Posts Made By: Michael Connelley

Hello:
I found the comet Satuday night from 9,300ft.on Mauna Kea with an 11", but it took a while and wasn't trivial. The comet appears to have returned to its pre-impact state. Less than an hour after the impact, the comet looked more or less as it did pre-impact (i.e. faint fuzzy ball) but appeared to have a 11-12th magnitude stellar core. I did not see that Saturday. I also looked at the Bug Nebula, and found it much easier to locate than the comet.

Cheers
Mike Connelley

Hello:
Without looking at some planetarium software, I couldn't say for sure, but I would doubt it. The moons of Mars are very faint and very close to Mars. The closer moon orbits only a few times the radius of Mars above its surface. I have seen one of the moons of Mars once, and that was with a 88" telescope. I suspect that what you saw was a background star. This has happened to me a few times. I remeber once I pointed my scope at Jupiter and it looked like it had 5 moons! Clearly one was a background star. It was quite handly to have the star there to compare the sizes of the moons to the point-like star.

Cheers
Mike Connelley

Hello:
In my opinion,you should not need to drill any new holes. You should be able to shim the focuser so that the focuser is pointing at your secondary. Your focuser doesn't *need* to be square to the tube for the scope to work well. I believe, and I may be incorrect on this, that laser collimators require that the light path make a right angle on the secondary. If so, then shiming the focuser would negate the usefulness of a laser collimator, but you would still be able use a regular collimator like a Cheshire eyepeice. When I built my dobsonian, I basically paid no attention to the angle of the focuser relative to the tube axis. I just made sure it was about the same place along the tube as the secondary and tilted the focuser to point at the secondary, and all is well. For an offset as small as yours, it should be possible to collimate the scope by tilting the secondary until the center dot on the primary is aligned with the cross hairs on a Cheshire collimator, but there may be some light lost. It's probably easier to just shim the focuser, then you don't have to worry about losing light.

Cheers
Mike Connelley

Hello:
I couldn't find anything about a Baker RC on the web, so it appears to be a name Takahashi concocted on thier own. I read that the Pan-STARRS telescope is a modified Paul-Baker or Mersenne-Schmidt design. The Paul-Baker uses three mirrors, whereas the Pan-STARRS telescope uses two mirrors and a corrector lens assembly. I don't know if the Pan-STARRS telescopes start with a RC design or not, but it does make sense. Email Klaus, since I think he has done a lot of the Zemax analysis of the Pan-STARRS optical design. I did some reading in the IfA library about field flatteners and correctors, since I was interested (for a little while) in helping Nick with the optical design. This was several years ago, so I don't remember the references, but a search on the ADS should bring them up.
I have found some info on flat field RCs. Apparently, if the primary and secondary have the same radius of curvature, the focal plane will be flat. This has the disadvantage that the secondary mirror will be very large, typically near 50%. The Sloan telescope is of this design. In this case, the dominant residual aberration is astigmatism, which may be small enough to be ignored.

Cheers
Mike

P.S. I am not sure there will be room in my office for a 10" BRC! The 12.5" cassegrain is taking a lot of space. First light should be soon....

Hello:
Depending on the size of the mirror in question, you may be able to wrap the mirror in kimwipes or large lens tissues. I recall hearing that kimwipes aren't so good for optical surfaces, so I think lens tissues are preferable. My 12.5" mirror from Discovery Telescopes arrived this way. We recently received a 16" mirror. A thick cotton pad was secured to a cardboard disk, and this was placed cotton side down to protect the mirror surface. The mirror plus cardboard were in a foam lined box. I think you could get the same protection by gluing a bunch of cotton balls to a piece of cardboard. Of course, you could just mummy wrap your mirror in TP.

Cheers
Mike Connelley

Hello:
I think it should work. The stresses are distributed over 8 poles so the force at each joint wouldn't be that much. A good epoxy (I have JB Weld in mind) should easily be able to handle this job. I'd keep the overlap region longer than the tube OD for added stability.

