The Hogsback Observatory started very simply – a telescope, mounted on a drive and connected to a tripod. The tripod sat on the concrete pad in front of the garage with a telescope pointing south. A small folding table and a folding chair completed the exotic installation. That was in 2009 and in the years since then total of six telescopes have been in use at the observatory. It is not my intention to give you advice on choosing a telescope for your astronomy adventures nor will I try to cover all possible astronomical telescopes. What follows is a quick survey of all six of those telescopes that I have used. For each one I will talk about their strengths and weaknesses, give you a thumbnail image of the telescope itself, and include a sample of some of the imagery produced by each telescope.
My first choice for a telescope back in 2009 happen to be a very good one. It came as a package from Orion Telescopes and Binoculars. The package included the telescope, a tripod and drive system to permit the telescope to track its target, and eye piece of reasonable quality, a finder scope (a small telescope that, like a telescopic sight on a rifle, allows you to accurately point the telescope at objects you want to observe), a mirror diagonal to permit more comfortable viewing, and a Barlow lens that would increase the image scale by a factor of two when needed. This was basically everything you needed to start using the telescope and required only the purchase of a relatively inexpensive astronomical camera to begin imaging.
When I purchased the system I thought it would be primarily useful for imaging the surface of the moon and it never occurred to me that it might produce useful images of the nearer planets. As you can see from the sample pictures it, does a surprisingly nice job on Jupiter, Saturn, and Mars. One of the major advantages of any telescope where a mirror is used as the primary optical element is that there is no chromatic aberration and such a telescope is well-suited to color imaging. Maksutov telescopes have a reputation for being very stable when it comes to holding collimation. If you handle the telescope with reasonable care there is no reason why collimation adjustment should ever be necessary. I still have this telescope (it is on extended loan to a friend), but, as my skills developed, I felt I could do a better job with a different instrument.
SV110ED Refractor
While my first choice for a telescope worked out well, my second choice was a bit of a step backward. Part of the problem was that I really like high-end refractor telescopes and to some extent I had bought into the mythology that surrounds such instruments. Time and time again you will hear owners extolling the performance of their telescopes and saying that they readily outperform other telescope designs, even if those other designs have a larger apertures. Apochromat (or APO) refractors are the gold standard for astronomical refractors, but APO triplets are VERY expensive as the glass for each of the three elements is costly and there are a total of six lens surfaces that must be figured and mounted to the highest standard.
ED telescopes are doublet (two lens) objectives that cam achieve color correction comparable to conventional APOs by using ED-glass (Extra-low Dispersion) for one of the elements. These telescopes are excellent for visual use since they achieve excellent color correction across the visual spectrum, they are less expensive then conventional APOs, and are lighter. Stellarvue manufactures a line of classic APO refractors, but also was marketing a 110 mm ED refractor in 2010. Some manufacturers refer to their ED instruments as “Semi-APOs” or even call them apochromats! Vic Maris of Stellavue called his ED scopes what they actually were – ED instruments that were fully corrected for visual observation. In addition to paying slightly over $1000 for the instrument, I also invested in a new mount to carry the extra weight (good refractors are heavy).
The SV110ED was a easy to use and gave lovely visual views of the night sky. The telescope (like virtually all modern refractors) had a relatively short focal length and a modest focal ratio (f/7). By itself, the instrument delivered wonderful wide-angle views of the night sky, but a 3X Barlow lens was needed for the moon, Jupiter, and Saturn and a 5X Powermate was needed to deal with Mars.
While the SV110ED was excellent for visual observation, color imaging was a disaster! Any attempt at color imaging produced pictures with a significant magenta cast. Given that small differences in color can enhance effective resolution, the inability to image in color with the SV110 was a deal killer. If my only experience had been with the 110, I might have given it a reasonable rating, but it was clearly not superior to the 127 mm Mak, which was ~1/3 of the price of the ED refractor. Vic Maris at Stellavue was able to offer me a good trade-in deal for one of the premier top-end APO refractors (the SV115T), which is how I ended up with my next instrument – a superb 115 mm (4.5-inch) refractor.
