Sky at Night Magazine

[wgater]

[center]If you are building the telescope which we had in our 'How to build your own telescope' features earlier this year, in the magazine, then why not post up some pictures and tips on how you are getting on? We've had a great response and it's great to see some pictures appearing on the forum. [/center]

[astromanian]

Here is one of three photo's of my mirror cell from Simon Lang's plans:

Ian


[image]local://104/2279CA0DA7B340D89B9BC468232A2F0D.jpg[/image]

[astromanian]

2nd mirror cell photo.

What I want to know at this stage, is how do I correctly mount the mirror to the cell?

Ian


[image]local://104/A70447F216A24433A9EEFC3B676DEA7D.jpg[/image]

[astromanian]

Just out of badness, here is the the third photo!

Ian

[image]local://104/C81D6BF771064EE686E9C62179F35F1C.jpg[/image]

[astromanian]

Back to school!

Now I remember what a protractor is for......


[image]local://104/EC77CB8639D24D4CB5CC56F2106777E5.jpg[/image]

[sallan]

Astromanian,


Looks like your project is coming on well.


I found some plastic right angle brackets X3 with a base with two screw holes ,and filed the mirror gripping part down if that makes sense. The brackets hold the mirror with the least overlap on the mirror that I could get away with.Also I had to use some wood to pack out from rear of mirror cell so the brackets could reach over mirror. I will send you a pic if you wish. Just to make sure as I was not to convinced how well that would hold ,so used some silicon mastic as well to give the mirror additional support. It will come off if needed in the future. Basically I made it up as I was not sure myself neither.

keep plugging away mate.


Simon

[astromanian]

Simon,I'm getting there slowly.

I decided on a few changes. Firstly, I have made the spider hub and secondary mirror holder from aluminium and I have been reading about the advantages of a curved spider. I began thinking about this when examining the strength (or flimsyness) of 2mm aluminium strip, I then picked up some steel banding about 18mm wide but much less than a mm. This is still flimsy across the thickness , but very strong across it's width. Not much good for supporting a lump of ally with a mirror attached to it.

Then I remembered a diagram in one of my ATM books which shows how to make a curved spider which will significantly improve the strength and stability of the secondary assembly (see below...)


[image]local://104/A545ADFD3DE242CF83414B951E16A45A.jpg[/image]

[astromanian]

[color="#0066ff"][font="verdana"][size="1"][color="#ffffff"]This type of spider is supposed to reduce the diffraction effect caused by straight arms - I like this method more due to its strength. We will see. If the whole lot comes crashing down on my finished mirror.............[:(]

I think i will reinforce the two fixings on either side of the spider against the tube assembly with maybe small square brass plates with nylock nuts and super glue the threads into the spider hub! - I'm giving this bit some more thought for now.
[/color]
Image: pg181 Handbook for Telescope Making - N.E. Howard 1962

[color="#990000"][color="#3366ff"]Astro[/color]man[/color][color="#ffffff"]ian[/color]
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[image]local://104/62A92E4FE84D4F198ACAA7D9BD103E61.jpg[/image]

[sallan]

Thats a brilliant spider hub there wish I had the gear to make that , counter balance later maybe lol


Have seen the curved spider in a few designs but not the two discs like that nice one as you are right mine is a bit flimsy and wobbles when I knock it slightly not noticed much diffraction though.

Any keep up the good work always good to see how someone else overcomes and adapts.

Cheers

Simon

[astrobits]

How to mount your mirrors onto the cell?

It is easy, just use glue!

I have mounted many mirrors with Silicone adhesive, even 16" ones. I chose the clear type for sealing, among other things, greenhouses. However, there are rules that you MUST follow with this mounting method.

1. Use only THREE blobs of adhesive. Using four or more can very easily cause distortion of your mirror. I know, it has happened. Position the blobs about 40% of the way from the centre of the mirror to the edge, this is the optimum to minimise sagging of the mirror. Any sagging of the mirror is spherical and only affects the focus position by a very small amount ( fractions of a mm). The blobs should be about 1" diameter ( for an 8" mirror) when the mirror is in position. ( for a 16" I aimed for about 2" dia ). If you make the blobs too large there is a risk of distortion again.

2. When the mirror is in position the silicone should be about 3mm ( 1/8" ) thick. This is easily achieved by using three spacers which can be removed when the silicone is set ( overnight ). This has three benefits. It allows the air to circulate behind the mirror for cooling, it prevents any distortion from the mirror cell affecting the mirror and it allows you to get a blade, saw or knife, in to remove the mirror when it meeds recoating.

If you think that the mirror might not stick very well I suggest that you try with a scrap piece of glass, or the mirror tool. Allow it to cure for a day and then try to separate the two bits. Be careful if you use thin glass for this as you might easily break the glass before breaking the bonds!

If you are going to use MDF as the mirror cell I found that the Silicone does not stick very well to this material unless you smear a bit of epoxy adhesive on first. When set just roughen up the epoxy surface and apply the Silicone.

Job done and no hassles with mirror clips.