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Posts Made By: Bob Chester

May 20, 2005 02:08 AM Forum: Telescope Making

perfect parabola - hard to grind?

Posted By Bob Chester

Marcin,
A 'perfect' parabola is impossible to grind. A really, really good one is quite doable. In an 8" mirror a parabola is the only way to go. Patience (which you say you have) is probably the most important ingredient needed.
Look for Jean Texereau's book 'How to Make a Telescope' or others that are available and don't be intimidated. Dive on in. Great joy and pride can be derived from viewing through a scope you know you made.
[COLOR="Red"]Caution!! [/COLOR] Amateur Telescope Making is addictive.

Bob

May 30, 2005 06:27 PM Forum: Reflectors

Re: Filter holder

Posted By Bob Chester

Hugh,
If the scope has a 2" focuser and you use 1-1/4" eyepieces you can use a 2" to 1-1/4" adapter that is threaded for 2" filters. Just be cautious using long barrel barlows. Of course this doesn't help if you already have a bunch of 1-1/4" filters.
Bob

June 3, 2005 03:20 PM Forum: Pictures of Me and My Telescope and........

All dressed up but nowhere to go...

Posted By Bob Chester

Sean took the words right out of my mouth but I'll say it anyway----Sweet set-up. May it provide a lifetime of enjoyment.
Bob

June 3, 2005 10:37 PM Forum: Equipment Talk

Is the optical shop dead?Has the net taken over?

Posted By Bob Chester

I was in Seattle last August and visited both ATWB (nice shop) and Captains Nautical. While in C.N. I mentioned I owned a Takahashi, the guy behind the counter said he had never heard of them. We didn't stay long.
Bob in Minnesota with 1 camera/scope shop and 1 ham radio/scope shop.

June 24, 2005 03:08 PM Forum: Takahashi

I finally did it.

Posted By Bob Chester

Couldn't be said any better than Steven said it.
Why don't YOU tell us why you don't miss the other three?

Another happy FS78 owner, Bob

July 18, 2005 11:57 PM Forum: Telescope Making

mirror re-coating

Posted By Bob Chester

I too can recommend Spectrum. Especially now that they are an A-Mart sponsor. I have used them and my mirror came back pinhole free, opaque, and with the figure preserved. A-1 communications with the man in charge also.
Bob

August 8, 2005 08:45 PM Forum: Telescope Making

Dew Heater Sizing for 6" Primary and Secondary?

Posted By Bob Chester

Mike,
Here is a page that may be of help. They like 1/2 watt resistors. The page describes the making of heater strips but the info should still apply to your design.

http://www.dewbuster.com/heaters/heaters.html

Bob

October 18, 2005 01:07 AM Forum: Telescope Making

polishing tool dilemma

Posted By Bob Chester

Marcin,

I personally had great difficulty with individual pitch squares. I couldn't cast uniform thickness strips from which to cut the squares. Cutting of the squares and fixing them to the tool was very messy. And too much channel maintenance during the first pressings just to get good contact.

SO, I ended up doing it this way.
I built a paper and tape dam around the tool. Poured in the melted pitch.

PROBLEM: Mirror is concave, cooled pitch is flat.
Several warm pressings were required to squish a little pitch out the sides and raise the center until good contact.
MY weapon of choice was a HEAT LAMP. They are a great way of softening the pitch with good control.
When good contact was achieved, I formed the facets with a PITCH FACETING MAT. A grid pattern pressed between the mirror and lap to form the facets. They eliminate a lot of channel maintenance though trimming of the edge of the tool is still required. At the end of each polishing session I would warm the pitch with the heat lamp, spray the lap with water, place the lap mat in place, press with mirror and cover with a wet towel. About 1/2 hour before my next session I would remove the lap mat and cold press with a little weight.
Newport glass sells pitch faceting mats, Willman-Bell used to and still might. I made one from the side of a laundry basket. Highly recommended.

Two tips:
1) Perhaps stating the obvious, but, before you start thinking parabola, bring the mirror to the most perfect and smooth sphere possible. Try to parabolize directly from this sphere. If you miss and corrections get too complex don't be afraid to work back to a sphere and try again. Patience me boy, patience.

2) Enjoy.

Bob

November 12, 2005 07:18 PM Forum: Equipment Talk

Soft bags for eyepieces/assorted bits?

Posted By Bob Chester

Here is a source for the grey foam.

http://www.foambymail.com/packaging.html

I own one of these cases and like it a lot. Fair price and plenty roomy for me.

http://www.buytelescopes.com/product.asp?t=&pid=6805&m=141

Bob

January 13, 2006 10:14 PM Forum: Telescope Making

My mirror - back with numbers

Posted By Bob Chester

Hey Marcin,
I was curious as to how things were going.
Soooo... you reversed that old oblate spheroid and over shot the parabola eh?
No need to despair, do as Dan suggests and you should be fine. Do you recall the stroke that created your original oblate spheroid? That would probably work also but keep a close eye out for zones.
I'm gonna put in a couple words in support of the sphere.
I personally prefer to parabolize directly from a sphere. I think it is the most assured way to achieve a smooth figure from center to edge. I probably spent 90% of my figuring and correcting time on producing as perfect a sphere as I could see. Then work for the parabola. If the numbers got too funky and required aggressive corrections I would go back to a sphere and shoot again. If you figure from a good sphere, monitor the numbers to see they are moving in the right direction together and arrive at a parabola without major corrections, you will have a smooth figure.

Also, with a sphere the entire surface goes gray with the knife edge. This gives a beautiful view of the mirrors surface quality. Dog biscuit, micro ripple, exposed bubbles, surface roughness in general, all come into clear view.

Good luck. Wishing you total success,
Bob