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Help needed with installing secondary mirror.


kbrown
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Gosh this is a lot more finicky than I ever thought it would be. I'm nearly there but....

So I went ahead and milled a matching size backing plate out of 3mm aluminum for the secondary mirror as per the idea in one of my previous posts. I also routed a channel for a dew heater wire on it:

IMG_20211227_143349.thumb.jpg.08f68572a7fcf81a84eacd4a673b157f.jpg

IMG_20211227_143338.thumb.jpg.659be795fa5023a0d6002f007ccb5568.jpg

Before gluing the heater wire in the channel I applied several coats of paint to act as the electrical insulation so the plate wouldn't short the wire. Here's the wire in place with some super glue dots:

IMG_20220107_115355.thumb.jpg.bfd11264cc158a657c08981cbe36c0ab.jpg#

I haven't got a photo of the finished thing but essentially I just filled the rest of the channel with industrial grade HV super glue. Then I attached the mirror onto the other side of the plate with three blobs of marine grade silicone with a 3mm gap between the plate and the mirror.

Everything seemed to work just fine except I can't really use the heater. Without the heater on, I get near perfect images with practically no astigmatism at all. But with the heater on, it starts to warp the mirror enough to get oval shaped stars. Here's an out of focus example:

Blink00008.png.8e858aca4c99e720a51cdfa88718a8fe.png

So my main question here is; How to heat the mirror up without  warping it?

 

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45 minutes ago, Chriske said:

Hold on,  you're saying(if I understand it correctly) : no heating and there's practically no astigmatic error at all ..

?

Yes, that's correct. With the heater on it slowly starts to show astigmatism again. I guess the heater causing the backing plate to bend slightly which in turn deforms the mirror. I haven't got a controller on the heater so it's always either on or off...

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You need to adjust the heat to just one, or two degrees above ambient. Just enough to keep the dew away from that SM.

But as your SM has already astig, when cold, there's another problem. Looking at the surface of the aluminium I think that sheet of alu is most probably hardened alu.
Meaning that sheet of alu is full of strain. That has to be removed first. Now as it is heating that sheet of alu the tension in the alu get worse and pulls on that mirror. You need to remove that strain by softening the alu. Sadly enough you need to remove the mirror first from that sheet of aluminium.
If you do not know how let me know.

BUT what I do not understand is why the ast has a perfectly elliptical shape. Maybe just maybe it has nothing to do with that hardened sheet of alu. at all.
First this :
Are you ab-so-lu-tel-y sure that both these mirrors are free of astigmatism...!!!  Maybe in the past you didn't notice..?
Can the primary mirror move freely in it's holder. Just a fraction of a millimetre is enough. The clamps/hooks (to hold the mirror down), are they not to tight.

Make drawing please, were are the silicone blobs located..? It's not a major issue, but it could reinforce the error you're dealing with right now.

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I suspect that the "hot" backing plate is simply heating the back of the secondary causing it to warp. If there are only three small blobs of silicone, with decent ( 3mm is fine) gaps to the secondary, then any warping of the backing plate should have no effect on the shape of the mirror.

Personally, I am against any heating anywhere near a telescope. Thermals of only 1 or 2 degrees will spoil a perfect image.

Nigel

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32 minutes ago, Astrobits said:

I suspect that the "hot" backing plate is simply heating the back of the secondary causing it to warp. If there are only three small blobs of silicone, with decent ( 3mm is fine) gaps to the secondary, then any warping of the backing plate should have no effect on the shape of the mirror.

Personally, I am against any heating anywhere near a telescope. Thermals of only 1 or 2 degrees will spoil a perfect image.

Nigel

If the backing plate is warping then it should not distort the shape of the mirror, as you said Nigel. That is correct. But what about the size of the blobs..? I'm afraid (tell me If Im wrong here kbrown) that the blobs are not to big. That's why I asked before to make an estimate drawing about that.
Heating a mirror, be it Pyrex, Duran or any other decent glass it should not deform at all, by the heating it that is. That's what all these types of glass are suppose to do, hold its shape when cooled or heated.
And btw 1 or 2 degrees should not warp any mirror at all.


Let's first find out what kbrown has to say about the quality of his mirrors.

First : Make a picture, same as above, turn he primary mirror 90° around it's optical axis in it's holder and make a second picture holding the same position of camera and scope. What happened with the pattern.? Did it hold it's position or not...?

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3 hours ago, Chriske said:

You need to adjust the heat to just one, or two degrees above ambient. Just enough to keep the dew away from that SM.

