I think that nearly all enthusiastic astrophotographers must have noticed the news about the hottest, portable, fast, affordable telescope from SharpStar. The rumors were spreading in 2019 and when I saw the specifications, this telescope landed on my “must-have” list. The specifications were incredible: F-stop 2.8, carbon tube, solid spider wanes, primary hyperbolic mirror 150 mm in diameter, robust 2.5-inch rack, and pinion focuser, and mainly the corrector, which should be able to correct the coma on the whole full-frame sensor (36×24 mm), WOW! I already had some trouble making the F4 Newtonian work on F2.8 by using the ASA corrector/reducer. This was quite painful, but SharpStar’s corrector is much bigger in diameter, so theoretically it should work.
Immediately as soon it was available, I ordered one at http://www.teleskop-express.de/ but their own rebranded version called TS-Optics hyperbolic astrograph. How does it perform? Let’s have a look.
I somehow expected that reaching the focus will be quite a challenge, therefore I ordered ZWO electronic focuser and easily attached it to SharpStar.
The secondary mirror has very nice spider vanes. This was my dream to get very stiff and perpendicular vanes. I have two Newtonians from TS and it was a bit struggle to get them perfectly perpendicular. On the other hand, the secondary mirror is not flocked/blackened. Moreover, there are many shiny screws. All this can cause some reflections, but we will see.
The primary mirror’s edge is covered by the ring. I really like this feature and all Newtonians should have it. A cheap solution is to clamp the primary by L-clamps, which cause ugly reflections/flares around the bright stars.
The corrector has an M48 male thread, so any 2″ extension can be attached. Included is only an M48 to 1 1/4″ extension, but I already have a couple of 2″ at home and these are not expensive.
The secondary mirror can be collimated even with the corrector attached.
But to collimate the primary mirror by a Barlow laser, the corrector must be unscrewed. The tool to do that was included and it was easy to unscrew it.
I was so excited to test the telescope outside. The spring season provides many deep space objects. All you need is just to point the telescope into the constellation Virgo or Leo and you will find many galaxies anywhere you look. In fact, this was the main reason I purchased this telescope – to have a focal length of around 400 mm and a fully illuminated 44 mm image circle. I don’t have a full-frame cooled camera, so I did the first light with an APS-C sensor size (ZWO ASI 071). The first target was the galaxy M105. After the post-processing, I was very disappointed. The stars in the corners are oval and they have a color shift. The blue channel is shifted towards the center and the red one outwards. See for yourself:
I pushed the saturation and the vibrance to magnify the aberrations. Here is the detailed picture of the corners vs the center:
I think we all agree this is not OK at all. If I see such ugly stars on a cheap telescope, I would not complain, but this one was not a cheap telescope. This test was done on an APS-C sensor, which means a diagonal 29 mm and not advertised 44 mm. I immediately contacted TS and asked what the hell is going on. I was told that probably it could be the back focal distance (BFD). It’s the distance from the corrector to the sensor. Normally, it’s 55 mm, but a bit shorter or longer could improve the performance. Fortunately, I have many M48 adapters and distance rings, which could cover the BFD from 58 to 51 mm, so I started the experiment. This time, no stacking, no image calibration by darks or flats. Just a single picture and cropped corner:
I don’t see any significant differences between BFD 53 – 55 mm, subjectively. The stars are still oval and if I would stack many images together, even the color aberration would pop up. Let’s put a subjective assessment aside and let’s use a deterministic method to analyze the data – CCD inspector. Here is the 55 mm, which is the recommended BFD.
The corners are obviously much worst than the center, but again, it’s an APS-C sensor and not a full-frame. I decided to plot the FWHM as a function of the BFD. I evaluated the center and each corner separately. If the telescope would be perfect, I would get the same FWHM value anywhere on the picture. Based on this diagram one can easily find the sweet spot of the back focal distance, which is in this case 55 mm. Things don’t get significantly worst if the BFD is reduced to 54 mm, or even 53 mm. Unfortunately, I didn’t have a 1 mm washer to test 56 mm so this distance remains a mystery.
When I showed the graph to TS they agreed that there is something wrong with this telescope and I have full right to return it. And this is what I exactly did. They didn’t offer me a replacement telescope (I can only guess that the product from the same batch would have the same optical quality). The sample I tested was a good-looking telescope with very poor optical quality. If you are not a pixel peeper, you better get a cheap Newtonian 150/750 mm f4 and you get the same outcome in the corners, maybe even better with a decent coma corrector. I really don’t understand how this can be advertised as a telescope for a full-frame sensor. I asked TS if some customer managed to get this “astrograph” working with a full-frame sensor. They replied that so far non of their customer used this telescope with such a large sensor (APS-C). Ha ha ha. This was a good one, but really this is what they told me. I would really appreciate it if other users have a similar experience as me. Please write me a comment below.