New GSO 10 inch RC - Page 4

gregbradley · #61 04-12-2009, 03:33 AM

Aluminium tube wouldn't be the end of the world but carbon fibre would be so much superior. One of the attractions of CF is stable focus.

My RCOS 12.5 inch with carbon fibre tube would retain the same focus point night after night. Amazing really.

However with colour "parfocal" filters being not really totally parfocal best practice would still be to refocus for each filter when imaging.

But if you use CCDsoft and you program in an imaging run of LRGB for 6 hours throughout the night its nice to know the scope will hold focus despite temperature shifts and perhaps you don't mind the slight difference of focus on RGB (its often very close but not perfect - ie Astrodons and Baaders, Astronomik filters).

Greg.

Paul Haese · #62 04-12-2009, 10:21 AM

I have to agree, my RC retains focus for nights on end. I go and check and there is not shift whatsoever. Carbon fibre is the way to go. Eric at the ASSA meeting the other night thought that going back to metal tubing was a budget issue. Carbon fibre tubes cost a lot to make. To me this is a mistake to not include carbon fibre tubes, they could raise the price a bit to cover the cost and everyone would still be happy.

Peter Ward · #63 04-12-2009, 02:23 PM

I've been advised by GSO that a carbon fibre tube would add around $A1000 to the cost of the RC10.

As I mentioned earlier, the front and rear cells of the RC10 are connected by two Aluminium Losmandy D-type plates.

These have no expansion joints and are bolted on tight....there is no way a CF tube will be any benefit in that configuration, other than perhaps being a tad lighter/stiffer.

RCOS mount their secondaries very differently, in either the carbon fibre turss or tube configurations only CF is used to mount the secondary, with no aluminium plates inbetween.

CF is also not the best material for refractor tubes. None other than Roland Christian at Astro-physics noted that a lens will shorten its focal length as it cools, hence aluminium tubes help compensate for this effect (by also thermally contracting)....this would not happen with CF.

Moon (James) · #64 04-12-2009, 04:10 PM

From Andrews:

Quote:

New RC-10" SDX astrograph. December airfreight shipment.
Available with white painted aluminium tube only. GSO confirms the carbon fibre tube version will be available about March, 2010.

dpastern (Dave Pastern) · #65 05-12-2009, 12:09 PM

Interesting.

Dave

Hagar (Doug) · #66 05-12-2009, 01:29 PM

Sounds to me like an excuse to increase the price considering the original specs for these scopes was to have a carbon fibre tube at the cost of the aluminium tubed version. The original prototypes came out with CF tubes.
Probable reason for the Al tube was to get the scope out before Christmas while waiting for the CF tubes to be manufactured.
CF is now a relatively cheap material and costs nothing like it used to cost some years back.
A classic example was the fishing rob industry. CF fly rods were only manufactured by premium rod makers at huge costs $1200+ for a 9' fly rod. With improvements in manufacturing techniques similar CF rods are now made for $150.

mick pinner · #67 05-12-2009, 01:57 PM

l agree Doug, $1000 extra for a C.F tube is utterly ridiculous regardless of it's precision.

dpastern (Dave Pastern) · #68 05-12-2009, 02:29 PM

Get used to it boys, it's called "profiteering". Apparently, in the name of 'free markets', it's acceptable. And it's socially unnacceptable to criticise it.

Dave

AlexN · #69 05-12-2009, 04:36 PM

buy the Aluminium one, convert the optics into a carbon fibre truss tube.. add a couple of grand for an instrument rotator, a FLI PDF focuser and some heaters to keep the optics dew free and then go photon bashing!!

cventer · #70 09-12-2009, 12:13 PM

Anyone got theirs yet ? I got note from Peter saying shippment had arrived but curious if anyone has it yet and what reports are on scope ?

Peter Ward · #71 09-12-2009, 12:37 PM

Quote:

Originally Posted by dpastern

Get used to it boys, it's called "profiteering". Apparently, in the name of 'free markets', it's acceptable. And it's socially unnacceptable to criticise it.

Dave

Admittedly these are a little fancier...

http://www.dreamscopes.com/pages/tubes-honeycomb.htm

Note the 12" OD tube price....$USD 1400

Waxing_Gibbous (Peter) · #72 09-12-2009, 02:13 PM

According to my business partner who is an actual, university graduated, certified, been employed by BAE aerospace and McLaren, composites engineer and licensed composite construction inspector,(smartarse), Mr. Christen is correct.
As CF does not expand or contract (to any meaningful degree). unless weight is the primary factor, any gains in rigidity will be outweighed by possible distortion of the optical train. This should not really be a factor at <130mm, but above that, it will start to become noticable unless the lens / mirror cell is well isolated. Preferably in Al or Ti which have roughly the same expansion properties as most glasses used in telescope making.
The larger the lens or mirror, the greater the effect will be, by optical standards anyway, you wouldn't notice it in real life.
Also it costs roughly the same to produce a CF tube as an Aluminium one, depending on the quality, but the rejection rate is higher with CF (bubbles and such in the resin etc etc), hence the higher price.
I'll take his word for it.

Peter Ward · #73 09-12-2009, 02:47 PM

Quote:

Originally Posted by Waxing_Gibbous

According to my business partner...........
.......... it costs roughly the same to produce a CF tube as an Aluminium one, ..................I'll take his word for it.

While this *might* be true for small OD tubing a quick web search of CF suppliers shown this is not the reality for larger OD's. eg.

http://www.carbonfibreexpress.com/index.php?cPath=22_41

I suspect there is not a great demand for large diameter CF tubing, as can be seen above, a good length of 70mm OD runs around $US960.00 .
300mm OD is not even listed.