But as your SM has already astig, when cold, there's another problem. Looking at the surface of the aluminium I think that sheet of alu is most probably hardened alu.
Meaning that sheet of alu is full of strain. That has to be removed first. Now as it is heating that sheet of alu the tension in the alu get worse and pulls on that mirror. You need to remove that strain by softening the alu. Sadly enough you need to remove the mirror first from that sheet of aluminium.
If you do not know how let me know.

BUT what I do not understand is why the ast has a perfectly elliptical shape. Maybe just maybe it has nothing to do with that hardened sheet of alu. at all.
First this :
Are you ab-so-lu-tel-y sure that both these mirrors are free of astigmatism...!!!  Maybe in the past you didn't notice..?
Can the primary mirror move freely in it's holder. Just a fraction of a millimetre is enough. The clamps/hooks (to hold the mirror down), are they not to tight.

Make drawing please, were are the silicone blobs located..? It's not a major issue, but it could reinforce the error you're dealing with right now.

Sorry for the confusion. No I don't think the SM has astigmatism anymore when cold. I think the previous astigmatism I had was due to the way I had it attached to the holder. With the new backing plate and three blobs of silicone that issue seems to have resolved itself when cold. Here's a 9 frame animation going from extrafocus to intrafocus with the SM cold. The collimation isn't perfect here though.

Here's the same exercise with the heater on:

1 hour ago, Chriske said:

If the backing plate is warping then it should not distort the shape of the mirror, as you said Nigel. That is correct. But what about the size of the blobs..? I'm afraid (tell me If Im wrong here kbrown) that the blobs are not to big. That's why I asked before to make an estimate drawing about that.
Heating a mirror, be it Pyrex, Duran or any other decent glass it should not deform at all, by the heating it that is. That's what all these types of glass are suppose to do, hold its shape when cooled or heated.
And btw 1 or 2 degrees should not warp any mirror at all.


Let's first find out what kbrown has to say about the quality of his mirrors.

First : Make a picture, same as above, turn he primary mirror 90° around it's optical axis in it's holder and make a second picture holding the same position of camera and scope. What happened with the pattern.? Did it hold it's position or not...?

Here's a rough drawing indicating the location and size of my silicone blobs. Maybe they are too big?

silicone_blob_locations.thumb.jpg.a76db73450502c8d1b430e4700f1b88d.jpg

I have checked the primary mirror and its holder multiple times in the past and made absolutely sure it's not pinched or slopping. I have also done the rotate around optical axis test and it did not affect the pattern.

@Chriske How would you remove the strain out from the aluminum plate? Heat it up slowly in boiling water or something then let it cool down on it's own (with the water)?

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Ok, some issues can be eliminated after that last post.

You need to concentrate on that SM or it"s holder.
a few possibilities there.

Before we go to removing strain out of that sheet of alu...this is most important not to do that yet.
You need to rule out some possibilities first.

- Do you have a reference flat..?
-Do you have a Foucaulttestes and know to use it..?

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5 minutes ago, Chriske said:

Ok, some issues can be eliminated after that last post.

You need to concentrate on that SM or it"s holder.
a few possibilities there.

Before we go to removing strain out of that sheet of alu...this is most important not to do that yet.
You need to rule out some possibilities first.

- Do you have a reference flat..?
-Do you have a Foucaulttestes and know to use it..?

I now do have a reference flat. I posted some results on this thread back in November (go back to previous page). I started building a focault / ronchi tester as well ages ago but unfortunately I haven't finished it yet.

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Carefully place the flat agianst the SM and see what result you got.
See that there are no dust particles between SM and reference flat...! do  not remove the alu-sheet at this point.
take a picture of that result.
Next remove that aul-sheet, and do exactly the same.
Compare these two pictures and post them both here please.

This way you can eliminate possible problems of that alu-sheet.

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6 hours ago, Chriske said:

Carefully place the flat agianst the SM and see what result you got.
See that there are no dust particles between SM and reference flat...! do  not remove the alu-sheet at this point.
take a picture of that result.
Next remove that aul-sheet, and do exactly the same.
Compare these two pictures and post them both here please.

This way you can eliminate possible problems of that alu-sheet.

I did some testing tonight but was reluctant to remove the SM from the backing plate just yet as I wanted to show this first. Here's the optical flat on the SM at ambient temperature under green laser light. Doesn't look too bad as far as I can tell.

IMG_20220116_234222.thumb.jpg.a908512fb984414034346874508bb3c1.jpg

Then without moving the setup at all I turned the dew heater on and waited about 20 minutes. Here's what I got:IMG_20220117_000001.thumb.jpg.7353ea5445b163190cea86f668fa8e16.jpg

As a sanity check I then turned the heater off and waited again about 20 minutes and it returned pretty much were I started:

IMG_20220117_002419.thumb.jpg.e33fc8cd89fc7fcf1e865e6118f023a7.jpg

 

So it's definitely heating the plate like this causing the warping. I tried to take a photo of the silicone blobs I've put in. Do you think these are too big in diameter?

