Collimation bolt PCD

[molland67]

Hi,
Is there someone out there who can help me obtain the optimal collimation bolt pitch circle diameter for my 10" f/5 mirror? I would like to use 3 point support. I am also considering using a 4mm aluminium backing plate to keep the thermoelectric cooling option open for the future... Using the Plop program looks a bit tricky....
Some basic info:
Material = BK7 glass
Outside Diameter = 250mm
Thickness = 34.5 mm at outside edge
I'm not sure what other info is required?
Regards,
Andrew.

[rockit]

Here is something I came up with after studying many inferograms some from RC Optical designs and lots of different sources. I get a little out side the square to test limits, and to practice before I build a 20" RC nesmyth. Here is a pic of my 15 point support, the outer circle is 305mm Dia you might be able to enlarge it to scale from. All the notches are to relieve the material so each point will flex with equal force applied.
Well I got this far and can not figure out how to post an image. Bugga:lol: :rofl: :sadeyes:

[ballaratdragons]

Under the box where you typed your post is a tab marked 'Go Advanced'. Click it. When the new page opens, scroll way down to 'Manage Attachments'. Click on it. There is a limit of 150kb per pic. :thumbsup:

[rockit]

I was aiming to high. Thanks for the heads up.
Here it is, the finished product is my avatar.

[Ambermile]

Have you tried PLOP?

http://www.davidlewistoronto.com/plop/

Arthur

[rockit]

Hi, Ambermil,
No I haven't, I make a lot of different items and find that if it looks right it is right.
The 15 point came about by trying to remove the flaring between the three point supports. Not very crucial on such a solid 12" piece of BK7, but its effects will benefit making a cell for a larger, thinner primary(I won't be using 45mm+ of Bk7--too heavy at 20" plus).
All this is in practical theory, you make it and test it! Not sell it off paper and hope it works, old school I know but 100% good.
With a 10" mirror that uses standard diameter to thickness ratio, the cell mainly needs to not impede circulation. With PCD of the collimation they work from equal distribution of mass, don't laugh but you could guess this by eye and only be a nats whisker off. It is the point that it rests with the most stability, but then in the scope its turned onto its edge.
Have a look at the primary holder on some EQ newts you couldn't call it a cell. Now owning a reflector I am thinking static electrical build up of dust more of a concern than cooling?

[Ambermile]

Well, I have to say I have a 27-point cell in the 22" (that's a big hunk of BVC) and am currently running up a 27-point cell for a 20" for a friend. If any of those 27 points varies, then each millimetre is worth 1/100 wave on the mirror. You can see it would be easy to ruin a good figure rather quickly here - but as you say, smaller mirrors = less required accuracy.

Arthur

[rockit]

I understand, but I look at as, structural integrity of the mirror. At 10" for it's weight I is very strong but over 18" they become heavy and less structurally stable. As a fitter and turner we had a piece of round bar 260mm Dia and 2m long(solid, good quality), the amount of flex in it was amazing, it was visually the best example of mass verses dimension I have ever seen. Gravity wins.
In the cell requirements, some of the very common EQ Newt's work great with only double sided tape straight to the primary holder, at least it won't shift when rotated in EQ fashion.
I went 15 instead of 9 to divide the the mass dependence of each point evenly. After guessing I sincerely recommend using a divider and stepping out your PCD, check twice cut once(or is that the other way?).
I hope I haven't stolen your post molland67, have we been of any help to you? At what stage are you in construction? Cheers

[molland67]

I was looking for a simpler reply.... eg. Make the 3-bolt PCD XXXmm diameter and you wont be far wrong... Is there anyone that is familar with PLOP and the properties of BK7 glass that can run the numbers for me?
Regards,
Andrew.

[GrahamL]

Would .4 (100mm) .. od ..be close enough ?

[molland67]

Thanks for your reply Nightstalker.

The Dobson plans call for a bolt PCD 2" smaller than the mirror diameter. This would be about 200mm PCD. This is quite a difference to your proposed 100mm PCD... Any basis for your suggestion?

[GrahamL]

Yes sketchy maths :P ..useing that program and plate glass (no bk7option).

Gives a figure of (.396001 x primary diameter) as your pc radius
which I think someone then needed to double :whistle: sorry bout that
pretty close to 200 mm like you mention.

could you post some pics when you finish the cell?

many thanks

.

[rockit]

HI molland67, have a look into rcopticalsystems.com . By searching through their site you will find well backed information, and some very nice kit.

[molland67]

Thanks everyone. I'll go with 0.396 x D. :thumbsup:
I'll definately post some pictures when I get the big picture sorted out.