Mounting a secondary mirror - correct way?

[gooseholla]

Hello,

I have three questions about mounting a secondary mirror that I hope someone can answer.

1. What is the best type of silicone to use?

2. Does the centre of the secondary mirror have to be directly under the centre of the spider's bolt?

3. If I put a template on the back of the mirror and mark the centre, is this where the centre would be on the front side?

Hope these make sense.

Regards.

John

 

[JRM]

Hi John,

 1- When I made my 10" they told me to use 100% pure clear silicone.

 2- as close as possible is best.

 3-  yes

for what its worth, my secondary mirror was attached back in 2002 and it is still holding strong.

Rick M

[Peter Drew]

Make sure that there is some sort of flat spacer between the mirror and the mounting plate, apart from ensuring the mirror sits squarely it also gives you an easier task to eventually remove the mirror for recoating. I usually use a 1p coin but if you're feeling flush a £1 will do just as well. 

[faulksy]

or some tooth pics depending on the size of the minor axis

[Astroscot2]

The secondary mirror on my OO newt looks like it was placed onto the holder by a drunk with a silicon gun. Its slightly skewed which becomes apparent whencentering the secondary under the focuser when collimating - the sligtht skew prevents the secordary showing a nice circle though the cheshire. Has no real effect on the perfomance but drives me nuts..

Mark

[gooseholla]

When you say silicone, are we talking about the kind of stuff you put round a sink, or is there another? Someone said something about glazing silicone used for aquariums?

Thanks for the replies.

John

[faulksy]

yes aquarium silocone is best. but has  be clear as it bonds better than coloured

[gooseholla]

So, for example, something like this? Just the first one on the list, not particularly the brand but the product, is this the right silicone sealant type?

Regards

http://www.amazon.co.uk/Everbuild-AquaMate-Fish-Tank-Sealant/dp/B0012RRFCQ/ref=sr_1_5?ie=UTF8&qid=1426088038&sr=8-5&keywords=aquarium+silicone#productDetails

John

 

[Astrobits]

I have always used any clear silicone that lists greenhouses/glass as a suitable application and have never had a problem.

Nigel

[faulksy]

as you say nigel has to say for bonding glass

[gooseholla]

Okay thanks. Now, this may seem silly, but silicone always seems flexible to me and quite soft when you poke it around the sinks. Does it stay this way when gluing the mirror to the secondary?

John

 

[Peter Drew]

Yes. That's why it's used. Holds safely but doesn't impart stress on the glass. 

[astrolunartick]

Thanks for starting this topic John.  Handy to see what every one suggests. Cheers guys

[faulksy]

also remember your offset

[JRM]

Pete and All,

    When I did mine I placed the holder in a vise grips at a 45 degree angle, placed three blobs ( not big but higher then

the spacers so the silicone would smush down some once secondary was placed on top) and three pieces of popsicle

stick spacers then placed the secondary onto the blobs, let it set for 24 hours before removing spacers, its been stuck

ever since.

Now forgive me if memory fails me but I recall being told not to use the stuff for aquariums forget the reason why but I did

not use it, but that was 13 years ago or so, the reason for three blobs was as Peter said, so as not to impart stress but also

three contact points hold better then one...

but out of fear of it falling off, I am going to purchase ( if I can not make) a holder for my 3.5" secondary, to much

time invested to have something like that occur

these look good..

http://www.astrosystems.biz/sechold.htm

Ric M

[gooseholla]

also remember your offset

Yup think mine is 0.2 of an inch or something the Kriege + Berry book says.

I remember that I have some clear silicone sealant out in the garage I purchased a couple of years ago. I'll check if its still in date.

Thanks for all the responses guys.

[Astrobits]

Yup think mine is 0.2 of an inch or something the Kriege + Berry book says.

 

I remember that I have some clear silicone sealant out in the garage I purchased a couple of years ago. I'll check if its still in date.

 

Thanks for all the responses guys.

If you opened it a couple of years ago then it might have set. If you can get anything out that is still "fluid" then it is still in date and useable.

Ric M, if you are worried about your flat falling off try to pull it off with your hands. IF you can pull it off then re-do it and try again after the silicone has fully cured. I once sent an 8" telescope OTA through a courier company who handled it so badly, must have dropped it from a great height, that the primary mirror separated from the MDF "cell" to which it was siliconed with 3 blobs. The failure point was within the MDF, the glass to silicone and silicone to ( treated ) MDF bonds were still intact.

Nigel

[gooseholla]

Thanks for everyone's replies. I siliconed my mirror onto the secondary about an hour ago. Will see if it is good enough tomorrow at 4pm. Last I saw the blobs had flattened down to just above the toothpicks.