SV115 Apochromatic Refractor
The SV115T turned out to be perfection in every category. The build quality was superb – heavy duty components, high-quality machining, and perfect fit and finish. The SV to- speed focuser was smooth as silk and generating comparatively little movement in the telescope tube. Color correction was dead-on and there was no sign of any spurious color even with very bright objects, such as the Moon, Venus, or Sirius. Assuming that seeing was not off the charts, images were tack sharp, the background was a velvet black, and the views had excellent contrast. Planetary images were too small to be useful at prime-focus (f/7, fl = 800 mm), but Jupiter showed innumerable features at f/21 ( a 3x Barlow) while smaller targets like Mars and Saturn required f/35 (5X Powermate). There is no question this APO refractor had easily equaled the performance of the slightly larger127 mm Maksutov, while leaving the 110ED instrument in the dust.
I have no intention of ever getting rid of this very fine instrument, but our very small lunar and planetary imaging group on the Internet push me to try still larger apertures. I am not yet finished with the business of refractor mythology, but I will get back to that after describing the next telescope in the history of Hogsback Observatory.
The boundary between a planetary dilettante and a serious image is generally agreed to be about 8 inches or 203 mm. Arguably, the Celestron C8 Schmidt-Cassegrain is an exemplar of such a transition telescope. Both Maksutov and Schmidt Cassegrains are compound telescopes. Speaking broadly, there are many similarities between them although the essential details very. A brief search on the Internet should acquaint you with the differences. The C8 is an affordable telescope on its own (generally a little above $1,000, depending on the features) and so many of them have been sold that it is not hard to get a good bargain on the use telescope lists. That’s precisely what I did to get mine.
The C8, as the name implies, has an aperture of 202 mm, a focal length of 2032 mm, and ways in at a modest 12.5 pounds. My first observing session with my used C8 was somewhat miraculous. The “seeing”was unquestionably the best that I have ever had and the result was the superb image of Jupiter on the C8 composite. The telescope certainly played its part, but since I never got another image anywhere near that fine, we can assume that could see was doing most of the work. But another sessions however, the performance of the C8 was quite consistent when operated at f/20 (2x Barlow) or f/25 (2.5x Barlow). Even Jupiter images are unacceptably small if you image at the native focal length (f/10). The image of Mars on the Mosaic above is almost identical to the image on the SV115T composite. The two images of Mars were obtained very close together on the same night. The only real difference between the two is that the C8 image is larger and sharper than that from the APO telescope (as would be expected given the greater resolution of the C8’s larger aperture). Given the normal quality of seeing in central Michigan, the C8 was most successful operating at f/20. The only real drawback I saw with the C8 was the need to pay constant attention to the matter of collimation. The top planetary imagers strongly suggest that Schmidt Cassegrain’s should be collimated at the beginning of every observing session to avoid the inevitable decline in image quality if the instrument is allowed to get out of alignment. Collimation was something that I did not enjoy doing and I began looking around for a large aperture instrument where I could avoid the chore. If I lived in a place with superb seeing, I would certainly have looked at a larger version of the C8. Celestron make Schmidt Cassegrains (with many options) with apertures of 9.5, 11, and 14 inches. Given the seeing problems at my site, one of these larger telescopes would be lucky to produce clear imagery once or twice a year. It was clear to me that an aperture around 8 inches was going to be optimum for my seeing conditions. So, in aperture around 8 inches and no collimation requirements might seem to suggest taking a second look at Maksutov Cassegrains. Before going down that path, I’d like to close out the promise discussion on the mythology of refractors and planetary astronomy.
Refractor Mythology
Earlier I made mention of the fact that many owners of high-end refractors contend that their instruments could outperform other telescope designs despite the larger aperture of the other telescope. There may be a good reason for this widely held myth. Back in the Fall of 2012, I was alternating between two telescopes during the 2012 Jupiter apparition. One was the 203 mm Celestron Schmidt Cassegrain (which was new at the time) and the SV115T APO refractor which I have been using much of the year before. When you have just a single mount, it’s a real chore to do imaging with two telescopes during the same session. It is so much work that I rarely do it. However, on the night of September 29, I did get images of Jupiter from both telescopes. In the case of the two images shown in the composite above, the Celestron and APO images are separated by about an hour. I ran across this set of images while I was preparing this page on the telescope history of the observatory. The image from the C8 is larger than that from the APO and that’s as it should be. The baseline focal length of the C8 is 2032 mm and, with a 2.5X Barlow, the effective focal length would be 5080 mm (2.5 x 2032). In contrast, the APO has a base focal length of 800 mm which, using a 5X PowerMate, would result in an effective focal length of 4000 mm. While the C8 image is a little larger, the two are quite comparable in size and, what’s more astonishing is that they show essentially the same detail. While the finest detail can be seen in a detailed examination of the C8 image, the difference is not great, while there is no doubt that the APO image has more contrast and more color. Unlikely as it may seem, the APO, with an aperture of 115 mm (4.5″) is in a virtual dead heat with the C8 and its 203 mm (8″) aperture. It is comparisons such as these that lead to the myth of the high-end refractor.
