Looking for a portable refractor that I could carry also on airplane, last September I stumbled upon this: http://www.omegon.eu...bon-ota/p,21266
The specs are impressive and matched almost all I was looking for. With a carbon fiber body, despite the 10cm/3.9" aperture, it weights only 3.5 Kg. With accessories and inside its aluminium case (which doesn't exceed IATA cabin luggage restrictions) it tips the scale at 7Kg. It has a generous 100mm (3.9") aperture (most of the other scopes I considered were 80mm) and, being F/6, it's quite fast which makes this scope useful also as a specialized high power telephoto lens. Last but not least, while it's only a doublet, it uses a combination of FPL-53 and SNBM-51 glass; at least on paper this sounds pretty good especially for a refractor with a fast focal ratio.
As I couldn't find a review online, I did some research on my own: Omegon it's a fabless manufacturer that put its brand on OEM scopes also sold under other names: for example, they have in catalogue a 127/952 refractor that looks similar to the one sold by Explore Scientific and a 5" Mak F/15 similar to the one sold by Bresser and now also by Meade. I could trace the origin of this 100mm F/6 refractor back to Long Perng, a well known Taiwanese manufacturer: on their site they list an OTA which is identical in all the characteristics except that it is said to employ FPL-51 glass in the low dispersion element of the lens pair. A few more days spent browsing the web and I came up with an old review of a Teleskop-Service refractor which also looked very similar to this Omegon. Even if it had a metal tube, the rest of the specs were identical to the Omegon refractor and it also used FPL-53 glass. While it's no longer listed on TS website, I could retrieve the original webpage via the Internet Archive: named TSAPO1006 I've no doubt it's the same scope manufactured by Long Perng and sold by TS, except now it uses a carbon fiber tube.
Satisfied by my investigation, I placed an order for the scope and waited.
The scope was delivered to me recently and put under test as soon as possible. The exterior finish and quality of workmanship is very good. The carbon tube is covered with a hard, glossy enamel coating that had no scratches or imperfections. The mounting rings are solid pieces of machined aluminium; the large screws that clamps the rings are also machined. A small but nice detail is the lens cap made of a thick aluminium plate and which is very sturdy. The rear portion of the tube, including the massive focuser, are all made of metal and finished with a black glossy paint.
The Crayford styled focuser is almost oversize for such a small tube; it has a 1:11 reduction ratio and it's very smooth in it's operation. It has a locking knob that allows to change the stiffness of the focusing mechanism or to lock the movement in place: a must when using heavy cameras. The complete focuser assembly is also freely rotatable around, which is a nice feature to have with equatorial mounts. On the eyepiece side, there is a M72 thread making the scope compatible with a lot of accessories made for Takahashi refractors (including the excellent Baader Clicklock 2" adapter). The scope also comes with M72 to 2" and 1.25" visual back; these use all-around brass compression rings to lock the barrel into place: a nice touch for those, like me, who are constantly worried to scratch the barrels of their accessories. In the end, after a detailed visual inspection, I was positively impressed by this scope. It was time to check if it delivered also quality views.
I put it on the mount before sunset so I was able to use it visually; the sky was still vividly bright so I targeted some distant tree branches that dotted the horizon. I tried with different magnifications between 15x to about 100x and the scope always delivered crisp views; I checked the trees and other silhouettes around the edges to see if I could pick up some chromatic aberration but there was none I could see. There is no doubt that this scope can be made into a good telephoto lens; Omegon also sells a telecompressor that works also as a field flattener and that is recommended if one wants to take pictures with this scope using large sensors.
The sun went down; in these months Venus is quite high and bright in the sky after the sunset and it's also the best target to check for chromatic aberrations. For this test I used the new TS UWAN, 7 elements and 82° eyepieces recently introduced with focal lengths of 7mm and 4mm (which would yield respectively 85x and 150x magnification). At 85x Venus is already visible as a bright, half moon shaped object; the borders are generally very sharp but there is a slight violet haze visible on the top half of the disk. At 150x, chromatic aberration becomes very intrusive with a distracting, purple-violet fringe present on about half of the visible borders of Venus. I must confess that I was really depressed after seeing this! I know: it's a doublet and, on top of it, it's also quite a fast refractor, but I was expecting a bit more from a lens system realized with what is supposedly the best non fluorite glass available! The first (irrational) reaction was to verify if the scope cell was somehow misaligned. I did a Ronchi test, then the "torch and cardboard test" but everything seems to be in order. To be at peace with my mind I would need to check if the ED glass is indeed FPL-53 and not the cheaper FPL-51, but I don't think there is an easy and accessible way to determine this. So, I must take the manufacture word on this or have the scope checked by some longtime refractor user to hear from somebody I trust, if my expectations were a bit too high.
After Venus, I pointed the scope towards Mars and this appeared like a nice, bright orange small disk. At least on this planet I couldn't see aberrations at the borders but I suspect these days it's too small and dim to be useful for a comparison. In ten days the moon will be again high and bright enough to be another good benchmark of what this scope can and cannot do. At least it didn't disappoint when it comes to DSO photography. I still haven't got the time to use it for a long time but I took a couple of shots of Orion from my Balcony with my DSLR. I also tested this scope with a CCDT67 telecompressor. While this focal reducer is designed to work with scopes having a ratio of F/8 or higher, many individuals used this on faster refractors - with generally good results. I attached the telecompressor to a 2" to T2 nosepisece with a 5mm spacer in-between, so that the optical path would be exactly 85mm for a 0.67x ratio. This way I have a 400mm F/4 scope. Field curvature is not taken care of as this is not a combined reducer/flattener; also the compressor reduces the image circle to 29mm which is still enough for an APS-C sized sensor but I loose the ability to take full frame shots. The compromises however are well worth, as I do sky imaging from an alt-az mount, so long exposures are out of question.
Overall, I've mixed feelings about this purchase. Built quality is surprisingly good. For DSO and terrestrial photography it looks promising. As a ultra-portable moon and planetary scope it's still unproven. At 1300€ new this isn't cheap and, for a few hundreds € more, one can buy a 127/952 triplet (from ES or somebody else) or a tried and true SW 120 doublet, which is universally kept in high esteem. These are not 'portable' the way I needed, though. On the other side, there are some refractors of comparable size and weight from premium brands, but the price gap widens considerably.
Edited by Marcsabb, 29 December 2016 - 09:51 AM.