alignment - it quickly shows if anything is grossly out of alignment but still collimatable.

However, your description of the star test problem sounds like strong tube currents. Did it look like example 8 in figure 96 of?:
http://www.telescope-optics.net/diffraction_pattern_and_aberrations .htm
or the example in http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix%20A-Testing.pdf
Could there have been a large temperature difference between the scope and the air? My 12 inch scope needs an hour or two with the fan on to stabilise if it is a long way off ambient temp.

regards ray

[nandopg]

Ray, James:
Thank you so much for all the kindness and technical tips.

Actually I am doing all the work with the scope horizontal, so it makes sense that once the primary mirror rocks on its base, the collimation trends to change.

For while I will try to stabilize the mirror using silicone as you directed. I don't want to drill holes on the cell to add some screws, because if I decide not to keep the scope I have to send it back as an original unit.

Among the procedures you recomended, one is from the Glatter's website, which is the one I have been following. The only part I skipped was the one that check and adjust the centering of the focuser and its orthogonality to the optical axis of the primary mirror. It might have been a mistake and I will have to do it.

Ray, when I said that looking through the Cheshire everything looks different, I ment that laser, autocollimator and Chishare, all indicated to a good collimation, but when the scope was pointed to a star, well a total s##%%t star pattern. But now there is a new perspective: I moved the scope from horizontal to vertical. One more detail: my star test showed me a uncollimated star pattern, not a pattern corrupted by tube currents.

OK, now having all the information that you guys gave me I will try a couple of things. I will post in this thread what I get.

Thank you very very much for all the help,

Fernando

[gasman1]

I don't believe anyone suggested an astigmatic secondary, or an improper secondary mount which would distort the flat. The secondaries are usually very thick compared to their diameter, but this is possible. If the star images from +out to -out of focus are symmetrical ellipses at right angles, then that's astigmatism for sure. If any astigmatism is baked into the primary, maybe you could anneal it out (fat chance) but you would have to re-aluminize or possibly even refigure the primary surface. Another possibility, if you're still reading this, is the eyepiece itself. Have you seen this bad images with more than one of your eyepieces? If you can get some eyeglasses with a very small astigmatic correction, say 1/2 cylinder diopter which is the minimum you'll find, then by rotating the eyeglass lens you may improve the focused star image somewhat. You can buy or borrow an opticians astigmatism only correction lens, i.e. zero sphere diopters, and use that for the test.

[nandopg]

Hello,
First I would like to thank everyone for all the precious information provided without which I wouldn't be able to achieve a good adjustment on my Newt.

I am writing a review with all modds introduced and the step by step procedure I used to get there. As soon as I am done I will post the file here.

To show the final result, I am posting three images:

Image #1: It is the view from the focuser down to the interior of the tube shown by a collimated webcam. In this image one can see all the circles against a nice target and reticle (credit to Astroshed tutorial).

Image #2: Star test showing a real unfocused star (Achernar). What I most liked on this process is that I hadn't to retouch anything at night to get the unfocused star presented.

Image #3: A view of the scope with the Moonlite focuser installed.

Best Regards

Fernando

[Shiraz]

hi Fernando. That's looking good - should be producing pretty good high res images as it is.

Star test looks fairly similar to what I get. comments:
- looks like a little bit of TDE - does the ring pattern definition change from inside to outside of focus? If so, this is probably fairly typical of GSO optics, but not a big issue for low dynamic range (planetary) imaging - might possibly be worth masking outer few mm of the mirror, but only if the focused images show significant halo.
- something is intruding into the beam - guess it is the focuser (from your earlier published shadow projection). if focuser you can probably move the beam across to get rid of it by slightly varying the in/out position of the secondary (I think it has to go further into the tube) and recollimating - would be worth fixing. Check using shadow projection to make sure the OTA does not cut off the edge of the input beam. the diffraction patterns due to the spider supports are not central, which possibly supports the idea that the secondary is a bit offset.
- there is some blurring of the diffraction pattern on the upper side. either due to tube currents or slight mirror distortion due to adhesive back mounting. not too bad, but worth fixing if it is anything instrumental.
- not quite circular pattern. collimation looks pretty good from the position of the central spot, so suspect slight problem with primary edge mounting (assume that you have loosened off the 3 mirror locking lugs). Not major distortion, but would be worth tracking down and fixing.

Really looking forward to your report.
regards ray

[astro_nutt]

Hi Fernando!
I have a friend who had a few collimation issues with his 12" dob. He was overtightening the lock screws on the primary mirror after he adjusted them. As soon as he moved the tube, the alignment was out. So now he tighten them ever so gently, no more than initial resistance is sufficient.
Cheers!

[2stroke]

Its a great tut the astronomy shed one for advanced collimation with a webcam, all hi tuts are great.

[nandopg]

I agree that is a great tutorial, but I found it doesn't apply completely for fast Newtonians. Some changes had to be introduced to succeed. I will be uploading my experiences hopefully over this coming weekend.


Yes, you are right. For this issue I followed the advices from Ray to use silicone in the place of the sticky lables and around the tuning screws. I experienced changing the position of the scope, checking the collimation and it remained unchanged.


Hi Ray, please see my comments in red above.

Best Regards,

Fernando

[Shiraz]

Hi Fernando.

Agree - looks like you are down to fine tuning and I would think that you should get good results as is - the optics look good. Additional tweaking should give you worthwhile, but relatively minor, improvements in resolution and contrast.

I get a very similar variation in pattern across an out of focus star image and have concluded that it is due to the adhesive mirror mounting causing minor distortions in the figure - have found it not to be a major problem when in focus in any case. Of course I could be wrong and it might just be thermal currents in the tube.

Really looking forward to seeing some images. Regards Ray