Help needed with installing secondary mirror.

[kbrown]

Hi,

As some of you might already know I recently replaced the stock 58mm secondary mirror with a 75mm one on my Sky-Watcher 250 newt. I've attached it to the stock mirror holder twice now and still cannot get round stars. I have double and triple checked it's not the primary mirror by rotating it in the OTA by 60 degrees and re-collimating everything (the distortion did not change). I also rotated the imaging train in the focuser which did affect the orientation of the deformation. Also tried with another camera without any filters etc. Still the same. So with this in mind I can only conclude the problem is with the secondary mirror.

Here's what the defocused stars look like before and after rotating the primary mirror (they look pretty much the same to me):

 

Initially I used a patch of double sided VHB tape to attach the secondary:

I didn't press it down to the mirror. I just let gravity to do the work for me:

Then after the horror test results I took the mirror off again and re-attached it with a ring of RTV silicone:

I 3D printed this two part spacer / template tool so that I get the holder in the right place and also act as a spacer while the silicone cures:

After about an hour I took the spacers off and let it cure for several days before putting it back in the scope.

 

To my horror this did not resolve the issue. The distortion is pretty much exactly the same (size and even orientation) as it was before. So my questions would be:

1. Is there something fundamentally wrong with what I've done above?

2. Suppose the silicone method is better in theory?

3. What would I need to be able to test the secondary mirror for astigmatism on its own before even attaching it to the stalk? Just to rule out I didn't receive a lemon from Orion Optics...

 

Any help would be appreciated. Thanks,

Kari

Edited October 20, 2021 by kbrown

[markse68]

HI Kari, I'm having secondary issues at the moment too   (but hopefully soon fixed :))

Do your irregular stars elongate with 90 degree separation as you go intra/extra focus? It seems quite normal nowadays to support big mirrors on a small stalk like that but maybe gravity is distorting the mirror? Did you let it acclimatise long enough and are you using the dew heater when you notice this?

Are these images from centre of view? Are you using a coma corrector and if so have you tried without?

Mark

[Merlin]

Maybe the flat itself has stress lines in it due to poor annealing. 

[kbrown]

55 minutes ago, markse68 said:

HI Kari, I'm having secondary issues at the moment too   (but hopefully soon fixed :))

Do your irregular stars elongate with 90 degree separation as you go intra/extra focus? It seems quite normal nowadays to support big mirrors on a small stalk like that but maybe gravity is distorting the mirror? Did you let it acclimatise long enough and are you using the dew heater when you notice this?

Are these images from centre of view? Are you using a coma corrector and if so have you tried without?

Mark

Slightly inconclusive as I don't get distinct ring(s) when at extra focus but I think they are in 90 degree separation. What does that tell us? Here's intra vs extra focus by the same amount:

Not 100% sure but I think I was actually using a dew heater as it was pretty humid. I pretty much used it always with the old mirror without problems. I will try without next time it's clear and make sure everything is acclimatised properly. My gut feeling is that it's not related to this though. Do you think I might need a larger stalk to hold the mirror?

27 minutes ago, Merlin said:

Maybe the flat itself has stress lines in it due to poor annealing. 

I certainly hope not. But I have no idea how I could test this?

[markse68]

What are you using for collimation and are those imaged through a CC? The 90deg thing is an indication of astigmatism- it's what I was getting and in my case I'm pretty sure was caused by the secondary not being perfectly flat but having a slight curve along its long axis. Can you check again with less defocus? I was seeing it clearest just a tiny bit either side of focus looking at how the stars just started to grow from a dot.

Mark

[kbrown]

45 minutes ago, markse68 said:

What are you using for collimation and are those imaged through a CC? The 90deg thing is an indication of astigmatism- it's what I was getting and in my case I'm pretty sure was caused by the secondary not being perfectly flat but having a slight curve along its long axis. Can you check again with less defocus? I was seeing it clearest just a tiny bit either side of focus looking at how the stars just started to grow from a dot.

Mark

Yes the images are through a CC but I did try with another camera without the CC and any filters and still got similar results. I will try again with less defocus as soon as I can.

For collimation I used several things. I 3D printed sort of a ruler/template which allowed me to centre the secondary in the OTA. I then put my small QHY camera in the focuser with a CCTV lens that allowed me to visually centre the mirror with the focuser tube. Then I used a laser collimator to adjust the beam from the secondary to hit the centre of the primary and also roughly align the primary as well. Final primary adjustment I did with a cheshire eyepiece. Then I took some flats to check for even illumination. I iterated this whole thing a few times to get everything agreeing with each other.