Cheers
Mike Connelley

Hello:
How about just a good Plossl and a barlow lens? I found I could see better detail in a Meade 15mm Plossl than in my 17mm type4 Nagler on the Moon and Saturn. I suppose it has to do with there being fewer air to glass surfaces and less glass in a Plossl. For high power viewing, contrast is often more important than field of view. You probably got a refractor rather than a 4" newtonian for the contrast, so why not get the most out of it? There's a review of planetary eyepieces on Cloudynights, and the wide field types (I don't remember if it was a Radian or a Nagler) didn't even come close to beating a good plossl. For as much money as one Nagler or Radian, you could probably get 2 plossls and a barlow yielding 4 different magnifications.

Cheers
Mike Connelly

Hello:
No, it wouldn't make a difference. You'd lose a little light at the edge of a wide field of view, but nothing that's noticable. You'd probably lose more light off of the edge of the secondary mirror than you would off of the front. For visual use, a loss of up to 50% may not be objectionable near the edge of the field. Even for phototographic purposes, this sort of thing can be calibrated out.

Cheers
Mike Connelley

Hello:
I should have mentioned that, as I envision it, the Newtonian secondary mirror would be kept to place the focus outside the tube where it can be accessible, and so that a conventional focuser can be used. This would require adding a spider since the corrector plate can no longer support the secondary. Field curvature is the main hurdle to making this idea practical. Since the field curvature depends only on the focal length of the primary mirror, one could reduce the field curvature by using a longer focus mirror. Unfortunately, this leads to a rather long focus system. For example, an 8" schmidt camera could go without a field flattener (or bending the film) if the focal length is more than 2.5m. In such a case, the curvature of the field would increase the size of a star 1" fron the center of the field by 10 microns. Thus, the tube would be 5m long and the camera would have a f/12.5 speed. At this point, the advantages of a schmidt system are lost.

Cheers
Mike Connelley

Hello;
There are several such telescopes, though they are more commonly known as segmented mirror telescopes. Telescopes like Keck, the Hobby-Ebery Telescope, the clone of Keck being built in the Canary Islands, all have segmented primary mirrors. When properly phased, such a mirror acts much like one large mirror, i.e. the theoretical resolution is as large as if the segmented mirror was a single mirror. This requires that the alignment between the segments be to optical tolerances, i.e. a fraction of a wave. One main drawback of such mirrors, at least for infrared astronomy, is that the emissivity of the mirror is much higher than for a monolithic mirror due to the gaps between the segments. The Large Biinocular Telescope will be an imaging interferometer, i.e. it'll combine the two beams from the two telescopes on the focal plane to get the angular resolution of a larger telescope in one direction at a time.
As far as I know, no amateur has attempted a segmented mirror. For telescopes like Keck, each segment is part of an off axis hyperboloid, and there are 6 different mirror figures among the 36 segments. For HET, the segments are all spherical making a large spherical segmented mirror (with the ensuing spherical aberration). Making a pair of off axis paraboloids isn't easy. For Keck, they used stressed polishing to get the proper figure. An then there's the alignment issue. The mirror support structure must be quite stiff. In the case of Keck, the actuators push on the segments one a second or so, and they need to phase the mirror at least once a night, To do this they have a wavefront sensor and some sophisticated software to automatically align the segments to eachother.
To do this manually, in an amateur telescope, would be challenging to say the least. Typical collimation is much too crude; you'd need much finer control of the mirror alignments. As mentioned above, making several off axis paraboloids isn't easy. It's just as easy to make one large paraboloid in a thin mirror, since off axis paraboloids are typically cut out of such a mirror. One could use several spherical mirrors to make effectively make a larger spherical mirror, but then you'd have to contend with a whole lot os spherical aberration. Even if it could work on paper, it would be impractical as an amateur telescope.

Cheers
Mike Connelley