Well I've come to the inevitable conclusion...MORE!
I've currently got a Skywatcher Black Diamond 8" f/5 newt on an EQ6 and I have aperture envy. I was toying with the idea of building a 10" f/4 with GSO mirrors, but then (predictably) I noticed that the 12" f/5 mirrors are almost the same price. My first thought was
"M42 barely fits in my 1000mm scope, am I going to be limited to smaller DSOs with 1500mm?". Obviously the term 'limited' is relative, I'm sure there's no shortage of joy to be had with 1500mm. Then an evil thought occurred to me... 12" f/4? Best of both worlds?
I want to build something permanent, something I won't have to upgrade any time soon and I don't mind spending a few months perfecting the design before I go nuts on the credit card.
What are the essentials? Ive been looking at the commercial scopes and using them as a catalogue to get ideas, Astrograph by numbers if you will.
I'm an Aircraft Structural Maintenance Engineer so precision fabrication and attention to detail come as standard
Any experience, suggestions, advice and warnings are gratefully accepted.
There's a program out there for setting the baffles size and placement, ill try and track it down. I think there was someone on here who also did a carbon fiber mirror holder as well. Sure the experts will respond anyhow http://www.dalekeller.net/ATM/newton...t/newtsoft.htm
Thanks for the information everyone, I can see I have a lot of reading to do between the links on here and the PMs. I'm glad there's no shortage of information and people willing to help.
I've been trying to add up the weight of all my accessories and see how much payload is left over for the OTA, that will ultimately be the most limiting factor in the build, especially considering the 12" mirrors are 3.5kgs heavier than the 10".
I would definitely recommend the 12" F4, but there are a few things to consider.
you'll need a coma corrector, precise collimation, the cats eye tools do the job well, a 2.5 to 3 focuser as the light cone is stumpier and you'd have vignetting at the base of the focuser.
it also helps to use a low profile focuser as that reduces the width of the cone at the base of the focuser by moving the focal point inward.
you'll need to leave sufficient room for a filter wheel, OAG as well.
you'll need a larger secondary, and a coma corrector for 2.5 to 3" focusers.
an OAG would be much better than a standard guidescope and guide cam as it eliminates flexure related issues.
for the OTA, it'll need to be very rigid with minimal flexure, and I would recommend either a carbon fibre OTA or a serrurier truss.
i've just completed a 10" F4 serrurier truss and it was inspired by Rolf Olsen's 10" F5 serrurier.
Advantages are very good rigidity, weight savings and ease of fabrication.
my 10"F4 with a guide camera, QHY8 CCD, dovetail, mirrors, etc, weighs 11kg's. I'm replacing the trusses with carbon fibre, so it should end up at 10kg.
Rolf is currently building a 12" F4.7 serrurier as well.
With your structural Engineering knowledge, it shouldn't be too difficult.
Look forward to your build progress.
James has done a good review of the GSO 12" F4 that comes at a very good price with a 3" focuser.
you could actually buy that, ditch the steel tube and build your own as you'd have most of what you need already. http://deepspaceplace.com/at12in.php
Thanks Alistairsam, I calculated (always a dangerous thing for me) a 12" f/4 with 450D, OAG, SSAG, low profile 2" focuser, carbon tube, carbon composite primary cell and aluminium spider it should come out at 12-14kgs, which is the absolute max I would push my EQ6 to. The 10" f/4 is 3.5kgs lighter.
Because my estimated weight is so close to the max payload I'm leaning toward the 10" f/4 but I don't really want to compromise, all the weight will be slung under the OTA towards the mount to minimise it's 'moment', I can do away with my guides cope in favour of an OAG (something I can't do now because the DSLR takes up so much backfocus), and luckily 'light and strong' is something we do quite well in aviation so I'm confident I can design something rigid enough an light enough.
Thanks for all the advice guys, it's all very welcome.
Hi
12-14kgs for a 12" F4 is very good. would you be purchasing a CF tube or fabricating?
I'm sure the EQ6 can handle well over 20kg's. especially if on a pier.
"Out of the box, the EQ6 is designed to accommodate telescope payloads up to 40lbs" http://www.skywatcher.com/swtinc/pro...1=3&class2=302
the HEQ5 pro has a max payload of 13.5kgs but 12kg's would be ideal.
the EQ6 can handle 14kgs very comfortably. so I don't think you should restrict based on that assumption.
edit: and I don't think the 2" focuser is suited for a 12" F4. just run it by the "NEWT" online calculator http://stellafane.org/tm/newt-web/newt-web.html
is aluminium for a spider a good thing? it could warp the ota when it cools down in the night. I know RC's use milled Aluminium spiders, but not sure if they have issues.
