Posts Made By: Michael Connelley

Hello:
Let's say you have a 3" high focuser and 1" between the tube and the mirror, which gives 9 inches from the optical axis to the focal plane. In order to get all of the light in the center of the field of view, you need at least a 1.8" secondary. Using one so small isn't such a good idea since you'll loose quite a bit of light at the edge of your field of view, and the edge of the mirror probably won't be very good. A 2.14" secondary will give you a fully illuminated fied of around 0.3", which is okay but kind of small in my book, especially since you've shelled out the big $ to see a 2" focal plane. So, a 2.6" secondary isn't too big if you're interested in wide field deep sky faint fuzzy work. If you're interested in mainly planetary viewing, then something smaller would be better. In the end, I doubt that there's any observable difference between a 2.1" and 2.6" secondary.

Cheers
Mike Connelley

Hello:
During tonight's star party, the seeing was quite good, so I decided to try to split Antares with my 12.5" cassegrain. The scope did this without too much trouble. While observing Antares at about 375x, I noticed what appeared to be a nebula near Antares. I have split Antares before, and have not noticed this. It was about 1/4 of a field of view away from Antares, which would be 2' to the NNW. It's shape was a bit like a comet, with a "head" to the south and a short faint "tail" to the north bending to the west, the whole this being about 0.5' long. It would say that it looked about as big and bright as a 10th magnitude galaxy. I observed this object in a 31mm Nagler, and a 17mm and 12.5mm plossls. It remained the same distance from Antares as I moved it around the field of view, and it also remained when I put Antares just outside the field of view, so I highly doubt it's a ghost. Also, the ghosts I saw from Antares were point like, not diffuse. It is was also not M4 or the other, closer NGC globular, as they are much father away, and I did move the telescope to those targets later on just to be sure. There's nothing at that location on SkyAtlas2000, there are no objects listed in SIMBAD, and there are no currently visible comets in the area. This was observed on May 28 at around 11h UT, which was around 1am local time. Thus I doubt it could have been some satellite's fuel dump unless it was in a very high orbit (and I also didn't notice it moving relative to Antares while I was observing it). So, does anyone have a clue as to what I saw?

Cheers
Mike Connelley

Hello:
The subject line pretty much says it. What's the thickness of the old Coulter 17.5" mirrors? I've been tossing around the idea of using voice coil actuators to push/pull a cheap old mirror into a good figure. If it's thin enough, one of those older Coulter mirrors would be a good test subject.

Cheers
Mike Connelley

Hello:
Just before sunset tonight, I saw Jupiter with my naked eye. The moon wasn't too far off, which helped me to locate it. After I found it, several other people could see it. The skies were crystal clear. I am at 13,700ft. elevation. which probably helped darken the skies before the sun was down.

Cheers
Mike Connelley

Hello:
Last night a few of us were observing Jupiter (among other things) from the 9,300 ft. level of Mauna Kea with a 12.5" cassegrain. Towards midnight the seening, and the scope, had settled down and we got quite a nice view of the GRS. I believe that I could get a glimps of Red Jr. during moments of good seeing using about 400x. There was a thickening of the brown belt trailing the GRS and to the south of it, and it looked like there was a pale tan/orange spot at the head of this thickening of the band. Has anyone else seen Red Jr. in the eyepiece?
The seeing was good enough to identify the 4 Galilean satellites by appearance (size and color). At 800x, Ganymede was a clear, and somewhat large, disk. If the seeing was a bit more steady, we may have had a shot at seeing surface details on it.

