I am in the process of rebuilding my 20" dob. Unfortunately, I am currently without access to a PC running windows, all Mac at home at present and not allowed to load unapproved apps onto my work laptop. I need to design a mirror cell for my 20" F5 mirror and without windows, cannot use PLOP.
Is there anyone out there who could run PLOP for me for the following:
Mirror diameter - 508mm
Mirror thickness - 51mm
F Ratio - F5
Substrate - Pyrex
Central obstruction - 89mm
18 point cell
Ian,
give Peter a call at SDM.
He has the bits you need already (laser) cut to size with all the hardware to match.
The price he charges is so reasonable that it really isn't worth doing it yourself.
No, if I remember correctly, there is a separate table with the radius data. The data on the components only provided the part dimensions via x/y coords.
A little off topic : From memory a single ring support at 55% or something works almost as good as an 18 point on a 2" X 20" , due to the ability to re focus out the slight overcorrection resulted from the sag on either side of the ring ( which reduces the error by a factor of 4 ) . My 14" X 1.5" is mounted like this - with a solid ring of velcro tabs on plate and mirror - no clips or slings - works a treat for many years
( Nice find in that article - I'm sure 60 to 70 % will be fine )
HI Ian - Its easy to calculate with plop - you just allow a single ring and put in loads of points say 36 to let it optimize the radius - I have some vague memory it was more like the 60% zone or something fro my mirror. Haven't used Plop for years so probably cant help you there. You need to make sure the `allow refocus ' button is checked somewhere.
No just used normal velcro but it was high quality stuff off a bulk roll but maybe not . I know that if I need to remove the rear triangle its mounted on it takes sustained pressure with a wedge for about a minute before it will start to separate ! The plate you velcro to wants to be nice and flat of course and varnish help the velcro pads to stick well .
From memory the 2.4 meter ATT telescope at Siding spring rests on collimating bolts but most of the weight is taken by just two ring shaped rubber tubes filled with air ( or mercury ) and probably pressure controlled by valves that are driven by counterweights that sense the attitude of the mirror and adjust the relative pressures in the tubes - a kind of astatic support.
This mirror is 1:16 ratio but this mirror would be I think 25 X more flexible than a 20" of the same ratio. Neverthless - amazing to think such a large mirror basically just rests on two tube annulus of air or mercury and no separate flotation points.
You could try an air ring support for fun - pump the tube up just enough that the 3 collimation bolts were able to do their job and most of the mirror floating on the air ring
A little off topic : From memory a single ring support at 55% or something works almost as good as an 18 point on a 2" X 20" , due to the ability to re focus out the slight overcorrection resulted from the sag on either side of the ring ( which reduces the error by a factor of 4 ) . My 14" X 1.5" is mounted like this - with a solid ring of velcro tabs on plate and mirror - no clips or slings - works a treat for many years
My 10" uses velcro strips on a 6 point design. Works extremely well, no detectable strain on the mirror at all and it holds collimation forever. Bought a box of 25mm wide, 10 meters long I think from Bunnings. Has other uses as well, cable retention up the pier eg.
I've used Marks suggestion of velrcro at the 60% zone for many years without any issues on my 12" I have three edge supports that barely touch the mirrors edge, Theres noticeable end float of the mirror sitting on the Velcro if you give it a slight lift (a good thing?) but its really not going anywhere,I'd happily transport my mirror box inverted if I had to and don't bother trying to remove it to clean as its pretty stubborn to separate.
There used to be a heavier duty "Velcro brand" available ,bunnings,,it had a load rating from memory,but I havn't seen it there recently,officeworks seem to list it.
I've used Marks suggestion of velrcro at the 60% zone for many years without any issues on my 12" I have three edge supports that barely touch the mirrors edge, Theres noticeable end float of the mirror sitting on the Velcro if you give it a slight lift (a good thing?) but its really not going anywhere,I'd happily transport my mirror box inverted if I had to and don't bother trying to remove it to clean as its pretty stubborn to separate.
There used to be a heavier duty "Velcro brand" available ,bunnings,,it had a load rating from memory,but I havn't seen it there recently,officeworks seem to list it.
cheers
I have hung mine upside down too, no problem. Loading for the industrial velcro is 1 lb per square inch ( Made in the USA of course ).
The Velcro allows a smidgen of movement due to the flexible nature of the locking hooks and material so no strain anywhere.
I wash mine in the scope, pull away the lightshield ( cardboard tube ) and just slosh around with a squeezy bottle with the mirror almost on edge vertically. I have pulled it clear once, took a bit of doing and required a thin blade to peel each of the three velcro areas one by one. Helped that the velcro had a bit of flex. Went back together no problem. I have a three side retainers with silicon and a 2mm gap just in case but not really necessary.
Heres the text output from PLOP btw
;18 point cell with fixed angles
diameter 508
thickness 51
focal-length 2540
n-mesh-rings 20
n-mesh-depth 5
rel-support-radii 0.385173 0.79345
num-support 6 12
support-angle 0 15
basis-ring-size 6
basis-ring-min 0
obstruction-diam 89
optimize rel-support-radii 0 0.01
optimize rel-support-radii 1 0.01
part triangle 6 point 0 0 point 1 0 point 1 11
part bar 3 part 0 0 part 0 1
many thanks Brent.
