Posts Made By: Gary Klein

I am looking for someone who can grind and figure my 50 inch classical cassegrain mirror set. I was hoping that Vaughn Parsons would do it, but his shop is shut down at the moment.

Gary Klein

Neil Barnett suggested that I describe my bino project to this group.

I had arranged to purchase 4) 30 inch x 1.5 inch pyrex mirrors from Vaughn Parsons a year and a half ago. I had asked for two of these to be F4.5 and two to be f6.

I had already purchased a set of 50 inch cassegrain blanks from him, which he was supposed to grind, polish and coat for me.

He completed one of the 30 inch f4.5 mirrors and I have it. Unfortunately for me, he has a consulting job in Hawaii and his shop in Salt Lake City is shut down. He also used the remaining 3) 30 inch blanks for other projects.

I would like to make a 30 inch binocular in f4.5 and a 30 inch binocular in f6. I think these will be a reasonable size to construct and transport in an air ride truck trailer. I think the scope can roll out the back of the trailer on a ramp with a winch or can extend on a pair of beams like a drawer slide.

As these have a long focal length, the veiwer will sit in an observing chair which moves with the telescope. I hope to make the optical structure and the chair support to be independent but move together.

For focusing, I plan to move the primary mirrors independently. The eyepeices will be fixed except for interocular distance.

I am also looking for someone who can grind, figure and polish my 50 cassegrain blanks.

I guess currently I am one tree shy of a hammock.

Thank you,

Gary Klein

Steve Swayze in Portland is very reasonable for testing and refiguring and does an excellent job.

You can do a knife edge test without any special equipment yourself. A better test is the Ronchi which takes a fine grid of lines (not very expensive) near the focal point and projects them onto the mirror and back to your eye. This shows the amount of correction, a turned edge or flat spot, and the smoothness of the mirror.

Gary Klein

I am contemplating building some large bino's. 22 inch up to 40 inch.
If the height of the "ladder" is not an issue, then what is the optimum f ratio for best optics? This is for visual use only.

If the focal length is long, then I will need to use a long eyepiece for low powers. This results in an exit pupil that will be larger than the eye, and probably reduced apparent field of view.

If the focal length is short, there seem to be several optical downgrades.

A larger secondary is required. More obstruction
The larger secondary is closer to the primary, more light scatter and less contrast.
The light is coming in at a severe cone angle. I suspect that some eyepieces will not be able to use the entire cone. IE, effective vignetted veiwing.

I am leaning towards f5, any thoughts?

Gary Klein

I accidentally got a little silicone adhesive on the front surface of my 11 inch flat (part of a diagonal for a large scope).

Then I compounded it by attempting to gently wipe it off with some cotton gauze, which of course smeared it badly.

I have tried lacquer thinner and isopropyl alchohol, flushing and wiping with the gauze, with no success. Am I totally up the creek?

From my cleaning attempts so far, there have been no visible scratches, so I think the aluminum got some Silicon Monoxide coating.

Any suggestions would be greatly appreciated!!!

Thanks,

Gary Klein

I am not an expert in the optics field! But before you start rethinking the Cave, I question your method of looking through the focuser to size the secondary. You did not tell us the minor diameter of your secondary, or the measurements of primary to secondary and secondary to your observation point, so I can't check it.

There are several sources of equations you can use to size the secondary. Any of the books on telescope making discuss it. There is a nice discussion on http://www.catseyecollimation.com/mccluney.html of effects of secondary mirror size on how much of the field will be fully illuminated.

Many of the drawings I have seen show the focal plane of the basic scope without eyepiece coming to focus on the outside of the drawtube (where the eye is). I am not sure that this is really the case. I think the focal plane occurs in front of the eyepiece and the eyepiece picks up the image and reconverges it into the eye. So the focal plane for the primary mirror is down inside the focal tube. This should be possible to check with a piece of translucent material directly focusing on a bright star or planet. By looking at the secondary from farther away than the focal point you may be cutting off the primary mirror's edges.


Gary Klein

I would use an electricians hole punch. They go through steel cabinets easily.

Next favorite is to back the tube up on the inside with a block of wood and use a hole saw at slow speed.

Gary Klein

I am pretty inexperienced at alignment, but I have noticed that several of my focusers introduce error.

When the optic or alignment tool fits loosely into the focuser, it can have a range of positions when you tighten down the locking screw. As the locking screw is near to the outside end of the focuser tube, it tends to cock the optic / alignment tool when you tighten it down.

If you are using a 1.25 inch to 2 inch adaptor, that adds another error component.

Another major problem I have had is when I put something heavy in the focuser, like a heavy optic or a binoviewer, the whole focusing tube cocks in the bearings rather badly. I have given up using my binos until I can come up with a better optic mount.

I think it would be better to have a holder that holds your optics 0 degrees with respect to axial alignment to your scope but displaced laterally half a millimeter, than have the optic centered but 1/2 degree off angle.

Gary Klein

Gary Klein

The jump from 18" to 30" is amazing. You don't look at the same old things. For example, instead of looking at M31's dust lanes, you can see the huge nebula in M31 and the very large individual stars in the nebula. You can't fit M31 in a normal eyepiece.

I use a 41 panoptic or a 40 mm TMB eyepiece most of the time. Then work my way down as seeing and the object require.

My 30 inch is f4.2, requiring a tall ladder.

I have heard a lot about Coma in the faster scopes. I can see it, although in my 16 inch F6 it varies greatly with the eyepiece used. Some of the cheaper ones have a lot of coma at the same power. At high powers, it does not seem to be a factor.

If you use a scope that is f4 with a 40 mm eyepiece, is the coma better or worse than a scope f3 with a 30 mm eyepiece? (same power)

I have noticed a difference in contrast between fast scopes and slow ones. The slow ones rule, especially as the light pollution increases.

Dear Paul:

It is possible that the catseye is causing an issue.

In larger scopes, the secondary wants to be visually centered on the focuser, but displaced slightly off axis to the primary, away from the focuser. This allows the secondary to grab the full cone of light, without being oversize.

As a result, if your scope is perfectly collimated, the secondary will appear to be off center in the reflection from the primary. This is because it is off center, intentionally.

The laser should nail the center of the primary and bounce back to the center of the laser. It will also appear slightly off center where it reflects on the secondary.

The secondary in this case is also actually displaced off axis slightly toward the primary, but appears centered in a cheshire.

In small scopes the displacement would be very small and not worth doing, so your secondary is probably on-axis in that one.

Gary Klein
Does this make sense?