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Posts Made By: Paul Preto

February 24, 2007 12:41 PM Forum: Reflectors

Off-Axis Reflector Astigmatism

Posted By Paul Preto

I have a DGM 5.5, that I have had for over a year. Generally, it gives nice crisp contrasty views, which are quite bright for the 140mm optics. My problem is I have never gotten the superb planetary views I bought it for, probably due to the trace of astigmatism that I see, even after tedious star test collimation. I spoke to Dan McShane a couple times about this, and he says it is the collimation. He also recommends using simple less well corrected eyepiece designs during collimation. I've done this, but still see the astigmatism. Can misaligned optics cause astigmatism in this type of optical system?

Since the primary is a segment of a larger f/4.3 mirror (per Dan), it has a thick end and a thin end. Dan says you point the secondary at the center of this primary section, though intuitively it seems like you would point it at the thin edge, which used to be at the center of the parent primary. I'm thinking that maybe I should be pointing my secondary somewhere other than the very center of the primary segment. Could this be my problem?

Or, what would happen if my primary segment is rotated a degree or two in its cell, so that the light cone is not hitting the secondary with the optimal geometry? I'm now considering getting a Astrosystems secondary holder to replace the non-adjustable holder that came with the scope so I can tweek the secondary during collimation (now you have to tilt the focuser to aim the laser beam). Before I waste my money, I'd like to get some feedback on my ideas and whether or not this could be a collimation issue.

March 21, 2007 02:59 PM Forum: Deep Sky Observing

Messier Marathon - the Agony of the Realm

Posted By Paul Preto

After last Sat and my 6th attempt at the Messier marathon, I've concluded that I just have to get something better for the Realm. Mind you, I'm a pretty good star hopper. I reached 109 in a marathon a few years back, and usually get over 100. But the method I'm using for the Realm (an old S&T article that star hops you through Virgo) failed me the past two times (probably because I didn't re-read it completely or practice with it beforehand). As a result I ended up spending way too much time in the Realm, to the point that I skipped some and moved on.

Anyhow, I'm wondering what other people use for the Realm. The thing for me, is I want absolute confirmation that what I'm looking at is the target M-object and not a nearby NGC. I know some people use a very wide field eyepiece, get to the right field, and then just take credit for all the M's in that field without picking out each one individually. That is not valid in my mind. I have pictures that help to some degree. But what I'm thinking would be great is a series of one degree circular charts to match the eyepiece view. These should overlap (very important) and move progressively through the Realm, with all objects accurately labelled of course. Does anybody know of such a chart set?

April 25, 2007 01:26 PM Forum: Telescope Making

Best 16" mirror for $2K?

Posted By Paul Preto

Has anyone else heard that Ed Stevens is taking a sabbatical? I have it from a good source that he will not be taking any new orders. So if you can't get a Zambuto for whatever reason, who would you pick for a 16" f/4.5? I'm leaning toward Pegasus or Swayze, in part based on price.

May 2, 2007 11:39 AM Forum: Telescope Making

Secondary - 3.1 or 3.5?

Posted By Paul Preto

For a 16" f/4.3, who votes for a 3.1" secondary (19% obstruction) and who votes for a 3.5" (21.9%)? For a f/4.5 scope, most agree that it should be a 3.1". But at f/4.3 my calculation shows that producing only about .45 deg fully illuminated. I should add that I spend 90% of my time deep sky observing.

May 11, 2007 11:32 AM Forum: Equipment Talk

Argo Navis vs. Sky Commander

Posted By Paul Preto

I hope this doesn't bring back the discussion of "greying" (or is it "graying"?). It may be a gray area. Anyhow, has anyone used both Sky Commander and Argo Navis? It seems Argo Navis would be the winner, but it costs almost twice as much for the whole package. So my quesiton is what are you getting for that much more? Which is easier to use? Also, for people who need reading glasses like me, can one be read more easily than the other?

May 30, 2007 10:12 AM Forum: AstroMart FAQ

Demise of the Marketplace?

Posted By Paul Preto

It just occurred to me that since membership is no longer free, we will have a diminishing buyer (and seller) pool. I tested it, and you cannot send a reply to a seller without being a member. I realize this protects sellers (and buyers) by insuring that members are checked out, but the reality never occurred to me before. Seems that since fewer people will sign up for a fee and older folks will slowly drop out of the hobby for obvious reasons, we will be left with a smaller and smaller buyer/seller pool, which of course means fewer sales. How can that be good for Astromart as a used equipment marketplace?

May 31, 2007 12:42 PM Forum: Telescope Making

Three Pole Lightweight Truss Designs

Posted By Paul Preto

Does anyone have experience or factual information regarding the Plettstone or Dobstuff truss designs? I can see how these designs would be lighter than a regular six pole truss-tube with a UTA, but I'm wondering if there are any major drawbacks. For one, it appears that your optics are much more exposed. I'm wondering about flexure, ease of setup/breakdown, or any other issues you may know about.

June 6, 2007 10:44 AM Forum: Reflectors

Blug Accuracy?

Posted By Paul Preto

Has anyone used the Howie Glatter Blug? I'm wondering how the collimation accuracy compares to the Catseye cheshire and autocollimator procedure for both accuracy and ease of use.

July 6, 2007 04:20 PM Forum: Equipment Talk

Re: Super Polaris - screw size needed

Posted By Paul Preto

I have a SP mount but have no idea what size the screws are. I would just pull one off the RA axis and take it to the hardware store with me. Also, wondering where you found that dec bracket?

July 6, 2007 05:13 PM Forum: Telescope Making

Sling or Cable?

Posted By Paul Preto

I'm reposting my question here, as it was getting limited response in the Dob forum:

For all you truss-tube dob owners, who uses a sling and who uses a cable to support their primary? For those using a sling, have you had any problems with sling induced astigmatism? What size is your primary? Is a sling adequate for mirrors below a certain size? Any other comments on this subject?