Posts Made By: Jeremy Schmit

Here is a little eye candy for all those who would rather be looking at the comet but are currently washed out. I finished this scope at the very end of November (I applied the final coats of varnish to the scope within a week of coating the mirror with aluminum.) I have only had a chance to use it about three times (I missed the december new moon due to a business trip), so I think this explains our weather.

The mirror is an f5.3 10" and tests at about 1/9 wave (although it has tested everywhere between 1/4 and 1/20 wave.) It mostly follows the Kriege and Berry design, but there are a also I few ideas I found here and elsewhere in the online world. The latches on the sides of the mirror box are the same as the ones used to attach the truss tubes to the upper cage. So, the upper cage clamps to the mirror box for transportation and storage. When I put the trusses in the pole bag with the shoulder strap and clamp the two pieces of the scope together I can carry the entire scope at one time.

A few months ago I found a trash pile at work that appeared to have a couple of large Newtonian objectives on top (I work in a physics department, so this is not overly strange.) After doing the initial double-take I found a ruler and went back for a closer look. They were 13" and 9", and both were mounted in adjustable cells. However, the cells didn't look like anything that was supposed to go at the end of a tube. I wasn't completely sure that these were being discarded, so I asked a friend in the Astrophysics group to inquire for me. Eventually, I got the word that these were indeed trash, but by this time the 13" mirror had disappeared. Since I already have a 10" scope I wasn't thrilled with taking the 9", but my friend convinced me it could be used to fund my eyepiece habit, so I grabbed the mirror and stuck it in the corner of my office

...Fast forward to present day...

I am now getting ready to move across the country, so figured it is time to get rid of all my junk. Since I don't have a Foucault tester handy, I figured I would just sell this glass as a blank and let the buyer have a pleasant surprise if the figure happens to be good. Still, to be thorough, I wanted to post the focal ratio so they buyer would know what kind of curve they were getting. Here is the punchline...this thing is completely flat! I think I might have an optical flat on my hands. IIRC, flats are worth a lot more than curved mirrors, correct? By how much? More importantly, how can I confirm that it is indeed an optical flat and check the quality? I don't want to sell this thing and screw the buyer over if it happens to have a crappy surface. Most of the reflective coating is missing (about 80%), but otherwise it looks like the surface is in pretty good shape. That protective layer of dust must have worked. Any suggestions for how to get rid of this thing?

Thanks for all the input, everyone.

Mark, it is exactly that bang/buck ratio that has me looking at orion.

Ken, I hadn't seen that review site yet. What a goldmine! It is exactly those kind of shaky reviews that have me questioning the antares. However, I haven't seen any reviews at all for the orion (or stellarvue). Speaking of the stellarvue, I checked out their site, and I didn't see a RACI finder. All they had was an erect image finder. Is this the same thing? (I've been assuming that the erect image is still flipped in one direction.)

My tool is about 1" smaller than my mirror, so I rarely bother to trim it. Even with the pitch overhanging the tool, the lap is still about 1/2" smaller than the mirror. The 2" strokes I described are the largest I'm doing. Most strokes are closer to 1-1.5", and at this stage of my work the strokes are faily slow and controlled. Therefore, as I work my way around the mirror, the outer 5mm on the sides of the mirror barely gets touched. Should I add some sideways movement to change this (Phil seems to say no, Norm says yes, is there a tiebreaking opinion?)

Norm, you seem to be suggesting a small "W" stroke with pressure on the edges as they overhang. This seems like it would make matters worse. Or, are to talking about mirror on top?

Thanks again,

Jeremy

It has been my understanding that the only lens caps worth having are made with depleated uranium (or was it enriched? Depleated might be too inexpensive for astronomical gear.) Either way, it makes it a real pain to balance your scope.

Here is a picture of the upper cage. The spider design is shamelessly stolen from a member of the local astro club and the secondary holder is from astrosystems. The stripes on the focuser board are the plys of the plywood. They are visible because I sanded the focuser board to match (somewhat) the curve of the sonotube.

I'm probably not the best person to answer this question, but there hasn't exactly been an outpouring of help here...

The idea is that washers can be used to very quicky hog out the initial shape of the mirror. To use this method, you need a mirror with the same size and focal ratio you are targeting (or alternatively the tool used to make the mirror.) Then you pour a plaster tool using the mirror as a mold. Now your tool has exactly the curve you want to generate, so you epoxy the washers to the surface of the tool and start grinding. The washers wear down much slower than the glass, so the initial hog-out goes very quick. I assume you can also use the washer tool for fine grinding, but I'm not sure. I also have no idea what the drawbacks are to this method.

Hope this helps.

Jeremy

I'm no expert, but I'd say if you are at f6.3 and you want to end up at f7, you should definitely start mixing in some TOT.

When you start parabolizing, the f ratio will change, but the total change in the radius of curvature is on the order of 1/32 of an inch, so this change is completely negligible.

Of greater concern, is the change in f ratio that can occur in the 120-220 grit range. On my current mirror (10 inch), I finished 60 grit at f6, but by the time I got to 320 grit I was down to f5.2 (I'm currently at f5.6-5.7 but that's a long story.) The other people in my class doing 10 inchers had similar results. I would guess that you will be at an even greater danger of this due to your undersized tool, so keep measuring your focal ratio until you get to 320 or so.

I haven't had a chance to try this EP out yet, so I don't know to what extent the particle will affect the views. In daylight, my clearest views of the particle are when my eye is about two inches from the lens, although it is still very obvious when I hold my eye at a more usual viewing distance.

I've tried tapping (and banging) the eyepiece against the palm of my hand, and this doesn't move the particle.

Lee, do you remember if the SW zoom was a easy or difficult to work with? I'm not sure how indicative this will be of my eyepiece, because I assume zooms are more difficult to work wi in general.

I won't do anything till I hear back from the seller, but right now I'm leaning toward a middle ground. I can unscrew the eyepiece from either end, and since a quarter turn at either end doesn't change the position of the particle, I can conclude that any lenses held in the endpieces are clean. Each endpiece holds at least one lens, (the bottom end looks like it holds more) so this rules out almost half the surfaces. I think I might just unscrew each end and chech the most accessible surfaces, and if they are clean, put it back together and forget about it.

Looks like a happy ending here. I unscrewed the barlow and gave it a blast of air and the particle moved. However, it was replaced by a new one. After checking that the lenses were secure, I inverted the EP and gave a couple more blasts of air, and everything is nice and clear now.

Thanks for the help everyone.

Jeremy