Waxing_Gibbous (Peter) · #74 09-12-2009, 03:18 PM

Quote:

Originally Posted by Peter Ward

While this *might* be true for small OD tubing a quick web search of CF suppliers shown this is not the reality for larger OD's. eg.

http://www.carbonfibreexpress.com/index.php?cPath=22_41

I suspect there is not a great demand for large diameter CF tubing, as can be seen above, a good length of 70mm OD runs around $US960.00 .
300mm OD is not even listed.

Sir is partly correct. If this humble servant can find the flippin' website, this humble servant will direct Sir to a firm that makes CF tubes for the oil & gas industry, where this humble servant notes there is rather keen demand for bloody-great CF tubing, begging Sir's pardon of course.

These, this insignificant worm, dares to point out, are a lot more rugged and and a darn sight cheaper ( about 1/2) than those available from "boutique" composite manufacturers.
This unworthy one will post the URL as soon as this unworthy one can find it.

Peter Ward · #75 09-12-2009, 03:40 PM

Quote:

Originally Posted by Waxing_Gibbous

Sir is partly correct. If this humble servant can find the flippin' website, this humble servant will direct Sir to a firm that makes CF tubes for the oil & gas industry, where this humble servant notes there is rather keen demand for bloody-great CF tubing, begging Sir's pardon of course.

....

Sorry, I don't get the need for sarcasm.

You'd probably need around 2mm wall thickness, 300mm OD...and given you'd probably not want your telescope to look like an industrial oil pipe...a nice high gloss cosmetic finish.

As I mentioned way earlier...GSO can get them, but the OEM cost will add around $US700 to the price per OTA....given the benefits, a needless expense IMHO.

If it was $100-200 it would be a no brainer... the CF version would look cool if nothing else.

Waxing_Gibbous (Peter) · #76 09-12-2009, 03:55 PM

Wasn't sarcasm. Sorry.
Actually some do have a nice high-gloss finish, inside and out, - normally white, zinc based coating as they spend most of their life in the sun.
The other stuff they use for drilling is not as nicely finished, but still pretty good.
My point being you don't HAVE to shell out a fortune for good CF. But really, Aluminium is as good or better for most applications.

AlexN · #77 09-12-2009, 05:23 PM

A mate of mine and I were discussing this just last night, the thermal expansion/contraction of an aluminium tube the size that we would guestimate the GSO RC10 has would be, Given a 20°c temperature change, the worst case scenario (assuming very poor quality Aluminium) was a contraction 45 microns.

The F/8 optics of the GSO RC10 give a CFZ (Critical Focus Zone) of 140 microns. Hence, given you focus well at the start of the imaging run, and don't experience more than a 20°c drop in temperature during the imaging run, your focus would not noticeably drift over the course of the run... Its a different story when you're imaging with a fast system, Ie, an F/4 scope with a CFZ of 35 microns.. but with the slower ratio's, your focus would very rarely shift.. not before you hit the meridian and have to flip the mount anyway, at which point, how hard would it be to just touch up the focus before recommencing the run after the flip?

Yes, CF would be lighter, Yes it would likely (depending on quality/thickness etc) be more rigid.. As Peter said, It sure does look cool... but will it make a noticeable difference in focus during the course of your average imaging run... Probably not...

If we were talking about a fast refractor, MAYBE. but then, even the glass in the objective lens will contract/expand during big temperature changes, causing the focus point to shift anyway, regardless of what the OTA is made of...

Yes Saying "0 focus drift due to temperature is a great way to sell telescopes" In most cases, it actually MEANS nothing...

Bassnut (Fred) · #78 09-12-2009, 06:01 PM

I have 2 OTAs, one Aluminium (12" SCT), the other CF (10" RC). The Aluminium tube requires focusing every time I use it and during a typical 6 hr imaging night. I usually dont bother refocusing during the night. The Aluminium tube SCT sub stars get noticabley blobier as the night wears on, especially if there is a large temp change.

Even though I always focus the CF RC on start up, I notice say with a typical 20000 count focus position, the new focus is never more than 10-30 counts off, and there is no visible change. The focus also does not change visibly during the night. Weeks can go by, with large temp changes, and the focus does not change visibly on reuse.

Unless there are other reasons for the SCT focus to change (the mirror is always hard locked, never changed), anecdotally at least, it seems to me, the CF tube is much better. Apart from other differences in these 2 OTAs, the focus in subs are always more consistant with the CF tube.

Having said that, with regular refocusing, the aluminium tube is fine, but unless you are fairly keen or have automated scheduled autofocus, regular manual refocus during long image runs can be pretty tedious, this is where the CF tube becomes very handy.

bmitchell82 (Brendan) · #79 09-12-2009, 06:09 PM

just something small fred, i notice you have a RCOS 10" and a Meade lx200r... they are in two totally different leagues! Meade made in mexico RCOS are hand made instruments (so im lead to belive). its like saying a skywatcher can proform just as good as a Takahashi Mewlon..? just my thoughts on that matter!

Paul Haese · #80 09-12-2009, 06:23 PM

I reiterate and agree totally with Fred. The 8" I have has not had to have the focus changed in 4 imaging sessions. I check of course on each occassion and mid way through the run check again. It just does not move. My Tak scope on the other hand has to be focused each session and has to have it changed during a session on some occassions. Coefficiency of linear expansion would suggest a metal tube will move if the temperature changes by as little as 5 degrees and it progressively moves more and more as the temperature changes. 20 degrees will have quite a large change in focus. Personally, I would be prepared to pay the extra.