130 mm telescopes compared

I write to ask a simple question and to invite comment, my question being stuck near the end of this long note.

Having spent countless hours at work and at rehearsals for a musical show this week, it was a relief and joy to get away (to my backyard) with a friend and observe with two AP 130 mm telescopes for a couple of hours Wednesday (9/1/4), Thursday and tonight. The low moon was more of a problem on Wednesday and less tonight. We used a 2003 f/6 Starfire EDFS (FL 780) and a 2002 f/8.35 Starfire EDT (FL 1085), on Losmandy G11s, with Televue diagonals (having only 1 AP and wanting to make the comparison honest). We used Nagler EPs (one 22mm 2 inch, which we shared; pairs of 13, 9, and 3-6), also a pair of 19 mm Panoptics, all of which we staggered to try to approximately match magnifications. Each night we simultaneously observed the Moon (the first two nights with Meade neutral density filters, tonight without), the Perseid Double Cluster, M13, M31, 3 doubles (Polaris, Mizar, Dub-dub), and Uranus. We viewed other DSOs randomly, without necessarily comparing. Tonight only, a Stowaway f/7 with an AP diagonal, set on a Gibraltar, joined us.

Both telescopes performed brilliantly. What was interesting were the minor differences I saw. Each night I felt the double cluster to be more satisfying with the f/8.35. I could see more tiny stars and the contrast was more brilliant. Lunar craters tolerated more magnification with the 8.35 (of course!) and this telescope never gave out, not even with a 3 mm EP (350x), wheras the f/6 got fuzzy at 3 mm (260x). The contrast between light and dark on the edge of the moon was more pronounced on the f/8.35. And to my surprise, M13 and M31 were brighter with the f/8.35. All three nights, each and every time we looked.

This I do not understand. Why did we both see more tiny stars and surface brightness with the 8.35? Given equal apertures, should not brightness--a function of light grasp--also be equal? Can anyone explain this to me? Does the greater refraction of the faster telescope cost brightness, or is this a random difference between telescopes? This last explanation seems implausable, as miniscule random variations ought not to change the light-gathering enough for the eye to detect the difference--no?

On Uranus, the f/8.35 showed a more clearly defined disc and I swear I could see a greenish color. When we brought out the Stowaway tonight, the view of the almost-half moon was as satisfying as with either of the bigger 'scopes. Could not get the same degree of clarity and detail, though, even with 3 and 4 mm eyepieces, as on the big boys.

Given these observations, my preference is for the f/8.35, but if I were stuck on a desert island (say, atop Mauna Kea) with only the f/6, I sure would be a happy man. Thanks so much, Mr. Christen, for your fabulous products.

Ciao

Robert

Thanks for your post.

The F6 was, of course made and optimized for imaging, especially H-alpha where the speed is necessary to capture faint data in long exposures. It is a thicker lens and would take longer to cool down vs. the F8.35 version. If you are going to do mainly visual astronomy, then the longer one would be more suitable.

I have done a lot of visual testing now of over 8 different designs for our new airspaced series. The 6" version will have a focal length of 1200mm, which is the point where planetary viewing seems to be ideal, yet the scope is not overly long.

This new design also will have twice as good color correction over the visual range at the expense of the far infrared. This is not of any consequence for CCD imaging since all the new chips are no longer optimized for IR, rather they have shifted response into the blue.

Roland Christen

I was surprised myself, I thought that a minor difference would be passed over by the human eye. My eyesight is NOT exceptional and I was biased in favor of the f/6 as being the easier telescope to physically handle. However, the differences in brightness between single samples of the 130 f/6 and 130 f/8.35 were consistent. We even swapped diagonals and EPs to be certain. But this is not to say that the f/6 was unexceptional; it is a magnificent telescope, if you had not seen the f/8.35, you would say, as I had previously said, that no 5 inch refractor could ever give better images than does this f/6.

Have others of you made such a comparison?

Robert Howe