Posts Made By: Joplin Motisher-Chittenden

November 27, 2002 04:18 PM Forum: Equipment Talk

Planetary scope specs

Posted By Joplin Motisher-Chittenden

I've found that an 8" to 10" newtonian is my instrument of choice for planetary viewing. If I had super steady skies I might consider something larger. For a refractor, 6" seems to be the practical size/ price limitation - they start getting really large/ heavy and expensive after that.
The standard rule is the longer the focal ratio, the better. For newtonians, the longer focal ratio has the advantage of a smaller secondary mirror which reduces the effect of the central obstruction. For refractors a longer focal ratio means less chromatic abberation. For an achromatic doublet in larger sizes,
f-12 is generally considered the minimum. A longer focal ratio also has the advantage of being easier on eyepieces and not requiring the use of a barlow in many cases. For newtonians f-6 is a good starting point. For achromatic refractors 4" and smaller f-10 and above. For lager achromatic refractors f-12 and above unless a chromacorr is used.

Joplin

December 21, 2002 01:54 AM Forum: Equipment Talk

Using a 1.25" Barlow on a 2"/1.25" ep

Posted By Joplin Motisher-Chittenden

The big question is whether you will get vignetting of the image or not. I have an Orion Shorty Plus 3 element barlow (which, as far as I know is basically the same as the Ultima model) that vignettes the image of some 1.25" eyepieces. The interesting part is that the effect has a lot to do with the position of the eyepiece's field stop in comparison to its barrel, as well as the eyepiece focal length (longer focal lengths are more likely to vignette). Eyepieces with field stops inside the element groups will also have varying degrees of vignetting depending on the design. Really, the only way to know for sure is to try the combination to see what happens. Televue's powermate devices elliminate this problem but are expensive. If no one else has replied regarding this particular combination, I will try the combination the next time I observe with my fellow club member that has a 22 panoptic. My guess though would be that the image will be vignetted to some degree.

January 4, 2003 05:31 PM Forum: Equipment Talk

parks newtonians

Posted By Joplin Motisher-Chittenden

Jacques,

I have a Parks 10" F6 Newtonian that I got off of Astromart with an older Parks1.5" shaft mount (both date from the 80's) - I paid about half of what the new 10" F5 Superior OTA sells for by itself. I can honestly say that it has the best optics of any 10" scope I've ever looked through (haven't looked through a 10" APO though :-). I agree that the pricing on the new 8" F6 Precision is way out of line though - the only way to get Parks scopes at a reasonable price is used. For the best bang for your buck, I would try and find a used Meade 8" F6 equatorial in good condition for about 2.5 times less than the Parks scope. Believe it or not, the mounting is probably better on the Meade scope and as a bonus it actually includes a clock drive (which is crucial, in my opinion). With the loads of cash you would save you could get some great eyepices and still have more than enough money left over to have the primary mirror replaced/ refigured with one of exquisite quality.

Joplin

January 4, 2003 06:02 PM Forum: Deep Sky Observing

M1 and saturn time.

Posted By Joplin Motisher-Chittenden

It is tonight (pretty much all night for North America).
I wouldn't expect to see the Crab visually; even with narrowband filters the glare from Saturn will likely overpower it. Our club tried with variety of instruments ranging up to 15" in aperture and numerous eyepieces and filters, no one could confirm seeing M1. This was last night under lousy seeing conditions and lots of light pollution though. Some members had seen the Crab in the same field of view as Saturn easily a few days back though. We just have to remember that Saturn is 250,000 times brighter than M1.

Joplin

January 14, 2003 02:41 AM Forum: Equipment Talk

Need a 10-40mm Zoom - is there such a thing?

Posted By Joplin Motisher-Chittenden

Sal,

The closest zoom you will be able to find is probably the Vixen LV 8 - 24mm zoom (same as the Televue 8 -24 zoom but less expensive). If you need additional eye relief, then the Pentax XL (also about 8-24mm) would be an option, but it costs twice as much. One thing to remember is that with these zoom designs, the apparent field of view gets smaller at the longest focal length setting.

Joplin

January 14, 2003 08:08 PM Forum: TeleVue

TV 11 mm plossl

Posted By Joplin Motisher-Chittenden

Jason,

The TV 11mm plossl is a great eyepiece, I use one with my 10" F6 newt. I would call the eye relief adequate (not great but not terrible either). It definitely doesn't have enough eye relief for people who have to wear glasses. I can't quite see the whole field of view without rolling down the eyecup ( maybe I just have deep eye sockets, though :-) One note, it does have a little vignetting when used in conjuction with an Orion shorty 3 element barlow (which I just ignore since the image quaility is still great). When I use the eyepiece, I usually back off just a bit anyway, since the whole field of view usually isn't required for viewing the planets anyway.

Joplin

February 14, 2003 03:37 AM Forum: Equipment Talk

Newbie Seeking Advise Please

Posted By Joplin Motisher-Chittenden

Mike,

In my opinion one of the most capable scopes for the money is the Orion XT-8 and similar scopes made in the same factory and sold by other retailers (an example would be the Hands on Optics 8XL). I have used a couple of these at star parties and have been quite impressed with all of them. Since they are a Dobsonian design they are easy to use and point and the 8 inches of aperture offers a good balance of light gathering power and portability. The 2 inch focuser is a great feature as well, it allows you to get a wider field of view which can be very useful for finding objects and looking at large objects. The XT-8 is a bit out of your range at $450 plus shipping but I think it's worth it to spend a bit more for the 8" scope as opposed to a 6" scope. Taking photos can get pretty involved in terms of equipment but it can be done with a dobsonian and a tracking platform (really short exposures of the moon don't require a tracking platform).

Joplin

February 15, 2003 04:36 AM Forum: Equipment Talk

2" or 1.25" wide angle

Posted By Joplin Motisher-Chittenden

Paul,

You are correct, the true field will be the same for an eyepiece of the same specifications (FL, design, field stop, etc.) regardless of whether it has a 1.25" or 2" barrel. The 2 inch barrel is more convienient to use if you have a bunch of other 2 inch eyepieces and don't want to fiddle with adapters for 1.25" eyepieces in a 2" diagonal (you could also just leave an adapter on each 1.25" eyepiece). The only time a 2" eyepiece is really required is when you need a large AFOV and long focal length (ex: naglers 17mm and up).

Joplin

February 16, 2003 04:55 PM Forum: Eyepieces

Criterion eyepieces

Posted By Joplin Motisher-Chittenden

Joe,

I would judge the value of those eyepieces to be in the $25 to $30 range. A dedicated collector of the Criterion scopes (Dynascope RV series, etc) might be willing to pay a bit more.

Joplin

February 25, 2003 12:31 AM Forum: Equipment Talk

Questions about FOV and degrees viewed

Posted By Joplin Motisher-Chittenden

If you know the eyepiece's effective field stop diameter (most manufacturers don't publish it; Televue does), you can calculate the actual field of view. Here is the equasion :

57.3 X field stop diameter in mm divided by telescope focal length in mm.

Joplin