MTS-SN8 mirror cell, corrector align questions

[MtnGoat]

My MTS-SN8 OTA is in pieces on my bench after deciding to do a teardown following various shenanigans and misadventures, mostly having to do with attempted collimation and the dumb move of rotating the corrector alone inadvertantly.

After reading up on all the SN stuff I could find, I decided to do the 'clipectomy' as I always wondered where my faint diffraction spikes were coming from. I also wanted to redo the mirror cell as I have oblong stars identical to some of the ones in articles concerning these mirror cells (and clip issues). While I have it apart, I may as well flock it.

So here's my corrector mirror combo..which are the proper marks when I put it back together? The one on the left side of the corrector is a "V" shape, and as you can see the other is a fat dark mark. There are no other marks on the primary aside from the label F-009, which matches what is inscribed on the corrector.

Why two lines on the primary?
http://www.flickr.com/photos/35709891@N07/4473289899/

[MtnGoat]

Now for the rest of my fun...this mirror cell lacks the bottom supports under each clip. It is supported solely by uneven sized blobs of silicone to each post, and the center support is three huge blobs on the back and a stack of fibrous disks. Further, it appears to be installed slightly out of true, one of the clip tops just kisses the surface and the other two are about 1mm above it.

Worse, it is also not centered..note the varying widths of the gap between primary and posts where the glue bond is. This would appear to make my attempts to center the corrector plate itself, useless. 

Do I leave this alone, or pull it and redo it? Having it totally floating at the edges with the three posts with no support under there at all seems nuts to me.

History...even at it's best, I have almost always had slightly egg shaped stars. Last winter when it was about 10F the focus was atrocious, simply could not find a decent place to even get the tiny eggs. Above 30F I was always back to the normal tiny eggs, well, until I started screwing with it anyway. Now it's in for a penny, in for a pound.

Here's a shot my supports..
http://www.flickr.com/photos/35709891@N07/4474067130/

[DSOMAN]

There is not much attention paid to the mirror placement at the factory and they are often not centered. If you are going to use the mirror posts then I would make a small shim so that the mirror is spaced the same amount from each post. A little dab of RTV at the base is plenty to hold the mirror in place. Don't worry if the mirror is not exactly level as that will adjust out when you collimate.

I have never seen the bottom supports missing - strange!! You will need to fabricate some spacers for that purpose unless the mirror can sit on the bottom of the cell and be level.

Since you have everything apart you might want to consider a floating mirror support:

http://tinyurl.com/lrq6qb

I built this for my SN10 mirror from aluminum "U" stock and it is self adjusting and provides excellent uniform support for the mirror without edge contact. A little RTV on each of the 6 pads works fine.

Don't worry about the orientation of the corrector relative to the mirror, people say not to move it but it really does not matter.  It is a precision manufactured mirror after all and is completely symmetric. The factory does not have the skill to do a proper corrector/mirror alignment even if it were necessary, which it is not.

Brent

[MtnGoat]

thank for the suggestions on the cell improvements and placement. those are some good ideas.

I'm still up on the air on the relative positions of the corrector and primary though. There are some examples of folks getting radically different Strehl numbers in experiments with rotating them with respect to each other. If they didn't intend for the orientations to be matched somehow I don't know why they make anything resembling index marks on either optic.

[DSOMAN]

That is a good question. The marks may just be a reference used in the optical component manufacturing process. In order to mach the corrector to the mirror one would usually measure the wavefront aberrations with a sophisticated optical test system and tune for diffraction limited performance or best point spread function [close to 0.8 Strehl as possible]. A perfect mirror has 84% of the light in the Airy Disk and 16% in the rings.

From a technical prospective I can't see any reason why the rotational position of the corrector should make a difference. The corrector has a slightly varying thickness [microns] and is completely symmetric.

I have been told by a tech at Meade that the scopes arrive from China and are stored then shipped out to buyers. They don't even open the box [that is probably why parts are frequently missing]. The China factory can't even align the motor gears correctly half the time so I think it is nearly impossible that they would do an optical alignment


My only suggestion would be to take your best guess and put it back together and run some star tests. If stars have a decent Airy Disk then I would call it good.

Brent

[MtnGoat]

One thing to keep in mind..this OTA is the old one, from the MTS days of fork mounted glory (Actually one of my favorite mounts, I'm gotoing one). I'm nearly certain it was made in the US and I think they probably were playing the match game.

[DSOMAN]

Give it a try. I will probably be fine whatever corrector position you use. It is important to have the mirror centered with the corrector.

The mirrors are mass produced. There is a minimum spec on curvature tolerance etc. which can be easily checked on an assembly line. All mirrors will meet a minimum standard but statistically some mirrors will be better than others due to set-up differences and tool wear etc.

Most correctors are formed then polished and coated with MgF2 etc. If you have to rotate the corrector to achieve optimal performance then I would guess that the corrector tolerances are poor. This is not good engineering but might be necessary for cost issues in very low cost Newts.

Brent

[MtnGoat]

I picked up a small scrap (bar stock, 1.5" diameter) of delrin stock from the machine shop at work. Is this a good material for making supports for the edges and center?

Will silicone stick to delrin? If not, what common material should I use..will PVC or aluminum work? I'm assuming the primary will not actually touch any of these surfaces, instead resting upon thin pads of the bonding silicone.

Should I bother cutting small shoulders for the pads at the mirror edges, such as the stepped design of the edge pads in the newer cells?

Here's what I'm thinking.

[DSOMAN]

If it were me I would try to fabricate some small shoulders for the mirror to rest on.

Delrin should be fine. You might try gluing a test piece with RTV to see how well it holds.

If RTV won't stick then I would use aluminum.

Brent

[MtnGoat]

I went back to the machine shop and did more digging in the scrap bin. I found what may be ideal, a couple blocks of some kind of high density closed cell foam. It's machine-able and light and used for prototyping. This stuff feels like a block of balsa but has no pores and is extremely fine grained and very hard.

I found out bonding to delrin requires special treatments or adhesives and I'd rather stick to what others know works, silicone.

I'm now thinking I may follow the scheme above, but only bond to the three center posts and leave the edges supported from beneath but floating.

[DSOMAN]

Sounds like a good material. Let us know how it works out.

Brent

[MtnGoat]

Went for it. One can spend only so much time reading and handwringing. Didn't want to be trapped in analysis paralysis.

[MtnGoat]

Glued down supports a couple hours prior to final assembly to make sure they didn't shift when I did the final stage

[MtnGoat]

side view. Spacing shims hold mirror 0.2" above supports to allow small layer of silicone in between.

[MtnGoat]

side view. Centered as well as possible by eyeball combined with plastic dial micrometer. Tonight after 24hrs I'll pull the shims and see what I have.