Hairy Stars

I believe this is due to one of your spider vanes being slightly bent.

andrea tasselli · Sep 7, 2025, 05:53 PM

You can always apodize your aperture but apart for aesthetics there is nothing to gain and aperture to loose.

I know and that’s my dilemma. In terms of figure the mirror’s edge is great so there’s no need to mask it from that point of view.

Rick Krejci · Sep 7, 2025, 09:50 PM

That looks like clips/mirror edge effects to me. They look like they’re 120 degrees apart. Do you have a mask around the mirror’s edge?

No, that’s one of the things I’m thinking about doing. I think the clips and mirror edge are what are causing the pattern you’re seeing.

John Hayes · Sep 8, 2025, 12:40 AM

Bond the mirror to the cell and get rid of the clips. Just be sure to bond it properly so that you don’t distort the optical figure. Before you do that, test the system with an aperture mask to insure that you know what is causing what in the PSF.

That double vane could be caused by a few things. Is there a focuser tube that extends down into the light path on one side? Are there any heater wires running along a spider vane? There is something with a straight edge at right angles to that pattern that is cutting into some part of the marginal ray path somewhere in the light path. You just have to figure out what it is.

John

No wires and no focuser intrusion so I think I just have to look closely at the vanes.

What would be your suggested method for bonding the primary? My first thought was remove the pads and clips, then apply 3 small blobs of silicone adhesive at the 70 percent zone. Along with that I would think it would make sense to remove the side pads and drill and tap for three nylon screws for centering.

📷 post-337676-0-68492700-1609709222.jpg

Tony Gondola · Sep 8, 2025 at 03:23 AM

John Hayes · Sep 8, 2025, 12:40 AM

Bond the mirror to the cell and get rid of the clips. Just be sure to bond it properly so that you don’t distort the optical figure. Before you do that, test the system with an aperture mask to insure that you know what is causing what in the PSF.

That double vane could be caused by a few things. Is there a focuser tube that extends down into the light path on one side? Are there any heater wires running along a spider vane? There is something with a straight edge at right angles to that pattern that is cutting into some part of the marginal ray path somewhere in the light path. You just have to figure out what it is.

John

No wires and no focuser intrusion so I think I just have to look closely at the vanes.

What would be your suggested method for bonding the primary? My first thought was remove the pads and clips, then apply 3 small blobs of silicone adhesive at the 70 percent zone. Along with that I would think it would make sense to remove the side pads and drill and tap for three nylon screws for centering.

📷 post-337676-0-68492700-1609709222.jpg

I like your concept. 70% zone with three “glue-points” sounds very reasonable. The best thing to do is to first experiment to make sure that you are using the right kind of adhesive. You don’t want it to fail and have the mirror fall out, you don’t want it to distort the mirror, and it should be safe over a wide temperature range. I don’t have a recommendation off the top of my head but there are a lot of optical adhesives on the market so a little research will go a long way. We used to use a small hard point (like a recessed ball bearing) to mechanically define the position of the mirror and put the “blob” of glue over the hard point. Then push the component against the hard point and let the glue cure. That way the mirror is held by the glue but the position is well defined by the hard points. That avoids having the mirror “float” on the adhesive.

John

I don’t think that the temperature during bonding is super critical. The important things are:

1) Strength of the surface adhesion with glass and the mount material (aluminum?)

2) Temperature stability. It shouldn’t fail when it gets hot or cold and it shouldn’t put a lot of stress on the mirror when the temperature changes. Low CTE is a good thing but that may be hard to find.

3) Some level of elasticity is good. There are a lot of adhesives that cure to be rock-hard but that’s not desirable. My recommendation would be to look at two-part RTV adhesives. Also pay close attention to any recommendations for surface prep. A few adhesives require a primer for best strength.

4) Try to keep the diameter of the glue “circle” as small as possible—maybe 0.5” - .8”. You just want it large enough to safety support the weight of the mirror but not much more. The larger the diameter of the bonding area, the higher the chance of distorting the mirror figure.

Before bonding your mirror, I recommend getting a scrap piece of glass to practice the procedure. You can also use that sample to do some pull testing to gauge the strength of the bond. That will provide a lot of peace of mind after you mount your mirror.

Come to think of it, you could also do this with double sided tape. That’s how Celestron mounts all of their secondary mirrors. Give me a little time and I’ll search my PC. I think that I might have some records on the kind of tape they use. It is totally permanent and if you mount it correctly, it shouldn’t deform the mirror at all.

John

John Hayes · Sep 8, 2025, 05:01 PM

I don’t think that the temperature during bonding is super critical. The important things are:

1) Strength of the surface adhesion with glass and the mount material (aluminum?)

