What is going on here?

[Paul Haese]

This problem has got me stumped. Take a look at the two images. One is inside focus and one is outside focus. This is taken with the Wynne corrector in place on the AG12.

On first viewing it looks like collimation is shifting from one side of focus to the other. In particular the secondary shadow shifts. For reference the camera is orientated so that the top of the image is toward the opening of the scope and down is toward the primary.

I am using an Atlas focuser which not only measures correct with callipers under load from one side of focus to the other but also shows no collimation shift watching the cats eye collimation.

Is the shadow shifting indicative of the secondary not being centred correctly? ie up or down the tube.

Secondly, the out of focus shapes seem to indicate astigmatism. However star shapes at focus are round stars. These shapes could well be either rotation of the secondary (the sight tube seems to indicate otherwise), astigmatism introduced by the Wynne or position (rotating the Wynne indicates otherwise) of the secondary either up or down the tube. Is this shape again indicative of secondary position?

I have owned several Newtonians over the years, mainly slow ones and never seen this sort of problem. It might well be that the oversized secondary is making this harder to assess and collimate the scope and my lack of experience with a fast Newtonian is the main problem. I would certainly like sort the problem and am seeking sound advice.

[multiweb]

That's what I thought. Definitely something with the secondary but I'm not sure if it's up and down the tube though. JasonD can probably chime on this.

[glend]

Hmm, it does look like the focuser tube may not be actually perpendicular and thus shifts as it is cranked in and then out. But if your convinced it is oriented correctly., the secondary orientation could cause it as well. Is your secondary centre spotted? If so a laser might show the shift. It's very similar to a problem I had with my RC08, which turned out to be focuser alignment.

[garymck]

Hi,
I had exactly this issue with my 8" F4 newt. I could never get things spot on until I used an adapted version of the procedure Vixen suggests for positioning and collimation of the secondary in an R200ss.

After using this procedure I was able to use Catseye tools to collimate perfectly - this was more accurate than a sight tube. For the first time I got a perfect star test, and perfect images over an APSC chip. Blew me away when I finally got it right.

PDF attached

Hope it helps
Gary

[Paul Haese]

Thanks guys for the comments and suggestions.

Here are some images taken at the observatory today. This should give a better idea of why I am stumped. This scope looks collimated to me, or very near to it.

Glen, no I have not centre spotted the secondary. I had it spotted a month or so back but rubbed off the spot. With the offset it did not appear directly under the cross hairs. Perhaps this is part of the problem.

[LewisM]

Paul, just a sideline: have OOUK been a part in the trouble shooting of anything to date, or have they washed their hands?

[multiweb]

It does look a bit offset in#3 at 4 o'clock. If your focuser barrel is 100% square to the mechanical axis then the secondary has to be spot on, otherwise you'd have to tilt your focuser base plate to compensate but that's harder to get right imho.

[Jason D]

Your collimation seems OK. You defocused the star little too much. When that happens, you will start viewing mechanical alignment. Proper star collimation is done at high mag with very few rings showing. If you see the spider vanes diffraction then Mechanical alignment will interfere with the reading. See attachment. I rotated one of the defocused photos by 180 degrees to undo what happens when your cross the focal plane. As you can see, all photos line up. You have a fast scope with a large secondary mirror, hence, the secondary mirror offset will be more prominent on both sides of focus in opposite directions.
However, I do see some astigmatism as you have already noted.
Jason

[Shiraz]

agree with Jason, it could just be the normal offset mechanical alignment. However, it is also possible that the light column may be intercepting the tube on one side. If this is the case, the most likely cause is that the secondary is too far in/out. This is nigh on impossible to get right by eye with a fast Newt - you can collimate it perfectly, but the light column may still not be exactly central.

FWIW, I ended up using back projection to get that aspect right. The linked image https://astrob.in/full/367081/0/ shows how to use a small led torch that is taped up to fit in a 1.25 adapter on the scope - doesn't need to be exact, because it is being used with a divergent beam. A bit of translucent tape increases the size of the light source, which helps a little. With a sheet of baking paper over the input aperture, back projection from the torch shows where the light column has to go (in the reverse direction) to get to the detector. A few minutes effort will quickly show up if you actually have a secondary positioning problem.

To illustrate what can go wrong, panel 3 shows a badly adjusted scope, with the shadow of the spider showing some distortion, the secondary shadow being clearly skewed and the primary light column being offset from the centre of the OTA due to poor collimation (by a lot). It is pretty easy to work out how to adjust the secondary skew, straighten the spider and then collimate the system to get back to something like panel 4 where the secondary shadow is now circular with a reasonable looking offset and the spider is straight.

However, the in/out of the secondary is still wrong as the light from the primary is still not centered. This is not adjusted by moving the primary, since that will screw up the collimation. instead the secondary is moved in/out and the the scope is recollimated to see what effect the movement has. Eventually gets to the result of panel 5 where the primary light column is reasonably close to central - not quite prefect, but this is as good as I can do with my old scope.

The primary rule for using this technique is that the results of moving the secondary can only be assessed after the system has been recollimated. ie move secondary - collimate - assess - move secondary - collimate - assess...etc.. If you don't recollimate after movement, the results are meaningless.

Hope this helps - cheers ray

edit: don't put the scope on the floor as shown in the illustration images - the collimation can be upset by the weight on the primary adjusters if they stick out - do it on the mount

also, the inherent asymmetry of a fast Newt means that the star skirts are not quite symetrical - FWIW I find that it is best to take all colour data on one side of the meridian to minimise multi-colour star skirts at high stretch

[Paul Haese]

Thanks Jason for the advice and images, it never occurred to me to check the reverse. Yes collimation should be checked slightly out of focus. With my RC and SCT I usually only go a ring or two. I had not considered that going too far out would affect the reading on this type of Newtonian. I have learnt yet another thing about fast Newtonians. :thumbsup:

It's good to know that the secondary location in the images are simply a product of the offset if you go too far out. I did not know if this would be the case. Your wealth of experience is invaluable and much appreciated.

Could the astigmatism be induced from the Wynne (maybe not likely given I have rotated the Wynne several times and get the same direction and shape or is this possible with a Wynne?) or perhaps not properly cooled optics? I only cooled these for 30 minutes prior to this test. Not enough but I wanted to check. I know it's not the primary as I have rotated it 45 degrees and get the same result. I did change the silicone on the secondary but perhaps ran it too thin when I reglued it.




Thanks Ray for you help too. I cannot move the spider arms but I can certainly check for illumination, rotation and in and out. That is a really great way of testing. Next time I am down I'll give it a go. I think I have rotation very near perfect but in and out is as you say near impossible to check with the oversized secondary. Thanks once again.

Fortunately, so far I have not found too much of a problem with the star skirts. Nothing moves on this scope once it is locked down, and star shapes remain consistent.

[Jason D]

It is hard to tell the source of the astigmatism based on available information but if it is not the primary mirror then the secondary is the suspect.
Jason

[Jason D]

Hello Ray,
Based on the collimation photos, there is not vignetting. Had there been vignetting by the OTA opening edge then the intruding OTA edge reflection would have appeared in the primary mirror.
Jason

[kinetic]

Great discussion!
Narrowing down issues with collimation myself. f5 Newt.

Focusor at 6 o'clock vs focusor at 2 o'clock vs focusor at 12.
3 different issues!



Steve

[Shiraz]

Hi Jason. Yes it does look pretty good.

The back projection test in this case would mainly be a confirmation that everything is spot on. It is great for peace of mind to see the light and shadows in the right places - possibly worth 10 minutes of effort.

cheers Ray