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Another stinky tale of secondary mounting woes

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#1 precaud

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Posted 21 April 2019 - 11:58 AM

Some background before we jump in. A couple years ago, I bought a new Skywatcher 350P 14" dob. After noting a few minor issues with it, I had a 1/2 hour session (in poor conditions) to make sure nothing was horribly wrong optically with it. A short time later, circumstances led me to take a break from observing, which lasted some 16 months or so.

 

One of the things I did during that time off was remove the secondary mirror and test it using a contact flat. See the photo in the next post. It shows a secondary with the typical hill in the center of the long axis, about 1/4 wave surface. Not a great result; definitely room for improvement there.

 

Last week I ordered and received a new Antares 3.1" 1/15 wave elliptical to replace the stock 82mm one. It arrived with good-looking data, but was thicker than spec'ed, 0.88" vs 0.70". This means I would definitely have to cut down the 350P's metal mounting stalk to use it. Not being well-set-up for such metalwork, I decided to take a closer look at the stock secondary before proceeding.

 

I had read posts from other SW collapsible dob owners who had removed their 2ndary, saying it was siliconed to the stalk around its perimeter. That made me wonder, how much of the mirror's error is actually on its surface, and how much is caused by the way it is attached to the stalk?

 

So I decided to remove the 2ndary from the stalk, test it again, and compare the results. The first pic shows the assembly in question.

 

A long serrated knife is usually a good way to break a silicone joint. So I dug in with one. Little white sticky chunks came out, but it wasn't silicone; it was the typical 2-sided foam tape so often used to secure secondaries to holders. And the tape was starting to lift up on the end opposite to where the most weight hung. See 2nd photo.

 

At this point, my enthusiasm for this project soared. Twice before I have remounted secondaries that had been attached with 2-sided foam tape, and the results were fantastic. So I sensed this would be no different.

 

I put away the knife, heated up the metal stalk with a hot air gun, and gently separated the stalk from the mirror. This exercise alone will convince you that 2-sided tape is not a good way to mount a large secondary. It didn't take that much heat to cleanly separate the stalk from the mirror. See 3rd pic.

 

Next step is to clean it up and test it. I gave the mirror some 6 hours to return to ambient temps before proceeding.

Attached Thumbnails

  • 2ndmont1.jpg
  • 2ndmont2.jpg
  • 2ndmont3.jpg

Edited by precaud, 21 April 2019 - 03:12 PM.

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#2 precaud

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Posted 21 April 2019 - 12:01 PM

I won't go into the complications of contact flat fringe testing. It's not easy to test coated flats, the interference fringes can be very dim when testing high-reflectivity surfaces. My reference flat is only 80mm in diameter, and this 2ndary is 82mm across the minor axis, so we're looking at the center of the mirror here. For comparison purposes, it's enough to know that a flatter surface will show straighter fringe lines equally spaced across the surface. I normally go for 1 to 2 fringes per inch for a good overall assessment. The spacing between the fringe lines represents 1/2 wave on the surface at the wavelength tested. In this case, the green channel was extracted and grayscaled, and the fringe lines were made more visible in a photo editor by adjusting contrast and gamma.

 

The first photo shows the mirror with the stalk attached, as received. I estimate 1/4 wave of error in the curved lines.

 

The second photo shows the mirror with the stalk removed. The lines are much straighter. I'd say in the 1/7 wave range.

 

This is clear evidence of the surface deformity caused by simply mounting the mirror to the stalk with a big piece of 2-sided tape. Unfortunately, this is the mounting method both Synta and GSO uses in their dobs.

 

My hunch is, in use, the error changes with temperature, worsening dramatically as the tape expands or shrinks much more rapidly than the metal or glass does.

 

After seeing these results, I've decided to remount the stock secondary properly with 3 silicone dabs and do visual tests before replacing it. Based on prior experience, I expect to see a significant improvement in image quality, and impacted much less by changing temperature.

 

It will take me a few days to do this. To be continued....

