Posts Made By: Ian Van Voorst

I noticed PFA film (.001" thickness) is available from McMaster Carr for a low price per foot. Is there any reason why this couldn't be used to make a dob bearing pad?

The only problem I can think of is that it might be difficult to attach the film to anything -- I don't know if any adhesive would work on the PFA and a mechanical attachment might be slightly tricky as I don't know how strong the film is.

--Ian

Thanks for the update. It seems that every ortho that comes out is assumed to be from the same factory in Japan. Perhaps this is not true, or the orthos from the same factory are not of all the same quality.

I recently wrote a post "UO Orthos Overrated?". I had begun collecting Orion orthos because they were said to be the same as the UO orthos but seemed to be parfocal. Up till know I have had a set of very inexpensive EPs collected while I was in high school and had no money. The Orion Ortho was the best of the set. However, I began to have doubts when a Celstron SMA with a barlow outperformed my ortho. I then read the April 96 issue of S&T where the Orion orthos were ranked last among several non-widefield EPs! So I began to wonder, are UO orthos overrated, or are the Orion orthos not the same. After reading the replies and other review, I think there is no way that Orion orthos are of the same quality -- even if they come fromt the same factory.

One other thought: You did not test the orthos in mono mode. Is it possible that either Antares ortho is fine on its own, but somehow mismatched (I don't know much about bino viewing) OR one of them is a lemon (it happens). Of course, if both are lemons, then there is a problem.

OH OH OH... one more question...

What about dew? Does this arrangement make the mirror more susceptible to dew? Does the movement of the air actually help prevent dew? I guess it's comes down to a big compromise between the ideal from the perspective of dew: a temperature just above the ambient temperature (I think that's simplifying it -- whatever), and the ideal from a thermal management perspective: a mirror at the ambient temperature. Of course if your primary dews over you can't see anything anyway... What about using a dew prevention system WITH the fan?

OK... I need to finish my scope soon, so I can stop obsessing about details and actually observe LOL!

Thanks for the info. How much refocusing is needed? Does it require a lot of in-travel (compared to, say, a short focal length plossl)?

thanks,

Ian

I guess I should have said that w70s are eyepieces costing about 50-60 bucks with a 70 degree AFOV. Helix has em, among others.

Ian V. V.

Here is a link to Dave Trott's website. Half way down the page are some refractors with a barn-door drives combined with a dobsonian type of mounts.

http://hometown.aol.com/davetrott/page6.htm

What I would like is to have a mount like a Unistar on top and a little motor on the tangent arm.

Still haven't found examples of the first mount I described.

--Ian

some thoughts:

If you have any sort of table, or platform with a view of the ecliptic, or if you could place one in your backyard, you would have the ultimate quick setup for the Nexstar or the ETX. You could just carry the scope out with out tripod and set it down -- no leveling or aligning with N (obviously you would still need to run the scopes alignment procedure).

I think most would agree quality control on the Intes scopes is better than the others. For planetary observing at this aperature, with this much obstruction, you must have all optical quality you can get. BUT the 6" on a GEM won't be THAT much smaller than the SCT.

--Ian

Great! -- thanks for the closeup -- really nice looking scope.

Don't be suprised if you see a few of using those connectors on our next project.

--Ian

I do enjoy building scopes, but I actually originally chose to make my scope for economic reasons. When I built my first scope in the early nineties, there weren't as many good, low-cost options for small/mid-sized newtonians. I wanted an 8 or 10 inch newt of known quality for planetary observing and that was not that easy to come by then. So it was very cost effective to buy a truly excellent mirror and build a structure for it.

My particular philosophy about scopes is that I wanted best optics possible, having a fancy focuser, or massive eq mount could come later. If I have the great mirror, I can spend time fixing up the rest. It is true that there are great values out there, building my own allowed me to spend 80% of my money on the optics. All scopes involve compromises. When you build your own, you get to decide what's important -- you get exactly what you want, which in the end can save you money.

There are now Teleports and Portaballs that have great optics, but they are really out of my price range. Compared to a Discovery/Orion/Hardin dob, my scope is not that cost effective. Compared to a 7" teleport -- a scope that is similar to mine in optical quality and portability -- my scope is an amazing bargain.

On another note, I think many people have NO IDEA how amazing a really good 8-10 inch newt can be if the mirror is really good, the obstruction is relatively low, temperature is managed, and everything is collimated. To few scopes actually satisfy those criteria.

--Ian

A Newtonian of the exact same aperture as a mak CAN perform as-well-as or better-than the mak provided it is collimated, tube currents are not a problem, and the planet is kept in the coma-free field of view (which may not be the case with a dob).

There is a weird mystique about Maks and Mak Newts that leads to an expectation that they can outperform larger Newtonian. The main advantages to the Mak are the lack of diffraction spikes, larger, flatter, coma-free field and possibly a sealed tube which may have an advantage in terms of tube currents once the scope is cooled. The non-optical advantage is that the tube is much shorter than the same-aperture newt.

I think when people look through a Mak and see those clean planetary disks without diffraction spikes, it creates a first impression that slants their overall opinion of the scope even if a newt shoes more detail in the disk.

That being said, the only comparison I have done side-by-side was with my 8" F6 Newt and a 4" Mak which isn't such a good comparison because of the aperture difference -- also, the Newt had better optics. What was sort of interesting was that the Mak was more sensitive to the seeing even though the aperture was smaller. I later found an explanation of this on the Internet somewhere -- due to the large central obstruction in the Mak more light is placed in the first ring of the diffraction pattern -- the seeing can cause the first ring to merge with the airy disk.

The 8" F6 and it can give planetary views that will knock your socks off and have spoiled me from using anything smaller. I also am so satisfied with the planetary views at this aperture that I don't really feel the need for anything larger.

-- Ian