Celestron C90 eyepiece advice needed (1 Viewer)

I just got a C90 (older style, black metal body, front plastic focus ring, no special coatings, in quite good shape) for $35.00 at a rummage sale. It has an Olympus microscope eyepiece (!!, labeled "GWH10x 23", and uses a spacer sleeve to fit well into the mirror diagonal) that gives quite good images, as good or better than good, regular, non-mirror spotting scopes I have looked through (but not as good as a Questar!).

As the mirror diagonal has an inside diameter of 1.25", I am guessing it would be compatible with any eyepiece of this diameter.

I am looking for a good fixed power eyepiece for terrestrial use that would yield somewhere around 25x to 45x, but as the scope is not the coated model, and I only plan on using it for informal astronomy with my young daughters, don't want to get an eyepiece beyond the capability of the mirror.

Any recommendations? Is the eyepiece I am using not to be appreciably improved upon by a different one? Thanks!

The Olympus is probably a very nice eyepiece. A "10x" microscope eyepiece has a 25mm focal length, so it's producing about 48x on the C90. A very low magnification (much below 40x) on this scope runs the risk of making the central obstruction shadow visible, especially in daylight when the eye is stopped down. A second eyepiece for high magnification (100-150X) might make more sense. The TMB Planetary series is inexpensive and quite good.

Thanks, Henry. Given that the resolution was so good out to the edges with the Olympus eyepiece, I was was wondering that I might not be able to do much better, at least not without a big outlay. Researching that model eyepiece, it is used with high quality Olympus stereo microscopes (and maybe others).

I have a Wild M5 stereo microscope with 10x eyepieces, and wonder how often people use their stereo microscope eyepieces for their telescope or spotting scope.

Apparently, the previous owner knew what they were doing using that eyepiece. Given that it doesn't make sense going less than about 40x with a C90, I might consider getting a more powerful ep. Thanks for the TMB Planetary series recommendation.

An inexpensive stick-on red dot finder (battery operated) came with the scope, so if I put in a new battery, I might be able to find the object faster, or my skill will improve.

Hello and welcome to BF. I have an Meade ETX 90 RA and have used it at 39x and above during the day. Not a wide field scope but the views are great and has close focus for looking at bugs etc about 10 ft. away. I do have a 60mm Nikon Fieldscope to use as well.
Regards,Steve

Just get a 40mm or 32mm Plossl eyepiece...good, cheap, and the mag you are looking for. That's what I use on my 90mm Mak.

Hi.

I am interested in the Celestron C 50 as inexpensive, lightweight travel scope. Looking for a wideangle eyepiece for the scope (c. 30x). How do you calculate the magnification when you only know the focal lenght of the eyepiece? Are there any recommended eyepieces (concerning quality)?

Does anyone have the comparison to Nikon's ED 50?

Kilian

Hi Kilian, The Celestron C50 has focal length of 580mm if you use a 10mm eyepiece you divide 10 into 580 = 58X I would think you would not want to go any higher than 20mm eyepiece would give you 29x. As far as the comparison to the Nikon 50ED I am sure that the Nikon would be a lot better and lighter at 16 oz. I don't know about good cheap wide angle eyepieces. Most of them would cost more than the scope.
Regards,Steve

The Celestron C50 Mini-mak works well with a 25mm plossl. This is based on brief in-store testing of this scope.

Rmel66.

Kilian:

The C50 not in the same class as compact 50mm ED scope (or the C90).

The tube in the Celestron C50 is the same as the Barska 50mm scope though they shipped with different EPs (the Barska having lower magnification). I have the latter (the first scope I bought).

There is also a problem of taking the catadioptric scopes to very low magnifications. As the exit pupil is an image of the objective it is "doughnut" shaped (a torus) with a hole in the middle from the obstruction. You really start to notice the hole at lower magnifications. It also leads to doughnut shaped bokeh for out of focus images (that some folks find annoying).

As catadioptric scopes tend to have a long focal length for their size they tend to have a smaller FOV (with any sensible EPs) and higher magnifications. As others have said a fixed 20 or 25mm EP is a better match than the zoom. At least the fixed EP widens the apparent (and real) field of view. Also as it's a 50mm outside diameter of the objective then the exit pupil will get rather small at high magnifications.

Although the CA is low as this scope uses mirrors not glass objectives the contrast is much poorer than one might expect probably because the 50mm objective has a 16mm obstruction. The rule of thumb is to subtract the obstruction size from the objective size to get a similar "effective" objective size. The effective aperture is 50-16mm or 34mm. Not good. The 65mm version of the scope might be a better comparison to a 50mm "normal" scope.

A comparison of the "C50" with a 25mm EP and a Pentax 65ED with a 12mm EP (at 30x to get a more similar exit pupil) is night and day. I suspect the ED50 would be similarly different.

Thanks a lot for your advice.

Perhaps I will test a C50 if I can get one in a shop but actually the Nikon ED 50 seems to be quite another class (yet more expensive but worth the difference in price). So I will go for Nikon..

Kilian

I am using a Meade 90mm ETX, without its mount on a video tripod, when i am going to be in one spot for awhile. Currently us a 25mm Plossl, for 50x. Nice and bright, and VERY crispy sharp.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380091311191

AM interested in trying one of these, but seller lives wrong end of the country.

hello to all...I'm using SkyWatcher 90/1250 with 3 eyepieces GSO: 15mm, 32 and 40mm..my pictures and video are on my blog, http://hobbydigiscoping.blogspot.com/ I hope it can help you...ah, forgot, the cam is Lumix ZX3 and Canon powershot 470...bye