79 replies to this topic

[Starman1]

Because clips should not touch the mirror in use.  Even the slightest pressure will deform the mirror's image at the focal plane.

[MeridianStarGazer]

Because clips should not touch the mirror in use. Even the slightest pressure will deform the mirror's image at the focal plane.

I did not say they would touch the mirror. They would just hover inside the gap, ready to catch the mirror if need be.

[Starman1]

I did not say they would touch the mirror. They would just hover inside the gap, ready to catch the mirror if need be.

That would work.  It wasn't clear from your post that that is what you meant, however.

I wonder if there would be other ramifications of the groove.

One solution to a loose mirror suggested to me many years ago was to glue a nylon strap to the top edge of the mirror

that was attached behind the mirror cell as well, leaving a little slack.  If the mirror tipped forward, the strap would catch it,

but because there was no tension on the strap in normal use, there would also be no optical ramifications.

[MeridianStarGazer]

I took apart a cheap 8" mirror cell. The clips were massive. A ring was used instead of a sling: possibly a smart design given that the average beginner might put their tube into their car upside down. The mirror cell had 3 contact points at 50% radius. And the collimation screws: I can see why why collimation comes loose. The 8" mirror was 20.5mm at the edge and weighed more than the cell.


Back to the clips, though. Each clip can have a short tapered end that engages the groove, and an inch long lever for your fingers.

The glued on top strap is easier than grinding 3 slots. The strap could be narrow and attach near the back of the top. It could even attach to a flange that limits the mirrors upward movement.

I would limit my scope altitude to plus 5 degrees, but should it tip further, the sling better be able to keep the mirror from falling out.

[rfiol]

I know this is an old thread, but wow -- what a great discussion.  Very helpful.  Has anything new emerged since 2018 when this was last discussed?

 

After digesting this, I think I am going to go with a) no clips, b) an easily removable ring/cover that I will keep on for transportation, but remove for seeing, and c) a safety strap (with slack) siliconed to the top edge of the mirror and attached to the mirror box.  

Edited by rfiol, 25 February 2021 - 11:33 AM.

[luxo II]

The method that seems to be missing is that used by F R Royce, and used in SCT's, Maksutov's and even large cassegrains:

 

1. Make the mirror conical, to lighten it and reduce thermal mass,

2 Drill a hole into the mirror and mount it on either a central stud (Royce), a central stud (cassegrain secondary mirrors) or even a central baffle tube (SCT's and Maksutov's).

 

It's relatively simple to mount a mirror this way without ugly clips protruding over the edge.

Edited by luxo II, 26 February 2021 - 12:30 AM.

[MeridianStarGazer]

I think if the bevel is 1/8", you can have clips that are not in the light path, and the bevel adds insulation to the edge of the mirror. That is my plan.

Another issue is the mirror coater will also have clip shadows. Well, GSO does. Anyone know about majestic?

[LU1AR]

The prolific telescope builder Ed Turco, made "The Ultimate Newtonian" with some improvements, that can be seen in this forum. One of its recommendations was to mask a couple of millimeters of the diameter of the mirror. He says that the worst configuration errors are concentrated in that area and what is gained in optical quality, is much more than what is lost by the aperture drop.
Ed Turco's telescopes competed in planetary quality with apochromatic refractors that cost a hundred times as much.
Without hesitation, I would end that problem with a black cardboard mask; glued on the periphery of the mirror holder.
Regards.
Edgardo

[Jeff B1]

The clips on my 16" (3" thick) mirror were very short because it was a permanently mounted scope; makes a different.  If one is careful even a portal scope can use clips that barely cover the mirror beveled edge out of the light path.

Edited by Jeff B1, 11 March 2021 - 06:25 PM.

[Jeff Morgan]

The prolific telescope builder Ed Turco, made "The Ultimate Newtonian" with some improvements, that can be seen in this forum. One of its recommendations was to mask a couple of millimeters of the diameter of the mirror. He says that the worst configuration errors are concentrated in that area and what is gained in optical quality, is much more than what is lost by the aperture drop.
Ed Turco's telescopes competed in planetary quality with apochromatic refractors that cost a hundred times as much.
Without hesitation, I would end that problem with a black cardboard mask; glued on the periphery of the mirror holder.
Regards.
Edgardo

The Takahashi Epsilon uses a circular mask that doubles as the edge clip. 

[Dan_I]

Very interesting discussion !

 

 

The Takahashi Epsilon uses a circular mask that doubles as the edge clip. 

This is also what I use on mine (5mm-thick laser-cut black Delrin). It also reinforces the mirror cell. There's about 0.1mm of clearance, adjusted with thin washers.

 

Edited by Dan_I, 12 March 2021 - 01:51 AM.

