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Messages - Don Pensack

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Equipment Discussions / Re: Mirror cell design question.
« on: November 06, 2017, 02:55:17 PM »
PLOP optimizes a 3-point cell for contacts at 40% of the radius from center, so the 3 points would have to be fairly close in (1.2" from center of mirror).
The 6-point design is a bit more forgiving.  For a 6" mirror (likely 1" thick or less), you could also simply cement it to a triangular plate.
But under no circumstances would you want a spring to press directly on the mirror or even on a pad the mirror sits on.
And keep the 3 collimation springs--don't use 6.

If you experiment with PLOP, you'll find there is a point where increasing the number of points of support makes almost no difference.
With my thin 12.5" mirror, that point was with 18 points of support (which is what the mirror has).  If you put in the details and get a significant reduction in error
with a change from 3 to 6 points, use 6.  Obviously, you designed just such a cell.  My only comment there would be that you need to significantly reduce the size of the contact patches unless the cell is used on an equatorial mount.  If used on a dob/alt-az scope, the contact points could be reduced to maybe 1/4" diameter.

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Equipment Discussions / Re: Comet filter?
« on: August 12, 2017, 01:13:37 PM »
Many inexpensive O-III filters also transmit the C2 lines, so make decent "comet" filters.
[You'd have to know the spectrum of transmission, and Lumicon will test a filter for free or provide you details
on a filter they've tested.]
And since these typically have much narrower bandwidths than many "CLS/Broadband" type filters, also yield more contrast.
Note that they only enhance the gas tail.  The dust tail is usually full spectrum and usually best without a filter.

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Equipment Discussions / Re: counterbalancing a truss dob?
« on: August 12, 2017, 01:09:12 PM »
For looks, add two small finger-shaped pins to the bottom edge of the top side of the mirror box.
Buy a few dumbbell weights of 2.5 and 5 lbs.  Add the weights to the pins as needed.
If you only need one weight and not above 30° altitude, a bungee cord between the bottom edge of the top of the mirror box
and the rocker box below it will suffice as a "virtual" couterweight.  Just make sure the length of the bungee is the right length
that it doesn't start getting stretched until the scope is getting low.  As the cord stretches, the equivalent counterweight increases,
which is ideal.  If your eyepieces vary in weight a lot, a second hook and second bungee cord may be needed.

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