I'll be fabricating a tube. Basically the rough plan is to make a 50" tube of four layers of 4x4 in alternate directions, the primary cell will be a sandwich construction of carbon and balsa wood with threaded inserts for the collimation screws. As carbon fibre has a tendency of corroding any metal it touches it will all be installed 'wet' with sealant, which should also help prevent vibration.
I don't know what I'm looking at with Newt Online, the parts I do understand are very informative however. Would someone who knows what they're looking at care to give me some feedback on this setup please?
I don't really understand the vignetting of the 75% ray? The 100% area is almost the same size as the sensor in my 450D so does it matter that the 75% ray is cut off a little?
Last edited by LAW; 28-12-2012 at 02:54 PM.
Reason: updated newtonline data
Haven't had a play with it myself yet but need to, did you try a smaller secondary?
A smaller secondary for an astrograph would be a bad idea, I think that he is better off increasing the size of the secondary to "future proof" himself for when he hooks this beasty to a 11002 chip.
Also the 100% rays are all you need to worry about, vignetting of the 75% rays dont mean anything if you are fully illuminating the chip. If you are planning on constructing a tube for this a 13 inch diameter for a 12 inch scope is a little ambitious with the cell and room to tilt the mirror for collimation.
A smaller secondary for an astrograph would be a bad idea, I think that he is better off increasing the size of the secondary to "future proof" himself for when he hooks this beasty to a 11002 chip.
Also the 100% rays are all you need to worry about, vignetting of the 75% rays dont mean anything if you are fully illuminating the chip. If you are planning on constructing a tube for this a 13 inch diameter for a 12 inch scope is a little ambitious with the cell and room to tilt the mirror for collimation.
Thanks Jay and Peter, I wasn't sure how big the tube should be around the mirror, I just put in 13" because it was bigger than 12"
The 88mm secondary is the one that comes in the GSO 12" f/4 kit, and it's the biggest they make. I think I'll happily continue playing with my DSLRs for now, a dedicated astro CCD isn't really on the horizon for me at the moment.
Would a bigger secondary move the focuser further down (shorten) the tube?
hi
yep a smaller secondary is not a good idea. I'm pretty sure the size of the secondary won't change the distance to the focuser from the primary. Just how much it is illuminated.
I'd suggest increasing the front aperture until that error disappears. working with metric units help.
your dslr should have a white line on the body that shows the position of the sensor. so best to calculate distance required with that as the reference.
in my case, I calculated the positions as much as possible, and then i built a jig with the primary fixed and the upper assembly adjustable.
I then went to an open area from where I had visibility of some buildings and trees two km's away. I put in the dslr and racked all the way in and then out about 5mm, focused on the distant object and marked the position of my UTA.
the qhy8 ccd focal point was well out. so I'm able to use either the dslr or the ccd on the same scope without moving the primary.
Location: '34 South' Young Hilltops LGA, Australia
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Don't do it
Quote:
Originally Posted by LAW
Thanks Jay and Peter, I wasn't sure how big the tube should be around the mirror, I just put in 13" because it was bigger than 12"
The 88mm secondary is the one that comes in the GSO 12" f/4 kit, and it's the biggest they make. I think I'll happily continue playing with my DSLRs for now, a dedicated astro CCD isn't really on the horizon for me at the moment.
Would a bigger secondary move the focuser further down (shorten) the tube?
Locating the primary mirror cell as close to the end of the tube as possible will shorten the tube.
On its own, a big secondary mirror size doesn't change the tube length by any significant amount. However using a big secondary and moving the focus out further will shorten the tube by the same distance you move the focus out away from but this isn't a great design strategy. You risk vignetting and are moving the weight of the camera out further possibly creating balance and flexure problems. Having weight hanging a long way off the tube makes balancing difficult and that long distance can cause flexure on the tube around the base of the focusser due to the torque imparted by the long focus distance. This is only going to save you a small amount tube length at most. You may as well design it with just the minimum amount of focus distance required for current (and future) cameras and avoid other problems that may be created by artificially trying to shorten the tube.
Locating the primary mirror cell as close to the end of the tube as possible will shorten the tube.
