Posts tagged 'Scope'

They Do It With (hyperbolic) Mirrors

Posted by on October 11th 2010 in Astrostuff, Shiny new kit, Thanks

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 🙁

A bit on the side

Posted by on October 18th 2009 in Astrostuff, Shiny new kit
Tags: ,

It took a while to summon up the courage, but eventually I got myself a little frac.
It's easy to mount and fits in nicely beside the big black 8-incher!






Knobs, flocking and scum

Posted by on March 12th 2009 in Astrostuff, Making stuff

The skies here have been bad for observing for many weeks, and on Monday there was no sign of it getting better during the next few days, so I decided that it was high time that I serviced the scope ready for the rest of the year. I knew that I was going to have to take the thing apart completely, so it was worth doing any major work in one go, rather than faff about doing it week-by-week.

The first job was the flocking of the inside of the tube. As supplied, the inside is sprayed with some sort of matt grey paint, and to be honest I've seen better paint jobs on Army Landrovers. Imagine Stevie Wonder repainting the stone-chips on your car using a paint-filled fire-extinguisher and you'll get some idea of what I'm on about. The paint does a mediocre job of cutting down on internal reflections, but it's not brilliant and if there are any scratches in it the reflected light can spoil an observing session.

So, I got hold of a couple of sheets of self-adhesive black flocking "paper" from Edmund Optics and set about dismantling the scope. The mirrors were safely boxed and stashed away from all danger, and the rest of the fittings were removed, labelled and stored accordingly. The flocking operation was fiddly but there were no major problems, aside from wrestling with the incredibly tenacious adhesive when the sheet wasn't quite in the right place.


A flocking brilliant job


The second job was the replacement of the collimation screws for the mirrors. I get annoyed that I need three tools to do the adjustments (the primary mirror needs a hex key for three grub-screws and a cross-head screwdriver for three countersunks, the secondary needs a different hex key for three other grubscrews) so I decided to replace the lot with something a bit more user-friendly. A lot of folk go in for the expense of "Bob's Knobs" which are thumbscrews and hence require no tools, but there's a problem at the back end of a Celestron Newtonian - the "pushmi pullyu" screw-pairings on the primary cell are pitched at only 12mm, so large-headed thumbscrews would clash. Besides, they're expensive for what they are, and I'm a cheapskate, so I opted for stainless Phillips panheads. The problem was, what size to get? I emailed the scope's manufacturer with the following question:

"Please can you tell me the thread sizes/specifications for the 3 sets of 3 collimation screws (6 on the primary, 3 on the secondary) on a 2007 C8N (the version with the thin-vane spider)?"

and received the following reply:

"Unfortunately the screws will be different because of various vendors that supply them. So we do not have specs on these.
Thank you,
Celestron Technical Services"

Wow, that was a lot of help. Not! Whatever happened to the idea of standardisation?

I had to resort to cadging some thread-gauges. M5 for the primary grubs, M4 for the secondary. A quick order to Stagonset resulted in the swift delivery of two dozen stainless screws various and change from a fiver. The six screws that I actually used (I didn't bother replacing the three cross-head countersunks) were 3-off M4 x 25mm @ 9p each and 3-off M5 x 12mm @ 12p each - simple, effective and way cheaper than posh knobs. Now the only tool required is the small Phillips screwdriver that was supplied with the scope. Sorted.


New screws for the primary

New screws for the secondary


Of course, using panheads instead of inset grubs for the primary meant that the rear mirror cover no longer fitted, so I ditched it and went for an open-ended setup. This is supposed to be a good thing anyway, as it allows quicker mirror-cooling and cuts down on thermal currents in the tube. The screw-holes for fixing the cover were opened out and tapped so that I could fit a set of rubber feet for the tube to stand on.


Showing some (gl)ass!


The last job before the rebuild was the cleaning of the mirrors. So many folk say that you needn't keep them fastidiously clean, but mine were caked with scum. They both had a bath in warm soapy water and a flush-off with distilled water before being rehoused. It was during that process that I confirmed that there was something odd about the primary mirror - the centre-marker wasn't central, as I've long suspected, as it explains why I've often struggled to get the scope collimated correctly. It needed to be fixed. I cut out a disc of paper the same size as the mirror and cut the centre out of it to make a template. Using it as a mask to protect the mirror, I gently peeled off the self-adhesive marker-ring and then stuck it back on in the right place - a full 5mm from where it had been. No wonder collimation had been a dodgy affair!

The properly-centred marker, complete with the scar where it was moved from.


Anyway, after the rebuild was complete I did a full laser-collimation within 5 minutes and the results were better than ever. All I need now is a clear night to do a star-test and all should be well.

Progress with the pier continues apace, but I'll tell you about that in a separate post.