IMG_20220117_004008.thumb.jpg.9c38b9a138ed36aa0d91b5c02a948ba7.jpg

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Remove that sheet of alu.

You can now continue without that heating system.
If you hold on to that heater you need to soften the aluminium to get rid of that strain.
I'll explain later how, it is not the same as softening metal...!!
The silicone blobs should be much smaller and very near the edge, almost peeking out of the perimeter of that SM.

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In my view the silicone blobs are too large. They are also too near the edge, they should be about halfway from the centre, or, as I mentioned previously, there needs to be only one blob in the centre. As I mentioned previously, my secondary has been held with a single blob for about 20 years. ( Maybe it's time I took it off and had it re-aluminised, then, maybe not).

Consider the last image you put up. If the Al sheet warps in a vertical direction then the silicone twists and puts a small "plus and minus" stress onto the back of  the mirror over the area of the blob but cannot affect the mirror beyond the blob boundary. Also, the more silicone there is between the Al and the glass the more the stress can be absorbed by the silicone and the less stress is transmitted to the glass. Thus the smaller and thicker the blob the better. However, if you heat the Al sheet then the sheet will expand horizontally and try to move the blobs away from the centre of the mirror putting a stress over the whole of the back of the mirror causing it to bend slightly compared to the front surface. In addition, the radiant heat from the heated Al will warm the back of the mirror more than the front putting further stress onto that back and causing more curvature of the mirror, ( all commonly  used glasses expand with temperature, Low expansion Borosilicate glass expands 30-50% of that for plate/float glass). Heating one side of a sheet of glass ( or anything else for that matter) will always produce a temperature differential from one side to the other as the thermal conductivity limits how fast the heat moves from one point to the next and the unheated side will always radiate the heat away keeping it cooler than the heated side.

To prevent dewing the mirror(s) should be shielded as in a solid tube (or shroud for open truss designs).

Nigel

 

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It's not a good idea to give different opinions while searching for solutions in a case like this. It's most confusing for kbrown.
So Nigel please you continue helping to solve this problem.
To be clear : no hard feelings here.

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13 hours ago, Chriske said:

Oops ...!  looking bad that heating system. How hot does that thing get...?  (about)

 

13 hours ago, Chriske said:

Remove that sheet of alu.

You can now continue without that heating system.
If you hold on to that heater you need to soften the aluminium to get rid of that strain.
I'll explain later how, it is not the same as softening metal...!!
The silicone blobs should be much smaller and very near the edge, almost peeking out of the perimeter of that SM.

I've had the heater on for couple of hours now while monitoring the temperature of the backing plate and the surface of the mirror. At the beginning both were at 18.9C and after a couple of hours later the plate was at 50C+ (oops !!!) while the surface of the mirror had only gone up to about 21C. Clearly this is not the way to go... I was going to build a closed loop heater controller (to keep the plate only a certain amount of degrees above dew point). But now seeing how inefficiently the heat is transferred to the other side of the mirror, I'm not sure if this is a good idea anymore.

 

6 hours ago, Chriske said:

And one more thing. Maybe you already removed the alu sheet. If not, While cutting, leave the silicone blobs on the SM.

I haven't removed it yet. I'm sort of reluctant to do that now as it does work without the heater. Out of curiosity why would I leave the blobs on the SM?

 

3 hours ago, Astrobits said:

In addition, the radiant heat from the heated Al will warm the back of the mirror more than the front putting further stress onto that back and causing more curvature of the mirror,

 

I think this might be the main culprit here given how much hotter the plate gets compared to the opposite side of the mirror...

 

3 hours ago, Astrobits said:

To prevent dewing the mirror(s) should be shielded as in a solid tube (or shroud for open truss designs).

Nigel

Hmm... haven't seen a design like this in a classic newt but I suppose it makes sense. Mine's a basic steel tube Sky-Watcher 250P.

 

@Chriske I managed to find out what alloy my Alu sheet is. It's 1050 H14 so it's pretty soft already. For 1xxx alu this page says: "Non heat-treatable. Technically annealing is possible, but it’s really hard and best avoided entirely unless absolutely necessary. To be honest, this stuff is so soft that it’s really unlikely that you’ll need to anneal it unless you’re really stretching and compressing it.". I'm not sure if I'd want to try to treat the plate I currently have since I'd probably have to strip the super glue and paint off as well. If I go down that route then I think it would be easier if I just milled a new plate. I'd be keen to hear about your method of taking the strain out of the Alu?

 

Edited by kbrown
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