[luxo II]

The edge of the ring is shiny, I would have made it with a step or shallow bevel to a sharp edge.

Edited by luxo II, 12 March 2021 - 04:28 AM.

[Dan_I]

The edge of the ring is shiny, I would have made it with a step or shallow bevel to a sharp edge.

I'm not sure it's really shiny, but I agree sanding down a bevel next time I clean the mirror won't hurt.

[Jon Isaacs]

I am revisiting this thread because I think I might be seeing the effects of the Obsession Style rotatable mirror clips on my 13.1 inch F/5.5

 

I will experiment with rotating them, both out of the way and at various angles.   I didn't see any Masculator results from straight clips at 45 degree angle, it seems like this might be similar to curved vanes on the spider... 

 

If rotating them out of the way does make a difference, I will put a large sign on the mirror cover reminding me to rotate the clips back in place before putting the cover on. 

 

Anyone thought about just using a single clip at the top?  

 

 

Jon

[Oberon]

I built (then abandoned) a single clip at top that automatically fell or dropped into place once the mirror was tipped, say, more than 75 degrees (or 15 deg from horizontal), and then reset to clear once tipped back closer to zenith. Its not hard to do and not hard to make reliable in normal use. But I didn’t consider it reliable for abnormal use (such as restraining the mirror in a motor vehicle accident), so abandoned it.

[MitchAlsup]

I might note that the profession guys put serrations (i.e., small amplitude sine wave around the inside of the ring)  to further minimize the diffraction of the ring (seen in coronagrpahs more than in more general optics.)

[Starman1]

I am revisiting this thread because I think I might be seeing the effects of the Obsession Style rotatable mirror clips on my 13.1 inch F/5.5

 

I will experiment with rotating them, both out of the way and at various angles.   I didn't see any Masculator results from straight clips at 45 degree angle, it seems like this might be similar to curved vanes on the spider... 

 

If rotating them out of the way does make a difference, I will put a large sign on the mirror cover reminding me to rotate the clips back in place before putting the cover on. 

 

Anyone thought about just using a single clip at the top?  

 

 

Jon

In my first 12.5" scope, I wanted to dispense with clips.

So I glued a nylon strap to the top edge of the mirror that was fastened to a buckle on the under side of the mirror cell.

The strap was just loose enough that if the scope moved below the horizontal the mirror would not fall out of the cell and would fall forward only about 1/8" before the strap would catch.

No clips in use on that scope.

In my current scope, I rotate them out of the way in use and return them to in place when transporting.

Edited by Starman1, 20 March 2021 - 11:22 AM.

[scotsman328i]

This is a great thread! They should sticky this post.

[Jon Isaacs]

This is a great thread! They should sticky this post.

 

I'm not so sure about that. The reason people use clips is to avoid using stickum to keep the mirror in place. 

 

 

Is that Bad or What?

 

Stupid jokes aside, a good idea..

 

Jon

[scotsman328i]

I'm not so sure about that. The reason people use clips is to avoid using stickum to keep the mirror in place. 

 

 

Is that Bad or What?

 

Stupid jokes aside, a good idea..

 

Jon

Hardy har har Jon! 

 

Stick to applied engineering, telescopic design and ocular physics as your day job. You’re MUCH better at that! 

Edited by scotsman328i, 20 March 2021 - 12:51 PM.

[GTom]

Absolutely useful thread! Recently unboxed my Quattro 150p, one of the first advice I got is to turn it into a 140p, LoL... I'll check if the stock clips could be removed on that one.

[Starman1]

With my dob, the 3 mirror clips (safety for travel) were small rubber-covered posts that protruded over the mirror surface.

They could easily be rotated out of the way so that, in use, there were no clips at all.

Since I remove the mirror and store the mirror in a separate case for transport, I finally decided the clips were unnecessary.

 

It's obvious that an EQ-mounted newtonian needs some form of clips.  That's where this thread comes in.

[Tom Stock]

Some people grind a channel around their mirror so a sling can go in there. Why not make it deep enough some short clips can reach in too? Then you have all the safety and nothing in front of the mirror.

It's been done.  It's the difficult way to fix something that was never an issue to begin with.

 

Also known as a solution in search of a problem.

Edited by Tom Stock, 26 June 2024 - 11:19 AM.

[Tom Stock]

In the 90's I used to just wrap the side of the mirror with painters tape to a plywood cell with thin carpet between.

 

I've come across many scopes since done the same way, but they were usually 10" or less.

Edited by Tom Stock, 26 June 2024 - 11:24 AM.

[CrazyPanda]

I wonder if there would be other ramifications of the groove.

 

Mike Lockwood grinds handles into his mirrors:

 

 

Even with a thin mirror, it doesn't have any negative effects. I don't see why a groove or recess couldn't be ground into the mirror to provide a place for safety catches.