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Posts Made By: Joplin Motisher-Chittenden

July 7, 2002 02:49 AM Forum: Equipment Talk

Need help picking EPs for new scope

Posted By Joplin Motisher-Chittenden

Jim - The focal length of your scope is quite important when choosing eyepieces. Your scope has a focal length of 640mm and to determine the maginfication you will get with an eyepiece divide the scope f.l. by the eyepiece f.l. ( eg, 640 / 16mm = 40 x ). The practical limit of magnification for most objects with a given scope is 50 to 60 times the aperture in inches. In a suburban location like yours, the lowest useful power on the Borg scope would be about 20x (32mm eyepiece).
University optics eyepieces are generally accepted to offer the best "bang for the buck" so I'd suggest starting with them. I would go with a U.O. 16mm Konig, 12.5mm ortho, 9mm ortho, 7mm ortho and a televue 2.5x powermate. That would give you a sequence of powers from 40x to 228x with no major gaps. I suggested the powermate since it is generally accepted that it is a superior design to conventional barlows. If you need to wear glasses while observing or prefer a lot of eyerelief, the Orion ED-2 series would probably be your best option. Also, if you don't already have one, it is a good idea to invest in a quality 2" mirror diagonal. Here is a link to a good explanation of magnification:
If you can, get out to a local star party and look through various eyepieces to see what you like best. Sometimes, if you are polite and offer views through your scope in return you may be able to borrow eyepieces to see how well they work.

Clear Skies,


July 13, 2002 04:57 PM Forum: Deep Sky Observing


Posted By Joplin Motisher-Chittenden

Richard, I hope this map helps, I produced it for local midnight. Arcturus isn't on it but would be just off the page to the left.

July 17, 2002 05:36 AM Forum: Telescope Making

F4 Optics

Posted By Joplin Motisher-Chittenden

The practical advantages I can think of are: wider maximum field of view, lighter weight, and smaller size.
The wider maximum field of view becomes less useful depending on how much light pollution you have.The disadvantages would be less contrast (due to larger secondary), more coma at the edge of the field of view, and harder to collimate. If you are considering a schmidt newtonian, that design at F4 is supposed to limit coma to the equivalent of an F8, but the high power performance still won't be as good as an F8 newtonian.
If you are considering newtonians 8 inches in aperture or less, I'd aim for at least F6 or F7. With apertures 10 inches and up, F ratios usually get smaller in order to allow use of the scope without a ladder.
Of course, portability will be more important to you if you need to fit the scope in a small car.


July 21, 2002 04:41 PM Forum: Telescope Making

8" F5 40"fl Perkin Elmer Lens - Need info

Posted By Joplin Motisher-Chittenden

You could collimate it pretty easily using a holographic laser collimator with concentric rings, there are also refractor collimating eyepieces made for the purpose.
Are you going to make full use of the huge sharp photographic field that thing has with a four inch or larger focuser? I'd be temped to find an old metal view camera with rack and pinyon focusing and attach that to it some how. How much does that whole assembly weigh ?

July 21, 2002 05:18 PM Forum: Deep Sky Observing

Satellites Crossing Face of Moon

Posted By Joplin Motisher-Chittenden

I've never spotted a a satellite transiting the moon's surface but I have seen aircraft and birds doing it and they all looked pretty sharp even though they moved past rapidly. I would guess that what you saw was a satellite given its shape and speed.


July 25, 2002 01:24 AM Forum: Equipment Talk

Mount for refractor??

Posted By Joplin Motisher-Chittenden


I have an older Vixen Polaris mount with a single axis drive that I use with my 80mm F8 refractor (about 7lbs with diagonal and finder). I find it easily handles the scope visually, even in moderately windy conditions. It is also good for piggy back photography but it's marginal for prime focus or eyepiece profection work. It is very portable and lightweight, though. The build quality is also much better than the current chinese mounts. For prime focus or eyepiece projection work I suggest a CG-5 (re-lubed and de-burred according to the instructions at with dual axis drives as a bare minimum. A Vixen Super Polaris with the dual axis drives would be even better if you can find one used at a good price. In my opinion, once you add the drives, the current Vixen Great Polaris mounts are overpriced. Most of the current import mounts will benefit greatly from a leg upgrade. When I'm using the Polaris mount for photography I use a huge set of heavy wooden legs under it.


July 25, 2002 10:16 PM Forum: Equipment Talk

which travel size scope to get?

Posted By Joplin Motisher-Chittenden

One possibility would be one of the closeout Meade ETX-90 RA's (lots of em on astromart in the less than $175 range in great condition also). That would be better on the planets than the short tube scopes since it doesn't have the false color problem and it has a drive built in. If you are more interested in wide views, the one of the short tube scopes would be good. I would consider getting the ETX-90 RA and a decent pair of 10 x 50 or similar binoculars. The ETX-90 is reputed to have better optics than the Apex 90.


July 27, 2002 02:13 AM Forum: Celestron

SCT Fork vs. EQ mounts

Posted By Joplin Motisher-Chittenden

One these should do the trick:

Fork mounts do have quirks too. It can be a pain to polar align one and certain diagonal / eyepiece combinations may hit the fork (particularly if you use an add - on focuser). While the eyepiece moves around a lot less, it is harder to use the telescope when it is aimed near polaris.

The other option would be a Giro 2 deluxe alt - azimuth mount which you could add the Tech 2000 dob driver system to. This system would behave the most like a dob and it is also the most portable by far. Here are some useful links to info about the Giro mounts:

A Giro mount with the C9.25 and a rich field refractor on either side would be a sweet combo :-). Since the motions on the Giro are so smooth anyway I would probably just get the mount and a sturdy tripod for a fairly inexpensive and portable setup.

I would try to check out one of the fork mounts at some star parties, etc. Also, once you get used to an EQ mount, they are pretty intuitive and actually offer pretty good access to all parts of the sky.


July 27, 2002 06:11 AM Forum: Equipment Talk

Polarizing vs. Neutral Density filter

Posted By Joplin Motisher-Chittenden

A variable polarizing filter is useful for viewing a number of very bright objects such as the Moon, Venus, and Jupiter with larger scopes. Usually the transmission varies from about 3 to 40%. It is different than a polarizing filter sold for photographic use (the light transmission of the photographic version hardly varies at all).Either type will reduce reflections from most objects and increase color saturation.
A neutral density (or "moon filter") is cheaper but is more limited in its usefulness as the transmission is usually a constant 13% or so, limiting use to the moon.


July 27, 2002 02:54 PM Forum: Eyepieces

Orion Explorer II EPs

Posted By Joplin Motisher-Chittenden

Here is an option that is almost as cheap and they're all plossl designs (some are even multi-coated!) :

Any plossl / orthoscopic / kellner eyepiece with a focal length less than 9mm is getting into the short eye relief range. I would consider a quality barlow instead of the shortest focal length eyepiece.