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Posts Made By: Joseph Babendreier

August 8, 2012 10:34 AM Forum: AstroMart FAQ

gift certificate

Posted By Joseph Babendreier
Having read a lot of comments on the new policy, I wanted to throw out this idea:

Perhaps Amart could have some kind of scheme for allowing members to give gift certificates.

The certificate would include a reduced price for signing up in order to use the certificate... and then the bulk of the certificate would apply to buying stuff from Classifieds or Auction.

Or Amart could offer a reduced rate for first time subscribers ($X instead of $15). I've noticed that magazines still use this gimmick to get new subscribers.

Clears,
Joe

August 23, 2012 07:17 AM Forum: Telescope Making

how to make a secondary

Posted By Joseph Babendreier
In one of the old ATM books I have, it explains how to make a secondary. I want to try to do that.

I got a f/4.5 17.5" primary (damaged but workable) and I was thinking of making a 3.5" secondary for it.

I was hoping I could get away with using 12mm plate glass. It's the thickest glass I can get here in Kenya.

I have more than enough grit to do whatever needs to be done, I was mainly concerned about the thickness of the glass. All the secondaries I've seen are thicker than 12mm.

Any thoughts on the matter?

Clears,
Joe

September 15, 2012 02:51 AM Forum: Prayer Line

Re: Mother past away

Posted By Joseph Babendreier
david goldman said:

Just for the record: My mother past away september 8.

I'll pray for her especially today. May she rest in peace!

Clears,
Joe

September 17, 2012 04:24 AM Forum: Telescope Making

need advice on movement of mirror box

Posted By Joseph Babendreier
I'm in the middle of building a truss dob for a f/4.5 17.5" mirror.

I finished building the assembly of mirror box / rocker box / platform yesterday.

Without putting in the mirror yet (which will weight 29 lbs) the mirror box is heavy to push and pull. I did not detect any stickiness in the motion. It turns smoothly. But I was surprised that I need to use a bit of effort to turn the mirror box.

The mirror box alone weighs about 20 lbs (without the mirror) and the rocker weighs another 20 lbs. I guess it's just the inertia of the 40 lbs.

Perhaps it is all too heavy. At least the seconday cage is light: a mere 5 lbs (without eyepiece or finder scope).

I nothing to compare with (except pictures in books!!! and my own home-made 10" dob). I was wondering if people who have a big dobsonian have a similar experience.

Clears,
Joe

September 18, 2012 01:29 PM Forum: Telescope Making

aluminizing a secondary

Posted By Joseph Babendreier
I will eventually need a secondary for the f/4.5 17.5" dob I'm building. But I'm a really tight budget. So I'm thinking of trying something in the DIY category, following some instructions in an old ATM book I have.

The secondary will be a 3.5" made from plate glass.

I was wondering where in the US would be a good place to have it coated.

Clears,
Joe

PS There's a deal available on eBay for $15 (plus another $16 for S/H). It's listed as item # 280475722223

October 9, 2012 08:29 AM Forum: Telescope Making

question about dob construction // secondary mirro

Posted By Joseph Babendreier
I'm presently constructing a dob for an f/4.5 17.5" mirror. I'm debating whether to use a 3.1" secondary or a 3.5" secondary. I've been checking out the design program at Stellafane:

http://stellafane.org/tm/newt-web/newt-web.html

I get this curious result. At least it's the opposite of what I expected:

Leaving all other parameters the same, the Stellafane program tells me that:

1) for a 3.1 secondary, the distance from primary to secondary (the program call it "mirror face to focuser hole") ought to be 65.5"

2) for a 3.5 secondary, the distance from primary to secondary (mirror face to focuser hole) ought to be 65.8"

Isn't a 3.5 secondary supposed to be CLOSER to the primary than a 3.1 secondary?

Perhaps Stellafane's idea of "mirror face to focuser hole" is more literal and includes the distance no only from primary to secondary but also from secondary to the opening of the focuser.

Any ideas on this?

Clears,
Joe


October 13, 2012 03:25 PM Forum: Telescope Making

question about using rouge for polishing

Posted By Joseph Babendreier
I'm making a 3.5" secondary mirror out of plate glass.

I cut three pieces of 6mm glass about 5" x 7" and ground them against each other in turns for about 4-5 hours total, working my way from #220 down to #800. The original glass piece was very old and had some minor scratches. The three pieces came out well enough with nice smooth surfaces. A very tiny pits here and there, but nothing I'mn going to fret.

I started using some rouge for polishing with a pitch lap (also a 6mm piece of glass for a base). It's my first time polishing anything.

The rouge leaves a general reddish hue on the glass that doesn't wash off. Is this normal? I've done about 10 minutes of polishing one of the three pieces and stopped.

I'm using rouge simply because it's all I have available currently.

Any experience on the matter?

Clears,
Joe

October 16, 2012 04:11 PM Forum: Telescope Making

are dealers reluctant to aluminize a secondary???

Posted By Joseph Babendreier
I've sent emails to six different companies asking for a price to aluminize a new secondary made with plate glass. All located in the US.

I've given several days to respond, in two cases several weeks.

NOBODY has written back to me.

Seems odd. Maybe it's such a small item that it's not worth their trouble.

Clears,
Joe

October 27, 2012 06:26 AM Forum: Telescope Making

progress on grinding for 12.5 mirror

Posted By Joseph Babendreier
Here's a picture of my 12.5" mirror. I finished #400 grit. I'll be moving on to the #800 soon.

I had gotten to #800 once already but got a nasty little scratch. I'll have to be more careful to clean out all the little crevices from the tool. I learned the hard way that the "sucking" action with such close contact between mirror and tool at that stage sucks some hidden grains of old grit into action.

But it was just as well. I decided to go back to #100 grit and grind down the sagitta a bit more. Now I'll have a f/5.9 mirror instead of a f/6.6 mirror.

Clears,
Joe

November 1, 2012 03:34 PM Forum: Telescope Making

HELP with optical testing for DIY 3.5" secondary m

Posted By Joseph Babendreier
I did my first test (ever in my life) looking at diffraction patterns. I'm hoping someone can help me sort out some ideas here.

I have three secondary mirrors (still in square shape) pretty close to final polish. Let's call them A, B & C. All three are about 5" x 7", aiming at making one usable 3.5" secondary.

I set up a red laser light shining thru a barlow and after the barlow an ample piece of ground glass. The red light is positioned to strike the target at about a 45 degree angle as the target lies on the table. I'm looking at the target from the other side, my line of sight also at about 45 degrees relative to the target -- just like in the book!

I laid A on C and saw about 40 lines, each line stretching from one edge of the glass to the other. All the lines where curved.

I laid B on C and got the same result.

I laid A on B and saw about 40 lines but all the lines were almost perfectly straight, stretching from one edge of the glass to the other.

Does this mean that both A and B are flat?

Clears,
Joe

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