Attached Thumbnails

  • 2ndfrin2.jpg
  • 2ndfrin3.jpg

Edited by precaud, 21 April 2019 - 03:09 PM.

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#3 Pinbout

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Posted 21 April 2019 - 03:04 PM

 

This is clear evidence of the surface deformity caused by simply mounting the mirror to the stalk with a big piece of 2-sided tape. Unfortunately, this is the mounting method both Synta and GSO uses in their dobs.

after I mount my secondaries, I test them.


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#4 precaud

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Posted 21 April 2019 - 03:15 PM

after I mount my secondaries, I test them.

 

I will do so as well.



#5 SteveG

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Posted 21 April 2019 - 05:27 PM

 

Last week I ordered and received a new Antares 3.1" 1/15 wave elliptical to replace the stock 82mm one. It arrived with good-looking data, but was thicker than spec'ed, 0.88" vs 0.70". This means I would definitely have to cut down the 350P's metal mounting stalk to use it. 

Why do you need to cut down the mounting stalk?



#6 precaud

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Posted 21 April 2019 - 06:00 PM

Why do you need to cut down the mounting stalk?

 

To square the mirror under the focuser. The stock secondary is 0.54" thick. Antares told me their 3.1" was 0.70" thick. Add to that about 0.10" for the dabs of silicone. Had that been the case, there should have been just enough space in the secondary collimation adjust screws to make up for it. But the one they sent is 0.88". So in order to square the mirror under the focuser, I have to shorten the stalk by nearly 1/2".



#7 SteveG

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Posted 22 April 2019 - 03:30 PM

To square the mirror under the focuser. The stock secondary is 0.54" thick. Antares told me their 3.1" was 0.70" thick. Add to that about 0.10" for the dabs of silicone. Had that been the case, there should have been just enough space in the secondary collimation adjust screws to make up for it. But the one they sent is 0.88". So in order to square the mirror under the focuser, I have to shorten the stalk by nearly 1/2".

That's too bad, you don't have much adjusting range. On mine (plastic), I'm using a large metal washer and (2) black Kydex washers where the tilt screws contact it. My 3 globs of silicon only raise the mirror 1/16". Even with the washers and silicon globs I have 5/16" of outward travel on my secondary (GSO).



#8 precaud

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Posted 23 April 2019 - 08:34 AM

I emailed Antares, described the problem, and asked to return it. Their explanation for the thickness variation between samples:

 

"Most of the secondary mirror blanks are molded. When they come out of the molds they get a little chipped up around the edges. We machine the main surfaces down until the chips are removed.
If a batch comes in that are in better shape (less chips) we don't have to remove as much so they're sometimes a bit thicker when finished."
 

He apologized for the hassle, and offered to go through his 3.1" mirrors and send me the thinnest one, no more than .725". I thought this was a very reasonable offer, and agreed. I'll return this one today.

 

I'm still going to remount the original SW one and use it first. It may prove to be just fine now.


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#9 nirvanix

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Posted 23 April 2019 - 11:44 AM

Seems like your stinky tale will turn out smelling like a rose. I've got one of those stories in my past. Simply remounting the secondary turned Jupiter from a fuzzball with two lines across it to something approaching a Hubble photo, so hope you get the same, and I'm glad you're sharing your experience with photographic evidence on CN.


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#10 precaud

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Posted 23 April 2019 - 12:22 PM

Thanks, I'm pretty optimistic about it. If nothing else, this should give an appreciation for the micro-geometry involved in getting high-quality images from glass. That a simple piece of foam tape could significantly deform the mirror flatness, stretches the imagination...


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#11 leoyasu

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Posted 23 April 2019 - 03:07 PM

Looking forward to see the results and thanks for the information! I have the same scope and a while ago I evaluated the possibility of changing the secondary mirror for one of Antares. Finally I gave up due to shipping issues and ended up buying eyepieces and a ZWO 120.

