Telescope Reviews: First Light: Garrett GT-100 Binoscope

The skies cooperated briefly tonight and I was able to manage about an hour of observing time. On a good night the sky at my home is mag. 5 with a decent view of the Milky Way. Tonight's conditions were near that at the beginning, but softened quickly as high clouds moved in. I would have liked better conditions and more time to test the GT-100's, but, nonetheless, here are a few comments.

Mechanics:

It was 36° while I was observing. The IPD adjustments and the helical focusers worked smoothly at this temperature. Maybe the Chinese have finally found appropriate grease!

The helical focusers have quite a bit of adjustment travel and very fine threads. They are a pleasure to use. Focusing on the crescent moon was easy. However, I'm accustomed to binoculars with more coarse focusing mechanisms, and I found myself playing with the focus on the stars from time to time. There is a larger-than-I'm-used-to range where the stars are pinpoints. After a while I just settled on one spot and enjoyed the views. (This is not a complaint; merely an observation.)

The GT-100's didn't loose any weight since I took them out of the box. At 26 lbs. my Helix Mount can handle them, but I wouldn't want to go much heavier.

I opted not to buy the finder that is made for the GT-100's because I wasn't sure it would be useable with my mount. As it turns out, it would fit, but I think I'll mount a red dot finder when I use the binocs at higher powers. The GT-100's come supplied with two bolts for mounting the finder to the binocular body.

My neck appreciates the 45° prisms!!

Bottom Line: There are really no bothersome issues mechanically with the GT-100's. Everything works smoothly as it should and the housing is substantial. Fit and finish is good.

Optics:

I tested the binoculars using a pair of 26mm TV plossls. As I stated in another post, the supplied 24.5 widefield eyepieces were too large for me and squeezed my nose when I adjusted the IPD.

I calculated that the 26mm plossls should provide ~23x with a FOV of ~2.2° and an exit pupil of 4.3. I tested the FOV using alpha and eta Cassiopeia which have an angular separation of ~1.7°. They fit in the FOV with a bit of sky outside each star, indicating that the field was very close to the expected 2.16°.

Collimation and side-to-side merging was excellent.

I used Vega and the crescent Moon to check the level of chromatic aberration. Since the GT-100's are basically two 100mm f/6 achromats, I expected to see some color. The Moon was very low in the SW horizon and it exhibited a faint greenish ring around the its limb. Vega showed a dim purple/blue halo, but it wasn't very obtrusive. In my subjective opinion the GT-100's did quite well in this regard, and it will be interesting to check them at higher powers on the Moon and planets.

I didn't notice any other optical aberrations. Stars were pinpoints across the entire FOV, from top to bottom and from side to side. Contrast was excellent.

Quickly I looked at M13, the Andromeda Galaxy, the Pleiades and the Double Cluster as well as the Moon. M13 was bright and distinct and the clusters fit in the FOV nicely. Of course, the galaxy extended beyond the FOV. M31's satellites were visible, but by then the sky was getting soft.

Bottom Line: The GT-100's have very nice optics.

In my opinion, these binoculars are going to be with me for a long time. They are well made and perform very well. Remember, I was using very good eyepieces in them, and I'm certain that they were a strong element in the equation.

By the way, I highly recommend Garrett Optical. I've been very pleased with both pairs of binoculars that I have purchased from them. Their quality control can be trusted!

Clear Skies,

Edited by Ed Kessler (10/26/06 11:20 PM)

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