I wish there was an easy way to check the secondary mirror on its own without having to go through all this as it's very time consuming...

Edited October 20, 2021 by kbrown

[markse68]

You can check flatness with an optical flat by looking at the interference fringes produced in monochromatic light when you lay the optical flat on the secondary. I bought a russian flat for this purpose and plan to check my old mirror with it. But i’m not sure i’d want to do it on an expensive new mirror- you have to be really careful to place it and remove it without moving it sideways and the surfaces should be scrupulously clean and completely free of dust. And optical flats aren’t cheap.

Mark

[kbrown]

1 hour ago, markse68 said:

You can check flatness with an optical flat by looking at the interference fringes produced in monochromatic light when you lay the optical flat on the secondary. I bought a russian flat for this purpose and plan to check my old mirror with it. But i’m not sure i’d want to do it on an expensive new mirror- you have to be really careful to place it and remove it without moving it sideways and the surfaces should be scrupulously clean and completely free of dust. And optical flats aren’t cheap.

Mark

Thank you. I found an 80mm russian optical flat on ebay for reasonable price so I thought I'd just get it out of curiosity if nothing else. But it's coming from Ukraine so god knows when it's actually here. For now I'll continue testing without it and maybe try to come up with an alternative way of holding the mirror on the spider...

Edited October 20, 2021 by kbrown

[Peter Drew]

Looks like classic astigmatism to me rather than any collimation error.  If you use a ring of RTV to attach the flat you should provide a hole or gap in the ring, otherwise differential cooling of the air trapped by the ring can cause distortion of the flat.  Regardless of differing opinions about OOUK I think it highly unlikely that they would supply an astigmatic flat, as they are easy to judge for flatness against a known reference.

[kbrown]

6 minutes ago, Peter Drew said:

Looks like classic astigmatism to me rather than any collimation error.  If you use a ring of RTV to attach the flat you should provide a hole or gap in the ring, otherwise differential cooling of the air trapped by the ring can cause distortion of the flat.  Regardless of differing opinions about OOUK I think it highly unlikely that they would supply an astigmatic flat, as they are easy to judge for flatness against a known reference.

Thank you Peter. I'm pretty sure (and really hoping for) it is my own fault and I can somehow fix it. I will "mind the gap" in the silicone next time. Do you think I should aim to have a larger stalk to hold the mirror as well as it has grown from 58 to 75mm?

Edited October 20, 2021 by kbrown

[Peter Drew]

I don't really know to be honest, small stalks seem to be commonplace these days with no general problems reported.  I've always preferred to make backplates the same size as the secondary mirror, specially large ones.    😀

[markse68]

A bigger stalk would offer a few advantages i think- more evenly distributed support at the very least but i’m worried about your dew heater- looks like it’s heating the stalk so you’re going to get uneven heating of the secondary. Glass isn’t a good conductor of heat which will lead to physical distortion. maybe that’s the issue here?

[kbrown]

14 minutes ago, markse68 said:

A bigger stalk would offer a few advantages i think- more evenly distributed support at the very least but i’m worried about your dew heater- looks like it’s heating the stalk so you’re going to get uneven heating of the secondary. Glass isn’t a good conductor of heat which will lead to physical distortion. maybe that’s the issue here?

Yes it's heating the stalk. Not ideal I know but I never had an issue with it with the stock 58mm mirror and it effectively kept the mirror dew free. I'll keep it off next time I'm testing...

[kbrown]

1 hour ago, Peter Drew said:

I don't really know to be honest, small stalks seem to be commonplace these days with no general problems reported.  I've always preferred to make backplates the same size as the secondary mirror, specially large ones.    😀

I have a desktop CNC router I could potentially use for making a backplate of matching size out of aluminium or even stainless steel sheet. I could then drill and tap say three M3 holes into the stock stalk and attach the backplate on it with counter sunk screws secured with loctite and then use a larger silicone ring (with gaps) to attach the mirror to it. I might be even able to put my dew heater on this new backplate instead of having it on the stalk. Does this sound like a good idea?

[markse68]

it’ll be more secure too- less likely to drop the mirror onto the primary. If you make it from aluminium then it’ll conduct the heat from your heated stalk and spread it more evenly across the back of the mirror i’d have thought.