 

Since this is a relatively easy modification, I would like to try it (especially now that I am starting in planetary imaging and this fired some alarms). One question, how did you reattach the mirror to the holder? I'm very bad with crafts lol.gif


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#12 precaud

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Posted 23 April 2019 - 06:49 PM

Looking forward to see the results and thanks for the information! I have the same scope and a while ago I evaluated the possibility of changing the secondary mirror for one of Antares. Finally I gave up due to shipping issues and ended up buying eyepieces and a ZWO 120.

 

Since this is a relatively easy modification, I would like to try it (especially now that I am starting in planetary imaging and this fired some alarms). One question, how did you reattach the mirror to the holder? I'm very bad with crafts lol.gif

 

Those are some very nice photos, Leo! I'll take some pics when I'm gluing the stalk back on. It's not difficult, just tedious.



#13 precaud

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Posted 23 April 2019 - 10:49 PM

Here's how to glue the stalk back on. Before you remove it and the 2-sided tape, mark where the front edge of the stalk is on the glass and measure distances to front and back edge so you can locate it there later. (Your marks will be removed when you clean the surfaces.)

 

: Remove all residue from mirror back and stalk. Goo Gone works great for removing adhesive residue. Let it sit for a couple minutes. Don't get it on the mirrored surface, though...

: Final clean both surfaces with acetone.

: Mark where the front stalk edge goes, and make guide lines for the centering the stalk's sides on the back.

: Mark where the silicon blobs will roughly go. One at the rear, two on the sides a little forward of center.

: Place three spacers in the gaps between the blob location. I ended up taping them down this time due to the weight of the stalk.

: Put down the three silicon blobs, and put the stalk in place on the spacers. Align it to the marks you made.

: Gently move it to a place where the silicon can cure undisturbed for a day or three. Doublecheck the alignment before leaving it.

: When you're ready to use it, remove the spacers and reinstall into the dob. Check centering and roundness under the focuser, then collimate.

 

Th-th-that's all, folks!

 

As you can see, one of my front blobs was a little larger and further forward than its neighbor. No biggy, they both have a good 1/2" of contact area underneath.

 

Attached Thumbnails

  • 2ndmont4.jpg
  • 2ndmont5.jpg

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#14 Jon Isaacs

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Posted 24 April 2019 - 08:30 AM

To square the mirror under the focuser. The stock secondary is 0.54" thick. Antares told me their 3.1" was 0.70" thick. Add to that about 0.10" for the dabs of silicone. Had that been the case, there should have been just enough space in the secondary collimation adjust screws to make up for it. But the one they sent is 0.88". So in order to square the mirror under the focuser, I have to shorten the stalk by nearly 1/2".

 

I believe there is also the root 2 increase due to the 45° angle. That's 0.48" all by itself.

 

It seems like you're on the right track.

 

Jn



#15 leoyasu

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Posted 24 April 2019 - 03:33 PM

Thank you very much for the instructions! These days I'll start with the modification. Luckily, I painted the back of the mirror with flat black paint so I already have everything marked. Anyway, I hope we get good results!


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#16 precaud

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Posted 26 April 2019 - 09:29 AM

after I mount my secondaries, I test them.

 

And here it is mounted with sili-blobs and ready to go. Looks very good - no curvature added by the mounting. Wish I would have removed it from the stalk and tested it before buying the Antares....

 

BTW, the fringes are a little weaker in appearance than the ones in post #2 'cuz I was experimenting with the height of the overhead white board diffusor. There's a tradeoff between lighting consistency and fringe contrast.

 

I have to say, I am very impressed with the optical quality of the SkyWatcher products I've bought recently. They have really upped their game.

 

Will the weather give me a chance to use it this weekend? Fingers crossed...

Attached Thumbnails

  • 2ndfrin4.jpg

Edited by precaud, 26 April 2019 - 09:34 AM.

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#17 leoyasu

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Posted 26 April 2019 - 04:38 PM

Don't know much about contact flat fringe testing but that sure looks better or straightened! In the meantime here, I'm trying to get silicone rtv 732...