Scope focuser upgraded

Posted by on September 10th 2008 in Astrostuff, Shiny new kit

I'd been considering upgrading the scope's basic rack & pinion focuser for some time. After various bouts of tinkering, I'd managed to reduce the "Synta Slop" image-shift in the original focuser as far as I could without major surgery, and was fairly happy that it was the best that I was going to get, the other option being to shell out £100+ on an upgrade that I couldn't justify.

Then I chanced upon a bargain Crayford-style focuser at a keen price from Astronomica. After a few emails to check that the thing would fit with the minimum of hassle, I took the plunge and ordered one.

And guess what? It's a little gem. OK, it's unbranded, but I'm not one for paying for a label. Silky-smooth action with no discernible image-shift, plenty of travel, enough friction to lift the D50, and the Skywatcher Auto-Focuser fits it as well. The only butchery required was the elongation of two of the mounting-holes in the scope OTA, but it was easy with the right kit.



I'll give it a thorough workout next time we get a clear night. Don't hold your breath, though - last night was the first (partially) clear one since we got back from Denmark, and the forecast says that it'll get worse before it gets better.

Playing catch-up

The blog's now well out-of-date, so it's time to do a bit of catching up. Luckily for you lot, I'm not a stats-whore, so I'll keep things brief and cram everything into one or two posts so that there's not a pile of individual entries to wade through. Now, where to begin...?

  • After a weekend-long posting-restriction imposed by the SGL forum admin, on Monday July 14th I was banned. And yes, the ban is justified. After some unjustified comments made by some of the admin, I reacted, broke the rules and got what I deserved. My crime was to react by removing the content from some of my posts. Just mine. No posts by other members were edited by me. The official version is different, and sounds much more sinister - according to the ban screen, I am banned for "attempting to corrupt the SGL database", which is a bit like saying that I've attempted to eradicate all life from the planet just because I swatted an annoying fly in our kitchen. Luckily, there's no sign that I'll be extradited to spend the rest of my days confined to the Guantanamo Bay facility. Anyway, I deserved the ban, and I don't much care that it'll never be lifted, but at least I'll no longer be hassled by some admins who think themselves beyond reproach and who think that they never have to apologise for anything, even when they have been proved wrong. It's sad that they get the good admins (who are in the majority) a bad press.
  • Talking of things astronomical, I had a bit of a panic a while back when one of the scope mount's tracking-motors packed in. After an online chat with the retailer, arrangements were made for the defective items to be investigated and replaced. Everything's now fixed and working fine, thanks to the great customer service provided by Adam at Rother Valley Optics (retailer) and by Optical Vision Ltd. (importer).
  • There was a clear night here a week or so ago, so I set up the scope to test out the replacement motors and to let the kids have a good look at Jupiter. They were suitably impressed with the view, seeing clearly-defined cloud-bands on the planet and seeing all four of the largest moons (Io, Europa, Ganymede, and Callisto - the Galilean satellites, after Galileo Galilei who discovered them in 1610). After sending the kids to bed, I returned to the scope and had a reasonable visual observing session, finding M8, NGC 6530, M20, M22, Cr 399, NGC 6822 and a few others. The ISS passed over twice during the session, and there were a few meteors around. I didn't set up for imaging, so there are no pics. Since then, it's been cloudy almost every night.
  • Regarding other outdoors activities, there's precious little to report. I missed out on the Outdoors Bloggers' Dartmoor Meet because it was the weekend of my birthday and I didn't feel that it would have been right to bugger off for the weekend. Suffice to say that I got loads of nice pressies. :mrgreen:
  • I managed to fit in a drinking session with my friend Dave, who's a casualty of the recent round of redundancies at Thwaites Dumpers. He's taking it well, but there are a few folk I know from there who've been hit hard by the job cuts. I hope that they all get sorted out soon.
  • Gear-wise, I've been quite reserved about splashing the cash. I supplemented the money that I received as birthday presents in order to buy a SkyWatcher Auto Focuser from Steve at FLO, so that I don't have to faff about twiddling the knob by hand (go on, you make the puns). Even though it's not really auto (it's a motor with a control-box on a curly lead), it's a great bit of kit. Fed up with buying numerous adapters for allowing all sorts of electrical gadgets to be powered or charged in the car, we got a power-inverter so that we can just use the standard mains chargers/transformers instead. I should have bought one of these years ago - I must have spent a small fortune on car-adapters in the last few years. We had a day shopping at Go Outdoors and I didn't get any outdoors gear for myself - that must be some sort of record! Mind you, we spent a small fortune kitting out the other three for our summer holiday.
  • Blog-wise, there have been a few changes here. A few more plugins have been added, if you're interested they're listed at The blogrolls have been updated - quite a few additions, some moves and a few deletions - if you spot any cock-ups, please feel free to let me know. I've a few more changes in the pipe-line, but I'll tell you about them another time.

Well, that's all for now. There's more, but it'll have to wait for later.

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