Celestron’s RASA 8 is a pure astrograph, a photographic telescope that may solely focus mild onto a digital camera chip – there isn’t a eyepiece. I’ve spent the previous 12 months imaging with the RASA 8 that belongs to Dr Paul Kummer, a retired laptop 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 effectively.
At first look, the RASA 8 resembles a Schmidt– Cassegrain telescope, however the main mirror sends mild to not a secondary mirror, however by means of 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.
Picture: Olly Penrice.
Focal ratio could be extraordinarily deceptive, so let’s minimize 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 lightweight amassing space of simply 5,674 sq. millimetres. Fairly merely, the RASA collects 4.5 occasions 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 count on all dialogue to finish right here, with the RASA 8 as the one short-focal-length imaging telescope anybody would ever need. Nevertheless, 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 practice free from tilt. A tilt between the chip and the incoming beam of sunshine signifies that solely part of the picture could 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 accomplish that. Because it can’t maintain an eyepiece, the one practicable option to collimate it’s to make use of a small, cylindrical digital camera equivalent to a information digital camera, sufficiently small to provide entry to the three pairs of antagonistic (push and pull) screws that regulate the entrance lens when you observe a defocused star on a pc display screen.
We would have liked to collimate Paul’s instance and the changes required have been really tiny. We settled, in the long run, for a great however not strictly good 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 isn’t exactly machined and so have to be tightened up in small iterations to keep away from stressing the focuser shaft. Make sure you observe the directions fastidiously. Our first focus motor expired after just a few outings however its guarantee alternative has labored effectively.
First mild revealed appreciable tilt, giving us good star shapes within the centre however unacceptable elongation alongside the brief sides of the picture. Tilt typically comes from the digital camera, so we constructed a take a look at 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 effectively 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} brief, 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 lightweight gradient in our photographs, however this was traced to a tiny LED on the digital camera lighting up the underside of the cable information.
In spite of everything these changes, we had acceptable however not good star shapes on the edges of the chip. Stacking numerous sub-exposures improves them and, to be honest, Paul’s ASI 2600MC Professional digital camera has a diagonal just a few millimetres bigger than Celestron’s claimed circle. Imagers who wish to ‘pixel peep’ won’t like what they see, however I used to be pleased with what we had.
Your selection of digital camera is restricted to fashions that may match throughout 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 recent CMOS cameras are well-suited to the 400mm focal size.
Rekindling a ardour
Finally we are able to now flip to the absolutely fettled telescope’s efficiency underneath 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 could be absolutely exploited and the celebs saved underneath management when eliminated. In my opinion it might be a disgrace to course of RASA knowledge with out utilizing Star Xterminator or, maybe, its rival, Starnet++.
After 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 tons of of clear nights every year. If I have been combating cloudy climate and lightweight air pollution, I might be much more persuaded by this telescope. It has one other benefit, too: it really works finest when used to take giant numbers of brief exposures, which means that the sigma-stacking routines will remove satellite tv for pc and plane trails extra successfully. One final final bonus: it appears that evidently ‘mud bunnies’ – the shadows forged 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 might not be good, it could not swimsuit everybody, however it has most actually received me over. I discover it a relentless pleasure to make use of.
At a look
Kind: 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)
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