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Alpollo

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Here is the telescope project I am currently working on.

The big picture is to have a 900mm Newtonian on a Bailey split ring equatorial mount. The rig will be used in a backyard mini remote observatory and also be portable. I may put it on a small trailer I have, I'll have see how "portable" it is first. I might be able to just put it in the back of my SUV if it's not too heavy, then just mount light duty wheels on it for carting it around the yard.

Most of the time it will reside in the backyard, under cover of a mini remote observatory, well that would be nice anyways. At the very least it will have a small enclosure to protected from the elements and to keep it ready for imaging.

I won't go into details on how I am going to control everything, because I don't know yet.

I do have a generally layout for the scope cobbled together. It is a Serrurier truss ~900mm f/6 150mm. I'm going to try to keep the trusses symmetrical front to back, but the COG may change depending on how things weigh up.

When my mirror blank gets here I will be able to start building my mirror cell. I've already got that part more or less figured out. I'll fine tune the drawing based off of actual measurements, right now I've just drawn a 6" nominal blank.  It is a moving mirror frame with roller edge support like what Lockwood and JPastro use on their big mirrors.  I've just made mine under-hung to line the COG of the mirror over the collimation points.

I am going to grind the primary and fashion a secondary elliptical mirror from Zerodur flats.

It has a 1.2" /1.92° diagonal 100% illuminated area which will cover a cropped DSLR sensor using a 2" secondary. The central obstruction is 33%.

I'm flip-fopping on the focuser right now. Ideally it's going to be stepper driven kind of flat Crayford dovetail sled thing. The nice thing about this concept is I don't need a T-ring or anything, perhaps just a field stop baffle, the camera would be mounted by the 1/4"-20 connection on the camera. This plan might be temporarily diverted in favour of a simple manual helix focuser. My DSLR is a Sony, so I can't go crazy just yet until I know how to get the Sony working with autofocus, remote imaging etc, if it's even possible. It's looking like I'll have to get a new camera down the road at some point and build the custom sled then. This means I will probably make the front half of the OTA removable to accommodate an upgrade, which is less then ideal but perhaps the pragmatic thing to do. The helix and T-ring would make attaching a collimation cap possible, I wasn't sure how I was going to collimate the DSLR on a sled using live view, though I'm think something could be sorted out when I go that route.

The focal ratio was chosen as a balance of field of view, tracking abilities, ease of mirror fabrication and dependencies on correcting optics. I was originally considering f/4 or f/5, but this requires a higher degree of skill and effort to make the mirror and makes the mirror tied to a comma corrector. I would prefer to have all reflecting optics only. I don't want my fancy pants mirror to be dependant on a refracting lens, that would be blasphemy. lol. I'll save the coma corrector purchase for a big fast visual scope one day, my mirror is small and not visual so why battle coma. Also with f/6 I'm hoping is going to have a decent field curvature plane vs a faster mirror for my DSLR sensor.

It will be made from 17.5mm/~3/4" Baltic Birch plywood.

The OTA below is ~900mm/36" long and 305mm/12" in diameter.

Baffling will be added and fine tuned later on. I'm really only concerned with 100% FIF but in reality it's very close to the 75% illuminated diameter in this configuration, so will just go the the 75% baffle diameters that Newt for the web gives me.

It is a fun project.

Proto.jpg

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Hi Alpollo

a 900mm aperture split ring? That's going to be some telescope. 

I built a 12" f/6 split ring some while ago and there were some lessons that had to be learnt:

Design around the centre of mass from the start. You will be surprised how close your mirror and pivots are. It needs careful weight and distance measurement but its worth it in a simple spreadsheet. 

This means your trusses will be highly asymmetric. Ot that you might care to introduce a central rigid box and shorter trusses either end.

I also found that the plywood top ring was very easy to build heavy - I built it as a rigid two-tier cake with focuser mount in-between. Then re-built it it with a single top ring and suspended focuser. Over time this has warped slightly. In other words it needs to be both very stiff and very light to not skew your balance and not go out of shape. 

There's some pictures here www.skybadger.net    but I'm sur you have looked at lots already.

Good luck 

Mike

 

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 "I do have a generally layout for the scope cobbled together. It is a Serrurier truss ~900mm f/6 150mm. I'm going to try to keep the trusses symmetrical front to back, but the COG may change depending on how things weigh up. "

 "It has a 1.2" /1.92° diagonal 100% illuminated area which will cover a cropped DSLR sensor using a 2" secondary. The central obstruction is 33%. "

 

He is doing a 150mm diameter telescope. The 900mm refers to the focal length.:icon_biggrin:

Nigel

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Yes it is just a little guy.

I have come across the Skybadger mount quite a few times, nice work. I look forward to seeing how it works out with the RA motor.

Originally this was going to be based around a 12" primary, but the physical size pretty much required a full size observatory. I would like a full size Obsy one day, but it's not feasible at this time.  Hence I thought I could fill the void with a little remote robotic observatory that only stands 4'-5' high.

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Doh!

Thanks Nigel . 

The scope I built was mobile with some heft and the wheel barrow wheel at the front. Bet one night I hefted, ran into a bump and then it went over the top. So the mount is in bits while I replace the horseshoe disk. I'v taken the opportunity to move the CoG so should be able to remove some counterweights.

I'll finish that one I've finished my ronchi double pass liquid mirror experiments...

Alpollo, The idea seems really good. Have you thought about folding a slightly larger scope to keep your size down while allowing a larger aperture ?

One of the projects on my books is a 15" f/8 that's going to be too long without folding... 

Regards

Mike

 

 

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I might do something like a folding OTA for a larger visual scope.

This scope is more of an "on demand" type of thing, so even if I had a way to fold it robotically, I suspect it would still need collimating.

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That isn't too bad, I could live with that balance point. 

That looks like an awesome scope, are you still using it?

I think my mirror only weighs about a pound, it should be here this week.

 

Hey SkyBadger, are you still using your mirror grinding machine? Would you build that style again? Would you do anything differently? I am for sure going to build a turntable, I'm just undecided if I am going to animate the over arm or not.

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