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Celestron’s RASA 8 is a pure astrograph, a photographic telescope that may solely focus mild onto a digital camera chip – there is no such thing as a eyepiece. I’ve spent the previous 12 months imaging with the RASA 8 that belongs to Dr Paul Kummer, a retired pc physicist from the College of Manchester, who co-owns a set-up with me at our distant imaging observatory at Les Granges Astronomy Holidays in southern France. All through that point, I’ve come to know the RASA 8 very properly.
At first look, the RASA 8 resembles a Schmidt– Cassegrain telescope, however the main mirror sends mild to not a secondary mirror, however via a lens and onto the front-mounted digital camera. The instrument’s headline function is its phenomenally quick photographic velocity of f/2, which is the place we will start.
Focal ratio might be extraordinarily deceptive, so let’s reduce to the chase: the RASA 8’s aperture is 203mm however, for the reason that digital camera sits within the entrance of the optical path, it has a substantial central obstruction leaving it with a real light-collecting space of 25,572 sq. millimetres. By comparability, a typical 85mm quick refractor of f/4.7 would have the identical focal size, however a light-weight amassing space of simply 5,674 sq. millimetres. Fairly merely, the RASA collects 4.5 instances as a lot mild as a generic quick refractor. Three hours within the RASA actually does equate to 13.5 hours within the refractor.
Given this outstanding saving in publicity time, we’d anticipate all dialogue to finish right here, with the RASA 8 as the one short-focal-length imaging telescope anybody would ever need. Nonetheless, its velocity brings problems.
Discovering focus
The sunshine cone of an f/2 system is steeply angled, giving a really shallow depth of subject. As a result of astronomical targets are at infinity, astrophotographers aren’t usually involved about depth of subject, however very quick f-ratios require disconcertingly exact focus, well-collimated optics and an imaging prepare free from tilt. A tilt between the chip and the incoming beam of sunshine signifies that solely part of the picture might be in focus.
Celestron declare that the RASA ought to arrive in good collimation, however house owners’ experiences recommend that it could or might not achieve this. Because it can not maintain an eyepiece, the one practicable technique to collimate it’s to make use of a small, cylindrical digital camera corresponding to a information digital camera, sufficiently small to provide entry to the three pairs of antagonistic (push and pull) screws that modify the entrance lens whilst you observe a defocused star on a pc display.
We wanted to collimate Paul’s instance and the changes required have been actually tiny. We settled, in the long run, for a superb however not strictly excellent collimation.
With the imaging digital camera in place, we turned to the difficulty of focusing, utilizing Celestron’s devoted focus motor. This was pretty simple to suit, however it’s not exactly machined and so have to be tightened up in small iterations to keep away from stressing the focuser shaft. Be sure you comply with the directions rigorously. Our first focus motor expired after a couple of outings however its guarantee alternative has labored properly.
First mild revealed appreciable tilt, giving us good star shapes within the centre however unacceptable elongation alongside the quick sides of the picture. Tilt typically comes from the digital camera, so we constructed a check jig following rules helpfully laid down on the web site of Starlight Xpress. Our simplified jig is proven within the {photograph} on the left. A lowpowered laser sends a mirrored image from the digital camera chip onto the bottom of the field. With a tilt-free chip the reflection will type a dot that activates the spot because the digital camera is rotated.
Surprisingly, the instrument holds focus very properly certainly, often needing a refocus solely after a meridian flip. Life can be simplified by the truth that warmth from the digital camera and air flow from its fan imply {that a} quick, unheated dewshield is all that’s wanted to fight corrector-plate dew. We discovered one of the simplest ways to route the digital camera cables in entrance of the corrector was to form a semi-circular information. We have been briefly perplexed by a light-weight gradient in our photos, however this was traced to a tiny LED on the digital camera lighting up the underside of the cable information.
In any case these changes, we had acceptable however not excellent star shapes on the edges of the chip. Stacking a lot of sub-exposures improves them and, to be truthful, Paul’s ASI 2600MC Professional digital camera has a diagonal a couple of millimetres bigger than Celestron’s claimed circle. Imagers who prefer to ‘pixel peep’ won’t like what they see, however I used to be proud of what we had.
Your alternative of digital camera is proscribed to fashions that can match inside the central obstruction, so DSLRs and filter-wheels are dominated out. We discovered that ZWO’s ASI 2600 MC is an effective match for the RASA 8 and the digital camera can be utilized with a a number of bandpass narrowband filter very successfully. The small pixels of contemporary CMOS cameras are well-suited to the 400mm focal size.
Rekindling a ardour
Ultimately we will now flip to the totally fettled telescope’s efficiency beneath the celebs. Let there be no mistake: it’s distinctive. Simply sixty exposures of three minutes can provide an thrilling and polished end result, even on faint targets like darkish nebulae. Bump that as much as ninety subexposures and the picture processor is in heaven. The information are a pleasure to work with, deep and clear. They do, nevertheless, profit enormously from processing with the software program Star Xterminator, (the up to date software program of which is given a trial run on this this challenge in Nik Szymanek’s Imaging masterclass on web page 76). By processing the nebulous element individually from the celebs, the RASA 8’s depth of seize might be totally exploited and the celebs saved beneath management when eliminated. For my part it could be a disgrace to course of RASA information with out utilizing Star Xterminator or, maybe, its rival, Starnet++.
Once I say that the RASA has rekindled my ardour for wide-field imaging and mosaic-making, keep in mind that I dwell at a darkish web site with lots of of clear nights annually. If I have been preventing cloudy climate and lightweight air pollution, I might be much more persuaded by this telescope. It has one other benefit, too: it really works greatest when used to take massive numbers of quick exposures, that means that the sigma-stacking routines will get rid of satellite tv for pc and plane trails extra successfully. One final final bonus: it appears that evidently ‘mud bunnies’ – the shadows solid on the chip by mud particles near it – are blurred out of existence within the RASA 8. As a result of they’re solely correcting vignetting, the imager’s flat fields ought to work for a really very long time with out being up to date.
At lower than £2,400, the RASA 8 is remarkably good worth. It is probably not excellent, it could not swimsuit everybody, but it surely has most actually received me over. I discover it a relentless pleasure to make use of.
At a look
Sort: Rowe–Ackermann Schmidt astrograph (RASA)
Aperture: 203mm (8 inches)
Focal size: 400mm
Focal ratio: f/2
Central obstruction: 93mm (46% of aperture diameter)
Decision: 0.57 arcseconds
Picture circle: 22mm
Weight of OTA: 7.7kg
Particulars: celestron.com
Value: £2,345 (OTA solely)
Olly Penrice is an astrophotographer who runs Les Granges Astronomy Holidays, which you could find extra about at sunstarfrance.com.
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