Logic dictates... 12" rebuild before equatorial platform?

[furrysocks2]

I recently retired my DIY 8.5 f/7.6 after finding a Revelation 12" f/5 on gumtree. I haven't actually been able to bring myself to remove the mirrors from the old scope yet.

In any case, the 12" is a beast (size and weight). Fits in the car with just one back seat down, so it's "portable" and I'm hoping to give it an outing over the next couple of weeks.

But I'd like it to be more portable - a suitcase mirror box perhaps, lightweight cage, truss design. Spotted a suitable upper cage assembly on astrobuysell last night... clamps, poles, focuser, secondary, etc. But would cost me more than I paid for the scope. And I'm buying EPs just now, so I can't justify that. I'd still have only half a scope, and I enjoy making stuff anyway.

 

I've just given up on my motorised EQ3 project and sold the ST102 - too much like hard work - EAA with a guide cam is not as much fun as it ought to be. I was never a visual observer really, but persuaded myself that the 12" should be the catalyst for that. And if I'm to go bigger later (and I have every intention to at some point), I better be able to justify that purchase with a bit more observing experience.

That said, I'd like to build an equatorial platform for the 12" - specifically for lunar imaging and the planets again, in time. And it would be a benefit for visual, too.

A 16" isn't going to appear in my life any time soon, so onward with the 12". The mirror is likely going to be heavier than my 8.5" and I intend not to replace the focuser and secondary if I can avoid it - purely on cost. But I'm not clued up enough to design first... it's likely to evolve.

 

The effort required to calculate and construct the equatorial platform is probably something I only want to do once. As I am planning to alter the construction of the dob itself, I might had better do that first. Alternatively, I could equally convince myself that given the subtleties involved in an equatorial platform build, my second build would be better than my first. Possibly not a great market for a probably-sub-par Revelation 12"-specific DIY equatorial platform, though. Setting myself up for two builds instead of just one might be foolhardy and I know how costs can grow but will try a bit harder this time.

 

I'm swithering... rebuild first, equatorial platform after?

 

 

[Beulah]

Enjoy the scope first whilst there are months ahead of generous astro-dark time. You'll be able to assess the mirror's performance. See how you feel around May time, when the nights move into much brighter times.

The EQ platform could be built first?

I started my 16" truss conversion at the wrong time and am regretting it a wee bit, as the other 12" can fit into the car with little persuasion and a lump hammer - yeh, I have a small car and went along the same logical path as you....need something more mobile and easy to carry around...

[furrysocks2]

2 minutes ago, Beulah said:

Enjoy the scope first whilst there are months ahead of generous astro-dark time. See how you feel around May time, when the nights move into much brighter times.

Aye, a rebuild is not a project for these months. I like having a pipeline, though.

 

[Beulah]

Yes, I have a pipeline too, lol. Think there's a binocular mount in there somewhere...

[furrysocks2]

I like the look of these 12" scopes by Jan van Gastel: http://members.ziggo.nl/jhm.vangastel/Astronomy/30cmscope/5_30cm.htm

Specifically, the way the trusses attach to the altitude bearings. I'm not big into the idea of a wire spider - expect to reuse the existing one. Undecided re: single/double ring for the upper cage. Uncertain how best to shroud the thing - perhaps just need to take it to darker skies, but for dew also...

 

From the upper cage assembly I saw on astrobuysell, I rather like the attachment mechanism.

I assume that the slot-blocks on the underside of the cage may need to be tilted, depending on the diameter and separation of the altitude bearings.

 

[furrysocks2]

Of course a camera could simply mount in the focuser, but a design based around a quick-release single-ring for visual and double-ring for prime-focus imaging could accommodate both. On an equatorial platform, this could be "fun" for planetary or bright-DSO imaging - it might provide a marginal reduction in obstruction and increase in effective aperture, and would eliminate any loss due to <100% reflectivity of the secondary. Gains are perhaps minimal, but I like the idea that one design could do both.

[Beulah]

I also looked at the extremely pared down scopes...they look fab.

I am not sure that I'd plump for something that reduced. And the wires fill me with fear...fair play to taking it to those extremes but I am not sure if I'd have the patience to collimate! how much shaking is there from the secondary when the scope is moved? As for obstruction, again I am not sure. If a normal spider edge is the thickness of the wires, what difference would there be? Would the tension from the strings be to much for the cage ring? (You're talking to an acoistic guitar builder who knows how much tension is in wire strings). I guess it's all been sussed out and the owners have no problems. It's great seeing such innovation in telescope design!

My design brief was for rigidity around the secondary and primary mirror (in the shape of a sturdy flex-ply cage with aluminium round tube 10 swg). The secondary is then protected and I will be designing a protective cloth bag. The mirror will be encased in a sturdy box. I've yet to install the lid.

My only concern is that my alt bearings are a little thin at 25mm thickness...we'll see how it goes.

Keep us up to date with your progress. I did start a build thread sometime back but gave up as progress is very slow due to an increased work schedule. Gotta eat, you know!

[furrysocks2]

Regarding the rocker design I posted above, the C-shaped altitude arcs are supported with lateral struts front and rear, but I have a slight concern that they could flex such that the front and rear truss attachment points on each side may move relative to one another. One intention is to incorporate an altitude encoder so at least one side may be a closed D-shape which will maintain the radius.

Intend to use the existing spider for the visual top-end if I can.

[furrysocks2]

I need to weigh the primary, measure the focal length, weight and measure the spider/secondary and focuser.

Then, decide whether I'm going to immediately replace the focuser with a dual-speed and build around its weight and travel instead.

It feels an easier proposition to build a box for primary and truss the secondary and then weigh/balance, and then attach the altitude bearings. By going with the design above, it feels like it might not be quite so simple to determine the geometry.

[furrysocks2]

Similar design principle at https://sites.google.com/site/thedobsonfactory3/...

As I see it, bringing the truss tubes further out from the typical mirror-box-corner position increases the "base" dimension of the truss triangles and for the front tubes at least, nevertheless decreases overall truss length. I do like the look of such a configuration.