I have been recently asked on a comment on performance of 71FL vs. 77EDII (because I am known for owning all Borg lenses and reducers). I have found this quick-test image somewhere on my hard-drive. It’s from November 2011 so kind of an old test. My conclusion is that again, biggest problem is helical focuser, second big problem was the bad design of the 0.7x reducer that was held in place by only 1 screw located on a side of the inner barrel which could (and also did) cause some misalignment that completely degrade the final performance, especially on a large (full frame 36x24mm) chip.
Since I did not want to spend more money on purchasing FTF and producing custom parts in order to place the reducer inside of the focuser in a smart way I have recently sold all my Borg scopes and reducers except of 77EDII F/4 SR combo (in the end I have converted to an expensive Newtonian system from ASA that brought my imaging to yet another level).
Here you can find one of the best results I got from this combo with a 34x24mm FF CCD (MII G3-11000 camera).
miniBorg 71FL + 0.7x #7870 reducer (F/3.9) + MII G3-11000 CCD
and the corresponding (uncalibrated – except for hot pixel removal and vertical column banding supression) light frame:
miniBorg 71FL + 0.7x #7870 reducer (F/3.9) + MII G3-11000 CCD
For star-field testing I am always using a H-alpha filter that shows the true shape of the stars (along with making them smaller). Using broadband filters makes the star shape differences much less apparent, but obviously works well for APO-chromaticity testing.
BTW all refractor manufacturers make the same design mistakes over and over again (I should start my own company that would deliver 100% perfect astrographs without a single compromise). Because I learned that I have to tune the refractors myself using plenty of custom parts, I have started using Newtonian systems in spite of the well known and much afraid of task called collimation. The BIG aperture and performance makes a HUGE difference in terms of light collecting. Plenty of refractors are slow (F/5 and even worse) and I simply do not have enough clear skies to make one picture in three clear nights.
Pavel,
Thanks very much for this test. Could you elaborate on what is wrong with this helical focuser? Any problems in holding heavy cameras?
Hi Ram,
as the americans say…
no it’s not in „holding heavy cameras“ as the focuser was mounted in front of the OTA, between lens and main OTA tube (and rings) while the camera was just screwed to the „back end“… I think (I think) it was the travel – like that it was not precise causing slight misalignment of the optical axis (like miscollimation) when the helical focuser was turned around… I can’t swear on this for 100% but I „think so“. Anyway I would get FTF for sure and even in this case, would mount it in front
just my 0.0000002 cents
Pavel
I have readen ,in a test from OPT , that there is an excessive deep violet chromatic aberration, but on the advice of Ted Ishikawa , it is necessary to put a hutech UV filter , this it does not sound very well for an APO fluorite doublet.
What is your experience ?
Best Regards.
Josè Mundo
Hola Jose,
it depends on which CCD camera you use. If you use some with low QE in the blue (UV) part of the spectrum (<400nm) like e.g. KAI-11002 / KAI-4022 then it should not be a big problem for CA, but if you use some ultra sensitive camera like with SONY chips (namely ICX-694/674/285/825) then it is truly a remarkable issue. Then you really need an UV filter. For me, the Baader one works very well (cuts below 420nm). Briefly said, I would recommend an UV filter too.
saludos
Pavel