Cheers
Mike Connelley

Hello:
Last night a number of us observed the NEA starting around 11:40 pm HST (9:40 UT) from 9,300 ft on Mauna Kea. It was quite easy to see in a 12.5", but it couldn't be found in a 80mm refractor. At the time, it was about 20 degrees above the horizon. The speed with which it was moving was quite remarkable. When I was trying to find it, if I couldn't find it after looking around its field, I had to check the S&T chart to see where it should be now, and then look there. Eventually, I found a bright star it was going to pass in a few minutes and waited. Right on que, it came by and after that I was able to follow the asteroid. You know how, when you have a dobsonian at a star party, you need to recenter the target after each person looks? That's what I had to do, even though the scope was tracking! Its motion was quite apparent, especially when it passed by another star, which frequently happened. It was expected to get as bright as 11th magnitude, and I'd say that it looked a bit fainter than that. Given that we saw it after closest approach, and at high airmass, it makes sense. I don't know how long we were following it, but by time we moved on to other targets, the scope had moved about 2 degrees from where I first saw it. Overall, about half a dozen of us saw the asteroid moving. Having an equatorial mount with motor control of both axes was definately a big help in following the asteroid. I just had to offset the scope to the NW, then figure out which faint "star" in the field was moving again, for the next person to take a look. Later on, we determined that the limiting magnitudes of the 12.5" was around 15.3, and for the 80mm around 13.5 near zenith. We're not quite sure why it wasn't found in the refractor. At such low elevations, there is more extinction and the sky is brighter, which would make it harder to find. Also, I believe the refractor was using lower magnification than I was with the 12.5".

Did anyone else attempt to see this NEA? Any success?

Cheers
Mike Connelley

Hello:
Here's a pretty picture of a nebula that we took two nights ago. Who can guess what object this is? The exposure times are 20 minutes in B, V, and R.

Cheers
Mike Connelley

Hello:
I recently finished the mirror cell for a 17.5" mirror, so I though I'd share what I came up with. It deviates from the standard Barry & Krieg mirror cell in a few ways. It has a triangular frame (an idea I shamelessly stole from another scope), where the collimation bolts will run though the corners of the triangle (haven't drilled those holes yet). Each of the triangles is on a see-saw bar, as I've seen done on a 6 point cell. One thing I didn't like about the Barry & Krieg cell design is how the bolts that hold the triangles stick through the triangle, and how the triangles sit on the flat top of a nut. I solved this by epoxying a machine screw to the bottom of the triangle, thus solving the problem of having the bolt pass through the triangle. The triangle pivots on the conical bottom of the machine screw head. The bolt passes through the see-saw bar to a lock nut on the other side. This lock nut isn't strictly necessary as the weight of the mirror will keep the triangles in place, but it's nice to know they're there. The hole in the see-saw bar on the far side of the triangle is oversized to allow the bolt to pivot about the upper hole. It was laid out by PLOP, then put together with nuts, bolts, and JB Weld, so it was quite quick and easy. Hopefully it'll work out well. Next: the top end ring. I welcome any comments or questions.

Cheers
Mike Connelley

Hello:

I am building a 17.5" truss tube telescope that will have a single top end ring. The focuser will be light, but the eyepieces won't, so the focuser board will have to be well supported. I plan on doing this by attaching the focuser board to the truss poles as well as the top end ring. Imagine a regular 8 pole truss with a top end ring. The focuser will sit just below the tips of the poles and look through the gap between the poles. This way, most of the weight of the focuser/eyepiece is carried by the poles, with the ring providing additional support. The job of the top end ring is mainly to tie the upper tips of the pairs of poles together.

My questions are: how wide must the top end ring be if it's 3/4" thick given this design and good plywood is used? I imagine it can be narrower than in a design where the focuser hangs off of the top end ring, but I have no experience with either design. Also, how does drilling light-weighting holes in the ring affect its stiffness? If I decide to use a relatively wide (2") ring, I'll be tempted to light-weight it.

Cheers
Mike Connelley

Hello:
When looking at a ronchigram, are the dark (or bright) bands 1 wave apart? It's tempting to interpret the ronchigram like a interferogram, and qualitatively you sort of can as I understand it. But is the ronchigram quantitatively like a interferogram? Or does the spacing between dark bands (in waves of error) dependent on the lines/mm of the grating?

Cheers
Mike Connelley