Posts Made By: Steve VanHyfte

Here is a poor photo of my cooling fan setup. The meade mirror cell had a 3" hole in the center, so I cut a circular mounting board with a router and attached it to the back of the cell via the 3 collimation locking screws that were next to the traditional spring screws. I cut a 3" hole for the fan and 4 mounting screws in the back of the plate. You can see the red on-off switch at 12 o'clock, then 25 ohm poteniometer fan-speed control at 4 o'clock, the mono headphone power-jack at 8 o'clock. The 3 collimation screws are at 2-6-11 o'clock, and you can see the 6 counterweights around the plate to offset my heavy university optics focuser and finder. The fan is held by 4 thumb-type-screws so it can be removed and flipped over in about 2 minutes. This way I can experiment with the fan either blowing up the tube or down the tube. So far I have only used it with blowing air down the tube and out the back. All the parts were purchased at Radio Shack. I cut a vibration-isolater pad out of a black Best-Buy mouse pad which is sandwiched between the wooden plate and the fan. So far I have not seen any vibration at 250x at any fan speed, 250x right now is my highest power. All of the components on the plate are "inside the tube" so if I happen to set the tube up on the mirror cell end, none of the components will be damaged. (nothing sticks outside the tube)

This fall I will be makeing some really nice telescope kits. But to make the kits cost efficient, I will need a quantity of focusers, finders, mirrors, ect at a very low price. If you look at E-bay and many other web-sites, there are MANY individuals re-selling telescope making components made mostly overseas for generally low prices, but that is still not low enough. I have tried many google searches for overseas suppliers for telescope parts with no success. Does anyone know how to contact these suppliers and for what prices/quanities.

My Dob with rocker box weights 60 lbs, so I used eboney star formica with 2" round virgin teflon pads for the azimuth bearing. The movement is very smooth, but still takes about 3x more effort than the altitude. At a recent star party, I compared my movement with the imported Orion and harding sytle DOBS and my dob is far superior, but against an obsession that is 2x the size of my scope, I would have taken a distant 2nd place. I went to the Obsession web site and they state they use glassboard and eboney star. I take it, they use the "glassboard" on the azimuth bearing. I looked up glassboard which seems to be FRP or fiberglass reinforced plastic, which seems to be a very high bumpy surface which is used as wall and ceiling covering for industrial and retail. Does anyone have any experience with FRP vs eboney star. Does anyone have an obsession telescope that can say for sure that the azimuth bearing uses a very bumpy surface?

I am working on an early 1980s Starliner 14" f/5 German EQ mounted reflector. The telescope has a declination motor, wiring and control box, but the Dec motor does not work. The RA motor works fine. The wires from the Dec motor go into the RA motor cover. I took off the cover hoping to find a loose wire or blown fuse for a "quick fix". But what I found was suprising.

At least 8 wires are not connected. And there is no schematic on the inside of the cover. Years ago, someone at the university mentioned that a relay may have been omitted when the telescope was manufactured. To make matters worse, Starlliner is out of business, and the telescope was purchased a short time before the company went under.

What I am looking for is any information to put the dec motor back in operation. An electrical schematic, a photo of a simmilar telescope that is wired correctly.

Thanks in advance.

I just finished collimating a 12.5" F/7.
How did I do?

This scope has been gathering dust for years!!, It is really big. The entire scope, is home-made, including the doublet lens which was ground sometime in the 1960s. A quick look at some trees did show some violet, but seemed less than my Synta 6"F/8. I wonder if anyone has any idea if the lens design for synta 6"F/8 would compare to a traditional Farenhoffer (spelling)8" F/15 ????, assuming the same sphere of 1/4 wave at focus.

I have question regarding the most accurate way to center spot a large mirror. The recommended way to find the center is to measure from the "edge" in many places, or to make a template with a compass the same size as the mirror. But most mirrors are really slightly larger than their advertised size to compensate for the beveled or champhered edge the mirror has so the mirror will not easily chip. This edge is not typically uniform and is variable around the circumference. So what edge is best to use, the actual outside edge of the mirror, or the outer edge of the figured top surface. I am not a mirror grinder, but it seems to me......
If the mirror was figured first, then aftewards the edge was ground off, seems the better edge would be the outer mirror edge. But if the edge was ground off first, then the mirror was figured, seems the better measurement would be the final edge of top mirror.
Am I thinking correctly, or an I all wet...
When is the mirror edge ground off, before or after figuring??
Thanks in advance....

I am considering working on a mothballed 8" F/15 project this winter. The lens was amature ground and I am guessing it is a crown flint combo. I have yet to look at any sky object through the lens, but does anyone have any idea how the final image should look like. Color diagrams or spot diagrams?, or say a 8"f/15 doublet would be comparable to an 8" F/6/8/10 of what other lens design?

Does anyone know how much backfocus is required for a Canon 20D to focus on a 20" F/5 reflector? Novaks' book Newtonian notes states the typical 35mm SLR needs at least 1 3/4" for the camera alone and typically more depending on the Prime focus adapter needed. Do the new digital SLR cameras need more or less backfocus than the older 35mm film cameras. Any help would be appreciated.

I have an 8" F/15 doublet lens with no coatings in a machined cell.

The lens and cell are both home made, the maker has long since passes away and it was dontated to our local club. If I hold the lens and cell assembly in my hands, and I look down into the sky lens (through R1 first), I can see the intereference pattern of R2 against R3 since the lights in my home are the new cork screw light bulbs.

As I rotate the entire lens/cell assembly or very gently press onto the sky lens retaining ring, the intereference pattern changes drastically. You see about 20+ sets of rings, and the center of the bullseye pattern is rarely centered. In some orientations, the center of the pattern is way over to the side of the lens, others rotations see the circular pattern change to an oval shape with oval rings all the way to the edge.

I removed the lenses for cleaning and found both lenses are really thin for 8" diameter. Both edge thicknesses are about 3/8". The sky lens is double convex with R2 ROC visually shorter than R1. The rear lens, R3 is concave to a point where the center thickness has got to be no more than 1/4", and just eyeballing R4 the lens has an extremely large ROC, just slightly curved.

The lens formulas I have seen for most Franhofer type prescriptions of this aperature typically call for lenses of about double what I see, even without measuring. ( I don't have a sphereometer or a caliper at the moment). A quick and dirty focalt test showed R3 with a ROC of about 37 to 38 inches.

Here are my questions. Should not the intereference pattern I see be fairly stable, with the bullseye fairly centered regardless of cell rotation?

Is it normal to actually see the R2 to R3 pattern just looking down the sky lens? (no coatings on any lens surface).

My guess is the lenses are way too thin and the pattern change I see is due to thin lenses.

Any thoughts?