On its own, a big secondary mirror size doesn't change the tube length by any significant amount. However using a big secondary and moving the focus out further will shorten the tube by the same distance you move the focus out away from but this isn't a great design strategy. You risk vignetting and are moving the weight of the camera out further possibly creating balance and flexure problems. Having weight hanging a long way off the tube makes balancing difficult and that long distance can cause flexure on the tube around the base of the focusser due to the torque imparted by the long focus distance. This is only going to save you a small amount tube length at most. You may as well design it with just the minimum amount of focus distance required for current (and future) cameras and avoid other problems that may be created by artificially trying to shorten the tube.
Joe
Thanks Joe, that's exactly what I thought might happen, I was just wondering if I had to shorten the tube with a bigger secondary or if I could leave the distance to the focuser the same. Newt Online was already giving me vignetting on the 100% ray with a taller focuser so I didn't really want to make it any longer if I could help it, but the 88mm secondary is the biggest I can go so it's not really a problem.
At the moment I'm trading tube aperture for illumination as the wider tube is making the light cone narrower at the focal plane. I'm sure I'll get my head around this eventually
What sort of objects are you planning to shoot Murphy? Have you played around with software like CCDCalc to get an idea what your new FOV is likely to be with a larger scope?
Pro: More light, faster subs, better resolution (hopefully)
Cons: Tighter FOV, tougher alignment if go to shorter Focal L, increased weight and focal length make tracking a LOT less fun.
Buying or building a light high quality short focal length astrograph is a serious undertaking that warrants plenty of up front research....
a 12" f/4 with 450D, OAG, SSAG, low profile 2" focuser, carbon tube, carbon composite primary cell and aluminium spider it should come out at 12-14kgs, which is the absolute max I would push my EQ6 to.
Murphy, I got a GSO 12" F4 steel tube newt on an EQ6 with almost 3Kgs of camera/wheels/filters/OAG on the focuser. It comes with an 88mm secondary as standard kit. It's very early days with this combo for me but it seems to produce round stars from inside a dome with 25Kgs of counterweights on the shaft. I'm waiting on adapters before introducing an 800gram corrector into the setup. I think the bigger the mirror the better if the mount can carry it.
Murph.
good project.
I do have reservations though. I too need to have you establish what you are trying to achieve. If it's a project, then why not simply produce a tube for the current 8" f5?
If that doesn't appeal, then I'd say get the 10" f4 mirror set and C/F a tube around that. I have a 10" f5, in a home-brew C/F tube, and it rides on an NEQ6, it is as big as I think I'd want to go. I know Robin has gone to the 12", and being F4 makes it the same length or so as mine, but I'm happy with mine.
Bigger mirror will simply give a longer focal length, all other things being the same. If you need more focal length for galaxies etc, then great, but sometimes a wider field can be handy, so a focal length of about 800mm is good. This is the reason a 10" F4 is so good as an all rounder. And getting a corrector/reducer like Robin has gives a very versatile rig.
If costs are something you need to consider, then go smaller than larger, say a 10" F4, and spend on a better focuser, plus corrector or corrector/reducer. Definitely build the C/F tube though.
Just some thoughts to ponder.
Gary
Thanks guys, I'm actually really enjoying all the different approaches to this project. I guess at the moment I'm mostly imaging larger targets like M42 and the region around Alnitak. But I'm starting to chase galaxies, NGC253 got me hooked on galaxies and I've been looking through the Formax group (without much luck) for a few weeks.
I do enjoy the wider fov of the 1000mm and I was worried about going to 1200 or 1500mm. The extra aperture of the 12" is tempting but lately I've been thinking the 10" f/4 might be a better imaging scale...? Although M42 and M45 are probably the only DSO that won't fit in a 1200mm fov so it's not really a big problem, hmm, decisions decisions.
Gbeal: http://www.iceinspace.com.au/forum/a...se.php?a=82911 that is gorgeous! That's almost exactly what I was planning on building. I was going to use two balsa wood core sandwich triangles with threaded inserts for the collimation screws.
Do you have any more pics of your setup or the build?
Murph,
it was a simple cut-out of two sections of "foam core", at least that's what I think it was called. It was about (I can measure a scrap I think I have left over), 20mm or maybe 25mm thick. I cut them out, and fitted/glued in the threaded inserts at the appropriate places for the collimation bolts as well as the tube attachment bolts. Then simply pained on the resin and laid some scrap C/F weave across. Another coat of resin after I smoothed off the splinters of weave sticking through the resin and it was done. Not elegant, but it works.
Gary
