Serious astronomers will always sneer at people using spotting scopes for astronomy. Way too many lenses, too much glass, with transmission figures in the high 80%s, instead of 90% and more as in proper astro refractors, therefore too much loss of light and contrast, and on top of that, spotting scopes usually come with a zoom eyepiece which is known to be sub-optimal optically when compared to a fixed focus eyepiece.
Assuming you are a birder who got a Swarovski BTX and wonder whether you could use this to occasionally venture a bit into night sky observations ...
The BTX has received acclaim and excellent reviews for birding, wildlife and nature observation, see e.g.
The BTX is a Swarovski ATX spotting scope with, instead of the regular ATX zoom eyepiece, a BTX binoviewer attached. The largest model is based on the 95mm objective module. With the ATX zoom eyepiece, you get a 30-70 x 95 spotting scope. If you attach the BTX eyepiece, you get a 95mm spotting scope with a 45 degree binoviewer and 35x magnification. With the optional ME 1.7x Extender, you get a spotting scope with 45 degree binoviewer and 60x magnification.
With the modular design of the Swarovski scope, you can therefore switch between the following configurations:
• 95mm (= 3.75‘‘) ATX spotting scope with 30x-70x magnification
• 95mm ATX spotting scope with 50x-120x magnification, using the 1.7x extender
• 95mm BTX spotting scope with binoviewer and fixed 35x magnification
• 95mm BTX spotting scope with binoviewer and fixed 60x magnification, using the 1.7x extender
Not with one word does Swarovski mention a potential usage of the ATX / BTX for astro on their website. It is only promoted for birding and nature observation. Nevertheless ....
I have been taking advantage of the nice weather the last few week to try out the BTX under the night sky. Seeing conditions at my location were average for deep sky but very satisfactory for the Moon, Jupiter and Saturn, so I focused on those three.
Some time ago, I had briefly used the BTX at 35x on the moon and found that it almost matched the optical performance of the Highlander 32x82 - not in terms of brightness, but contrast, detail recognition, absence of CA, edge sharpness were outstanding in my eyes.
This time, I used the BTX with the extender providing 60x, and intermittently, I switched to the ATX eyepiece (still with the extender) for some „high mag“ views at 100x and beyond.
To compare the Swaro, I used it side by side with both APM 70 ED and APM 82 ED BTs, the 70 with the Vixen SSW 7mm eyepiece (providing 57x magnification), and the 82 with the DeLite 7mm (providing 67x). With the BTX at 60x, I thought that would allow some sensible comparisons.
The BTX is almost the same length as the APM 82, but this one weighs – without eyepieces - 9.9 pounds, the APM 70 weighs 8.3 pounds. The BTX including extender and everything weighs 6.6 pounds, which makes its handling easy and the demand on mount and tripod modest.
I used the Vixen and Omegon fork mounts for the APM 70 and 82, respectively; Swarovski offers aluminium and carbon tripods of its own, and I used the heavier (but cheaper) aluminium tripod. Swarovski also offers its own mount which, together with an optional mount plate which allows the balancing out of the BTX, allows for a very smooth operation.
Moon (observed over a number of days)
The immediate impression: The BTX is slightly less bright than the 82 and the 70. This is not because of the size of the exit pupil (it’s 1.6mm in the BTX, and it was 1.2mm in each of the two APMs), but due to the “binoviewer effect”: with only one tube and a split light beam, there is less light for each eye.
The field of view is slightly smaller than in the 70 and roughly comparable with the 82, in the BTX the full moon disk and a surrounding circle of dark sky are visible. Edge sharpness is excellent in the BTX and at least on par with or better than the one in the APMs (for which I had chosen very good eyepieces, I believe). CA is well controlled, there is just a tiny hint of it in the outer parts of the FOV.
Central sharpness is also excellent. Rupes recta was no challenge for either instrument, Rima Birt, however, was beyond any of them.
(( Remark: switching to the one-eyed ATX configuration and using higher magnifications at 100x up, Rima Birt could just be detected ))
Rima Hyginus was again well visible in both the BTX and APM 70, but for some reason barely visible in the APM 82.
Vallis Schroeteri (best visible 3 nights before full moon) was again very well visible and clear with lots of detail in the BTX, followed in image quality by the APM 70 and then the APM 82.
Overall, as mentioned, the view in the BTX is slightly less bright in the BTX, which could explain why I was seeing so much detail on the surface of the moon (less dazzling than in the APMs), which could also mean that the BTX might be less ideal for deep sky observations, where light is usually a “scarce resource”. For the “moon hopper”, however, the BTX – both at 35x and at 60x – seems to be an excellent companion at this range of magnifications.
Jupiter (observed on three consecutive days)
Same immediate impression as on the moon: the image of the BTX is just slightly less bright than in the APMs. And same impression regarding central sharpness, contrast and absence of CA. Several horizontal bands are easily visible on the planet surface, the main ones – NEB and SEB – quite prominent. The BTX even shows the swirly nature of SEB quite nicely, I have hardly seen it that clearly in this instrument size.
The image in the two APMs is generally similar, but I recognized the swirls in SEB only after having seen them so clearly with the BTX and having specifically looked for them.
The large moons are obviously well visible, they are very point-like in the BTX and the APM 70, a bit “fluffier” in the APM 82.
Saturn (observed on two consecutive days)
Similar impression as with Jupiter, the BTX fares very well. I tried to see the Cassini division, but this was more guesswork than anything else in all three instruments, so I would say it was a “not visible”.
((Remark: just to confirm, I switched over to the ATX configuration and went up to 90-100x, then the division became visible))
It seemed to me that I could see the slightly brighter or “whiter” zone around the equator of the planet body, with the northern and southern parts of the planet darker than the central part. It was best visible in the BTX, which provided again very good contrast, and almost as well with the APM 70, a bit less so with the 82.
The BTX competes rather well with similar sized binoscopes on objects like the moon and large planets. Whether its slightly darker image would represent a bit of a handicap for deep sky observations would remain to be seen (I would suspect it), but maybe large bright objects such as M42 would be manageable with it.
Is this a recommendation to buy a BTX for astro usage ? No. The only thing I wanted to find out was whether the avid birder with his or her BTX could also use it with profit for an occasional evening under the sky, instead of buying additional gear. In my opinion, the BTX performs well under the circumstances described above, plus it’s easy to handle and very light weight.
The keen astronomer, however, will probably be better off buying dedicated astro equipment, such as a binocular telescope, not least because of the price (the BTX with the mentioned accessories costs about as much as a same size Binoptic cost at the time, and roughly twice as much as e.g. an APM 82 with 2 sets of very nice eyepieces), but also because of the higher flexibility of a BT (the BTX can only be used with 35x and 60x).
For what it’s worth.
The image below:
The APM 82 ED and the Swarovski BTX side by side
Edited by Pinac, 27 July 2018 - 04:31 PM.