Posts Made By: Jim Mahon

Most big dob owners I know have fabricated an off-axis stopdown mask of some sort. Some nights it just pays to have it, no matter how good your mirror is.

I used black foamcore. Light, durable, and nonreflective. Cut it with a razor knife insert on a regular compass, and have a nice 6 inch unobstructed scope working at f 13.5 when I use it.

Jim Mahon

I do not have any first-hand experience with the EQ-6 - the impression I got from the Cloudy Nights review and the new review in S&T seemed to indicate that there were a few nits about perceived durability of accessories like the battery box and lack of a bubble level on the tripod, plus a few limitations which would affect only those doing imaging. Seems like a cost effective big GEM for visual work, though.

I've got a Nexstar 11 GPS and can report that I'm very happy with it for visual use. Setup/alignment is very simple and the go-to and tracking very accurate. I do a great deal of public astronomy under suburban skies and it is great to be able nail DSOs which would be tough for me to find in my dob because hop stars are lost in the skyglow. Not being a longtime SCT fan, I have been very pleasantly surprised by the optics in mine. Very sharp and holds collimation quite well. The upgraded motor controller software is great and tracking and slewing are much quieter than the LX-200s I've been around. The supplied tripod is adequate for visual use but would be too shaky for imaging with a wedge IMO.

Kim - I'm quite envious at your seeing that level of detail in the rings! Which instrument?


Clear Skies!

Jim (Mahon)

Pete, it's still functioning for me....

Jim

I recently made a simlar upgrade but wound up going from a CG-5 to a GM-8. While I think that the GP-DX is a fine mount, I do a lot of public astronomy and the clutch arrangement on the Vixen mounts was a big negative for me. I lost a motor on the CG-5 when somebody tried to "help" me by centering an object I'd intentionally offset in the field. The need to disengage the slip clutches on the GP was going to eventually cost me motors, either from another helper or my own inattention (I hadn't done it yet, but it seems like a setup for a fatigued user, and I pull some all-nighters.)

I thought the GM-8 clutches would be a pain but the dovetail can be slid fore and aft for quick balance adjustments on the C-8 - keep the safety knobs installed on the dovetail and you have a fairly idiot proof arrangement. The stability is extremely good, and I found that setting up clutch tension is much easier than I anticipated. Tracking is very smooth and accurate, and I really appreciate the four rates available since I do a lot of lunar and solar work.

I don't miss the slo-mos at all, which really surprised me. I can set the tension so that the scope can be slewed easily, yet stops where I leave it and picks up tracking. I do have large hands, which probably makes tension setting on the GM-8 easier. The optional fancy clutch knobs Losmandy sells have clearance issues on the GM-8, and I have mine only installed on the ascension axis. The dec knob extensions will rub on the ascension motor cover. I have no problem with the ascension knob, but I'm at latitude 34 and I understand that much above 45 is a problem for the fancy ascension knob.

It's heavier, but I never carry it far. I bought a big new Pelican case (1660, I think)with wheels, and it holds the mount head, the counterweights and shaft, the electronics boxes, my solar filters, and a Stellarvue 80mm I use for solar.

Just my 2 cents - not sure I'd switch if I already had a GP-DX and didn't do a bunch of public astronomy.

Clear Skies!

Jim Mahon

Sad news indeed! I'm selfishly glad that I have a fairly complete set of their visual filters. Who fills this market niche best now that the de facto standard is gone?

Opinions, anybody?

Jim Mahon

I witnessed a scary incident about 10 years ago which was a Finagle's Law example rather than an outright failure of a filter.

Several of us were doing solar observing and showing projected images in the midafternoon for a local elementary school at a science fair. A guy about 30 feet from me had a slip-on glass filter on the front of his refractor, which he was using for direct viewing while projecting through another scope. He was a few feet from his scope talking to a student when another kid walked by, and carelessly ran their hand along the scope tube as they passed. They managed to knock the filter off the objective end, and it hit the concrete and rolled merrily away on edge. The owner shouted a warning and took off after the filter - BUT WITHOUT MOVING THE SCOPE OFF OF THE SUN. I slapped my scope away from the the sun, tightened the axes down and started for the other scope, more to prevent anybody from looking into the eyepiece end than any other thought, but before I got there - maybe 10 to 15 seconds after the filter came off - a tiny crackling noise announced the fact that the guy had a cemented element eyepiece in the diagonal, and that it had been cracked by the suddenly expanding, outgassing cement vapor.

Lots of obvious cautions there.

Jim Mahon

Ron, I have a 10" f6 tube dob which I built myself, and it is the absolute upper limit for me on the sonotube designs - but I'm 6' 2" and the 8" f6 dobs are a tad too short for me. My 18" is a truss scope. I personally find that a 10 to 12.5 inch truss dob makes a wonderful traveling deep-sky rig and have several friends who are in love with these medium aperture truss instruments. That might be a better path for a future deep-sky investment for you. Around 12.5 inches you really get to the point where a wealth of detail is available in DSOs.

I'm also a TV refractor owner (a Genesis SDF) and certainly understand your love for your TV-102. Having my bigger scopes set up alongside my refractor gives me the best of both worlds!

Clear Skies!

Jim

I've owned 18" f4.5 and 10" f6 newtonians, both as it happens with rather good early Galaxy mirrors, for quite a few years now. In the f6, the University Konigs of both 32mm and 40mm focal length work pretty well - there's coma, but it's not really noticeable until you're out to about 85% of the way to the edge, and then it really is something you have to be thinking about. Really not annoying at all. My premium eyepieces like the Panoptics and Naglers are nice in this scope, but not necessary to obtain a pleasing view.

In the f4.5 scope, though, the coma is quite obtrusive and very annoying in the Konigs from about 2/3rd of the way to edge on out. Might as well not bother with these in the f 4.5 scope unless you have a paracorr IMO. I never have bought a Paracorr - I use either a 35mm Panoptic or a 31mm Nagler for wide field work in the f4.5, and the coma is well enough corrected to not be terribly obtrusive. The Nagler is better in this respect, but it ought to be for the money!

Only you can decide what your tolerance for aberrations at the field edge is, but most people can readily see the difference and definitely favor the premium eyepieces in the 18. I would urge you to try to test drive your candidates if possible in your f5 scope at a star party if at all possible. I personally think that you'll be happier with a design corrected for the faster scope. Such eyepieces are pricier, but IMO the satisfaction and beautiful wide field views are worth the investment. Most of us are in this hobby for the long haul, and when you consider how long you might be using a high quality eyepiece, the difference between $150 and $300 invested in an essential tool like your widest field eyepiece isn't really all that huge.

I've seen a couple of 35 Pans on sale in the last week for under $275. The Pentax 40mm is nice too, but would give you a rather oversized 8mm exit pupil in an f5 scope...

Clear Skies!

Jim

I can view it on either IE for OS X or Netscape 7.02 for Mac OS X. Just now opened it on Netscape and cruised several pages - no problem. (New i-Mac, 800mhz G4, 256MB RAM)

Jim Mahon