[kbrown]

Cool. I shall see if I can actually pull it off... Something like this:

[Chriske]

The two attempts, described higher up, to glue that sec. mirror are not 'standard' procedures. (in fact completely wrong, sorry)
During course we always glue the sec mirror with only three small dots of silicone(near the edge). Distance between mirror and holder about 2mm, to allow better and evenly cooling of the mirror.
During our nearly 40 years of course we've glued literal hundreds of sec mirrors. never had any issues.

Look at the shape of the defocused stars and look at the shape of the silicone ring...  Seems familiar...?
 

[Peter Drew]

I agree with Chriske's comments and would try the 3 blob silicone approach at the next attempt.      🙂

[kbrown]

3 hours ago, Chriske said:

The two attempts, described higher up, to glue that sec. mirror are not 'standard' procedures. (in fact completely wrong, sorry)
During course we always glue the sec mirror with only three small dots of silicone(near the edge). Distance between mirror and holder about 2mm, to allow better and evenly cooling of the mirror.
During our nearly 40 years of course we've glued literal hundreds of sec mirrors. never had any issues.

Look at the shape of the defocused stars and look at the shape of the silicone ring...  Seems familiar...?
 

 

2 hours ago, Peter Drew said:

I agree with Chriske's comments and would try the 3 blob silicone approach at the next attempt.      🙂

 

Thanks guys. I will try that next as well. The idea of the silicone ring came from this thread. Someone said they fixed astigmatism by doing so but maybe in my case it's doing the opposite.

 

[Astrobits]

Having more than three points of contact will almost certainly distort the mirror. If it eliminates some astigmatism then it is very lucky that the mirror was pulled in the right direction!  Also, make sure the mirror is about 3mm away from the backing plate. That way it will be easy to remove it when necessary and provides a cushion against any distortion on the backing plate.

It is not necessary to have three blobs on the secondary. Mine has only one small central blob and it has been that way for nearly 20 years.😀

Nigel

[kbrown]

On 24/10/2021 at 17:28, Astrobits said:

Having more than three points of contact will almost certainly distort the mirror. If it eliminates some astigmatism then it is very lucky that the mirror was pulled in the right direction!  Also, make sure the mirror is about 3mm away from the backing plate. That way it will be easy to remove it when necessary and provides a cushion against any distortion on the backing plate.

It is not necessary to have three blobs on the secondary. Mine has only one small central blob and it has been that way for nearly 20 years.😀

Nigel

Thanks Nigel. I'm curious about how big is your secondary? Do you have a matching size backing plate? How big is the blob of silicone you attached it with?

[Astrobits]

10 hours ago, kbrown said:

Thanks Nigel. I'm curious about how big is your secondary? Do you have a matching size backing plate? How big is the blob of silicone you attached it with?

It's 80mm MA. 16" f/4.8 primary ( which is held by three blobs in case you were wondering). Done in the early 2000's and still perfectly O.K.

No. As long as the backing plate is as large as the blob of silicone there is no point in having anything bigger for the sake of it.

Just gone to take the attached photo which shows the flat from the side, so it is about 113mm long in this shot.

Nigel

[kbrown]

Still haven't re-attached the secondary as life's gotten in the way. However out of curiosity I tried testing the flatness of it using a second hand optical flat I got off of fleabay and a 50mW green laser diffused through a ping pong ball and reflected off a white card. Never done this before so I have no idea if I've done something wrong or not. I basically cleaned the surfaces as good as I could then placed the mirror on the table, a piece of optical cleaning tissue on top and the optical flat on top of this. Then I slid the tissue out whist making sure the optical flat didn't move. This is what I got time after time:

I can see a little bit of waviness on the fringes but I don't know whether this is due to my method or perhaps because my optical flat isn't big enough to cover the entire mirror. I'd be grateful if someone more knowledgeable could give me some pointers and/or feedback.

Clear skies,

Kari

[Astrobits]

Looks good.

There is a slight curvature to the fringes, particularly noticeable to the right hand side. I suspect this is caused by viewing geometry. With flats you need to be as far away from the surfaces as possible otherwise the gap between the mirror and reference surface becomes greater away from your direct line of sight. Ideally your view should be perpendicular over the whole surface but you would need to be at infinity to do that. Get very close and you will see a bullseye pattern 🙀 and not straight lines even though the flat is perfect.

Nigel

[Peter Drew]

Looks good to me as well.      🙂