They Do It With (hyperbolic) Mirrors

Posted by @ 10:09 pm on Monday 11th October, 2010.

I wouldn't have bought this if it hadn't been on offer and if I hadn't managed to get a further significant discount.
It takes up less room on the mount than the big black 8-incher does and it doesn't catch the breeze so much, so it's a lot more stable.
Add to that the facts that it's lighter, more portable and gives a much bigger flat-field for imaging, and it was a no-brainer.



For those with an interest in such things, it's a GSO GSRC6M 6" f/9 Ritchey-Chrétien Astrograph as supplied by Teleskop Service (as opposed to the Astro-Tech version marketed by Astronomics). The Ritchey-Chrétien design is favoured by many professional observatories (including the Hubble Space Telescope) and by some high-end amateurs for many reasons (the absence of any refractive elements, the fixed primary mirror, the coma-free image capability etc.) but until recently they had been expensive beasts compared to other Cassegrain designs. I've wanted one for many years and when the chance to get one came along I grabbed it with both hands.

FWIW, here are some of the specs:

  • Design: True RC (Ritchey-Chrétien) with a hyperbolic primary and a hyperbolic secondary mirror. No glass corrector plates or lenses in the optical train
  • Aperture: 6" (152mm), Focal Length 1370mm, Focal Ratio f/9
  • Primary Mirror: BK7- surface quality 1/12 Lambda or better, 99% dielectric high-reflectivity coating
  • Secondary Mirror: BK7, 99% dielectric high-reflectivity coating. Robust collimatable cell. Complete obstruction = 77mm
  • Construction: Steel tube with alloy primary and secondary mirror cells. Total weight 5.4kg
  • Focuser: Axially-rotatable 1:10 dual-speed Crayford focuser for extremely smooth focusing with no image-shift, accepts 2" and 1.25" accessories

Just in case you were wondering, it is currently on offer discounted from 898 Eur to 499 Eur including tax... suffice to say that a polite request to TS resulted in a favourable deal at a much-reduced total cost and including a GSRCV50 50mm spacer placed between the focuser and the telescope. At this point I must thank Wolfi Ransburg of TS for the great deal - thanks, Wolfi!

Typically, we've had cloudy nights here ever since the thing arrived 🙁

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4 Responses to “They Do It With (hyperbolic) Mirrors”

  1. Fraser says:

    Well it's certainly impressive looking. How many scopes do you have now?

    Any tips for the first time scope buyer? Was thinking about a Skywatcher Explorer 130P...

  2. BG! says:

    Only three scopes, Fraser. I can't think that I'll ever need any more.

    Tips... hmm, let's see...

    Much depends on what you want to see. With most scopes you won't see big planets, colourful nebulae or detailed galaxies - such images are usually digitally-derived. Regardless, it's worth bearing in mind that an all-rounder scope will give a taste of many things but won't excel at anything. Speciality scopes that are good for, say, planets, are generally bad for galaxies (and vice versa). Some cheap scopes are diamonds and some expensive scopes are lemons. 'Tis a minefield!

    And then there's the mount to consider - a good scope on a crap mount won't perform well. Driven "equatorial" mounts are preferred as they track the sky without the effect of "field rotation" as per "alt-az" mounts. AstroTracs are becoming increasingly popular with folk who prefer to use a DSLR with photo-lenses rather than scopes, and who prefer a portable setup. These gizmos will also accept small scopes.

    If you are into imaging, you need to match kit to target - for planets use stacked short exposures from a webcam, for wider fields use a DSLR or a dedicated astro CCD camera for longer exposures.

    Probably the best tip I could give to a first-time buyer is to join a forum and ask the question there. I recommend Astro-Chat but there are many others. There, you'll find folk who have much experience of a wide range of kit.

    If I had to start again from scratch, I'd probably opt for the AstroTrac with my DSLR and then later add a small apochromatic refractor, then work up to what I have now.

    Be careful, though. This hobby will soon empty your wallet if you let it!

    Hope that helps.

  3. Fraser says:

    I was actually thinking of buying as a gift for my SO. I doubt she'd be interested in imaging, but I might. I think the mount for the 130 can take a motor addon. I'll check out Astro-chat, hadn't seen that one before...

  4. Hello man, this is very appealing! I've only discussed this on Facebook! This is actually a good read. I believe you might find the Skywatcher 130P webpage interesting!

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