#18 precaud

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Posted 26 April 2019 - 05:20 PM

I used this one, it came recommended for this, and may be more widely available:

 

https://www.homedepo...08570/303769575


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#19 Pinbout

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Posted 26 April 2019 - 05:43 PM

If you want a piece of tissue dm me



#20 precaud

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Posted 26 April 2019 - 05:55 PM

If you want a piece of tissue dm me

 

Tissue for what ? I;m not crying... yet  ;)



#21 Ed D

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Posted 26 April 2019 - 08:19 PM

The factory secondary on my Skywatcher 10" showed worse curvature without any mounting stresses.  I bought an Antares secondary and machined the mounting stalk because the Antares was twice as thick as the factory secondary.  In my case, it was the single best improvement I made on my Dob.

 

I'm glad everything worked out for your 14".

 

As for the tissue, maybe Danny is offering a piece of the correct type of tissue to use to protect the Antares secondary.

 

Ed D


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#22 precaud

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Posted 26 April 2019 - 08:50 PM

The factory secondary on my Skywatcher 10" showed worse curvature without any mounting stresses.

 

That is weird...

 

I bought an Antares secondary and machined the mounting stalk because the Antares was twice as thick as the factory secondary.

 

Does "machined" mean, you ground it down to length?

 

  In my case, it was the single best improvement I made on my Dob.

 

Seems to be a very consistent theme...

 

I'm glad everything worked out for your 14".

Thanks, but we're not quite there yet...

As for the tissue, maybe Danny is offering a piece of the correct type of tissue to use to protect the Antares secondary.

 

I think he's responding to a different thread I have going. But he's fun to play with so why not?  :)



#23 Ed D

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Posted 26 April 2019 - 09:34 PM

Not weird, just a horribly warped secondary, more to one side than the other.

 

I made a threaded shaft so I could mount the secondary stalk on my mini-lathe and take off enough material to center the new secondary under the focuser.  I'm sure others have used different methods, but this is what worked for me, with lots of patience.

 

The tissue bit reminded me of the Geico commercial with R. Lee Ermey playing the role of a therapist:  LINK

 

Ed


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#24 precaud

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Posted 27 April 2019 - 09:30 AM

I made a threaded shaft so I could mount the secondary stalk on my mini-lathe and take off enough material to center the new secondary under the focuser.  I'm sure others have used different methods, but this is what worked for me, with lots of patience.

 

The tissue bit reminded me of the Geico commercial with R. Lee Ermey playing the role of a therapist:  LINK

 

I remember that commercial. Hilarious! Still is! And I had my share of time with therapists in the past...

 

Mini-lathe... I'm jealous. If I have to reduce the length of mine, I'll have to use much coarser methods... like a hacksaw and bench grinder...


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#25 precaud

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Posted 28 April 2019 - 03:23 PM

I just installed the remounted 2ndary, centered and collimated the scope, and did some terrestrial viewing of various features on two power poles, one about 300 feet distant, the other about 450. Besides the sun glints on the ceramic insulators, I like reading the fine print on various components to see if clarity gets blurred with magnification. It's a bit windy today, gusts to 20mph, so patience is sometimes required waiting for the still air moments.

 

Remounting the secondary has turned this into an excellent scope. I could always tell that the primary was beter than the 2ndary, the scope had "focus snap" but it didn't deliver the goods when it got there. Now it does. When conditions support it, it will go into the 400x range comfortably.

 

Would it be even better with the Antares 2ndary? I don't know. Maybe I'll try it some day. It almost doesn't matter at this point.

 

2mm is an important number for me; it's the exit pupil above which my dominant eye becomes increasingly astigmatic. With this scope, a 9mm eyepiece gives a 2mm exit pupil and 183X magnification. With a Paracorr, make that 10.2mm eyepiece and 186X. Despite NM's "stellar" reputation, there are many nights here that do not have the seeing to support that power. But for now, I can't wait to get it out under the stars, good conditions or not!


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