2) Temperature stability. It shouldn’t fail when it gets hot or cold and it shouldn’t put a lot of stress on the mirror when the temperature changes. Low CTE is a good thing but that may be hard to find.

3) Some level of elasticity is good. There are a lot of adhesives that cure to be rock-hard but that’s not desirable. My recommendation would be to look at two-part RTV adhesives. Also pay close attention to any recommendations for surface prep. A few adhesives require a primer for best strength.

4) Try to keep the diameter of the glue “circle” as small as possible—maybe 0.5” - .8”. You just want it large enough to safety support the weight of the mirror but not much more. The larger the diameter of the bonding area, the higher the chance of distorting the mirror figure.

Before bonding your mirror, I recommend getting a scrap piece of glass to practice the procedure. You can also use that sample to do some pull testing to gauge the strength of the bond. That will provide a lot of peace of mind after you mount your mirror.

Come to think of it, you could also do this with double sided tape. That’s how Celestron mounts all of their secondary mirrors. Give me a little time and I’ll search my PC. I think that I might have some records on the kind of tape they use. It is totally permanent and if you mount it correctly, it shouldn’t deform the mirror at all.

John

That would be great, certainly the simplest approach. Would you have the same size constraints when using tape verses RTV?

Once you fix the issue then please post back and show us, i have similar problem once with my 8” Newt, i didn’t use my 6” yet but i believe it will have same issue as well, i had an issue of collimating my 6” and not with 8”, but i think i will modify both then will use them again and see, waiting to see your solution.

Good to hear, can’t wait

Tony Gondola · Sep 8, 2025, 05:42 PM

John Hayes · Sep 8, 2025, 05:01 PM

I don’t think that the temperature during bonding is super critical. The important things are:

1) Strength of the surface adhesion with glass and the mount material (aluminum?)

2) Temperature stability. It shouldn’t fail when it gets hot or cold and it shouldn’t put a lot of stress on the mirror when the temperature changes. Low CTE is a good thing but that may be hard to find.

3) Some level of elasticity is good. There are a lot of adhesives that cure to be rock-hard but that’s not desirable. My recommendation would be to look at two-part RTV adhesives. Also pay close attention to any recommendations for surface prep. A few adhesives require a primer for best strength.

4) Try to keep the diameter of the glue “circle” as small as possible—maybe 0.5” - .8”. You just want it large enough to safety support the weight of the mirror but not much more. The larger the diameter of the bonding area, the higher the chance of distorting the mirror figure.

Before bonding your mirror, I recommend getting a scrap piece of glass to practice the procedure. You can also use that sample to do some pull testing to gauge the strength of the bond. That will provide a lot of peace of mind after you mount your mirror.

Come to think of it, you could also do this with double sided tape. That’s how Celestron mounts all of their secondary mirrors. Give me a little time and I’ll search my PC. I think that I might have some records on the kind of tape they use. It is totally permanent and if you mount it correctly, it shouldn’t deform the mirror at all.

John

That would be great, certainly the simplest approach. Would you have the same size constraints when using tape verses RTV?

I made a trip out to my workshop where I’m sure that I have a spare sample of the tape; but as hard as I tried, I couldn’t find it. It will probably turn up as soon as I stop looking for it! I’m pretty sure that it’s a 3M VHB tape. It was black and about 2-3 mm thick. 3M makes a wide variety of VHB tapes for different applications and temperature ranges. You’ll have to look through the 3M specs to see which type would meet your requirements. I’d recommend using three round pieces of tape—probably ~0.7” in diameter. Again, I’d pull test it before mounting the mirror. The stuff that I used on the Celectron secondaries was super strong. It was virtually permanent. You could remove it with a lot of heat and some hard work but there was no concern about it letting go.

John

Jochen Schambach · Sep 9, 2025 at 07:21 AM

This has already been discussed in German forums. Most attempts using different kinds of glue or tape ended up introducing astigmatism, especially when the scope was pointed close to the horizon.

That’s probably because the mounting wasn’t done properly. We used to mount reference flats in the range of diameters from 4” up through 12” for interferometers using potting compound and that was a super critical application. Any surface distortion greater than 1/40 wave (~ 12 nm) over a fairly wide temperature range would exceed the specified optical accuracy for the component. Adhesives are used all the time to mount ultra-precision optical parts but to work well, it has to be done properly. As I’ve said, Celestron uses tape to mount SCT secondary mirrors and the primary mirrors are all bonded using potting compound. I’ve spent a bit of time working with the folks at Carl Zeiss in Oberkochen and they were very well versed in this stuff so I’m not sure who is participating on the German forums but I would have to wonder about their level of expertise.

John