The problem is the comparison is not a fair one. The Jupiter image on the C8 mosaic shows what kind of resolution the telescope can deliver when seeing is exceptionally good. One look at the C8 image in the comparison above should indicate that seeing was nothing like that when these two images were obtained. The fact is, a larger aperture telescope will always be impacted to a greater extent by poor seeing in a relatively smaller aperture. The results above probably represent a situation where we are comparing a significantly degraded image from the C8 within an APO where seeing has had very little impact. Under any conditions where seeing would become incrementally better, there would eventually come a point where the limited resolution of the APO objective (no matter what its other admirable qualities) will steadily lose ground compared to the resolving capabilities inherent in the greater aperture of the C8. There is one thing however that the comparison does suggest – it debunks the notion that short focal length refractors are not effective for planetary imaging. You will have to push the instrument with 3X or preferably 5X Barlow’s, but you can get a useful image scale and very satisfactory resolution of cloud or surface features. You could build your observations around the use of such a refractor, but should a session of really good seeing emerge, you are unlikely to make the best possible use of the situation. The moral here is that use it if you have it, but don’t expect a Sky and Telescope cover shot!
180 mm Maksutov Cassegrain
The problem was to locate a telescope with something around a 200 mm aperture where constant collimation would not be required – basically another Maksutov! My success with my first telescope, the Orion 127 mm Mak, encouraged me to look at other Maksutovs in their product line. No one seemed to have an 8-inch Mak, but Orion did have an affordable 180 mm model (7″) that looked as if it was worth trying. This particular model is hard to find in the Orion cztelog, but if you do a search for the stock number (#09969) you will find it. Why they are so shy is a mytery, as I think it is the premier telescope in their high-end line and with a retail price just above $1,000, it is, in my opinion, the most affordable instrument available for quality lunar and planetary imaging. The telescope appears to be a product of the Synta factory in China and it is probable that similar products from other sources may be the same instrument in different tube colors and logos.
The design focal ratio of the instrument is f/15 and, with the addition of the electrical focus unit and mirror diagonal, the working focal ratio ended up at ~f/17. This works well across most my range of seeing and image sharpness and detail are excellent at prime focus (f/17) as shown by the Jupiter sample image. If seeing will support it, a 1.5X boost to f/28 produces a better image scale in the case of Mars. 2X and 2.5X might be useful in the event that a session demonstrated exceptional seeing, but that has yet to happen. More powerful Barlows and PowerMates, so useful with short-focus refractors, are of no use when starting from an f/17 native focal ratio.
200 mm Maksutov Cassegrain
One of the biggest lessons I learned was that chasing telescopes with larger and larger aperture (the well-known “aperture fever” in amateur astronomy) was not going to work given the less-than-ideal seeing here in the middle of the Great Lakes Basin. As I began thinking about the telescope I wanted to use in my retirement, several characteristics served to define my search:
1. A maximum aperture of 8-inches (203 mm)
2. A long focal length to provide a reasonable image scale without the use of a lot of accessory lenses. This would mandate the use of some sort of compound telescope as my modest dome could not accommodate a very long instrument.
3. Thermal stability in terms of both focus and collimation.
4. Peerless optics to optimize performance no matter what the seeing during any particular session.
The Holy Grail of my quest turned out to be the OMC 200 Maksutov/ Cassegrain from [Orion Optics U.K.] in Britain. With an aperture of 200 mm and a focal length of 4000 mm (f/20), this is a dedicated lunar and planetary instrument. While the standard for most commercial telescope optics is 1/4-wave, the OMC200 optics are finished to 1/8-wave accuracy. It’s not cheap, and you may have to wait a long time to get one, but it is a telescope that dreams are made of. Unless otherwise noted, all the images on this site were captured using the OMC200 instrument.
