Single-truss 12" Newtonian for an EQ mount

[GreatAttractor]

As I've just had a successful first light, I'd like to present my new main telescope.

I've been considering this kind of structure ever since I read about Lazzarotti's Gladius, but I had doubts if it could work in a Newtonian. That's what happens after watching too many pictures of high-end deep sky setups, with their multi-kilogram focusers, wide filter wheels and heavy cooled cameras. But – for the Sun, Moon and planets I only need a Barlow and my Chameleon (<200 grams in total).

I'm thankful to fellow astrophotographer wessel, who demonstrated that the approach with a quite narrow rectangular aluminium truss is rigid enough (mine is also 100x20x2 mm). Additionally, I've recently obtained a very cheap GSO 12" Dobsonian – all that's left was to start the work.

The stock solid tube has been converted by one of the previous owners into a 6-truss, but the lower OTA was still very heavy (and unlike wessel, I didn't have tube rings for it; new ones of this size are quite expensive). Then I thought I could use a cage welded from aluminium tubing (20x20x2 mm); the cost wasn't high, around 300 PLN (~£60) in a local welding shop.

I cut everything manually with a miter saw:

Trying on the primary mirror cell:

 

Primary collimation method is a reverse of GSO's: the screws are screwed into the cell from top, with knurled steel knobs screwed onto them from the bottom. I've also changed the springs from 1 mm to 1.4 mm:

(Recently I've got even thicker ones, perhaps I'll be able to ditch the locking screws altogether.)

Belatedly I realized that the locking screws have nowhere to screw into. I made tapped support pads for them out of a 20x6 mm beam (after anodizing I'll probably glue them to tha cage with epoxy):

The cage is attached to the main truss via four M8 screws (the front pair also crosses the dovetail). The dovetail (a massive WO piece) uses two more M6 pairs. Like in wessel's build, all these 8 screws pass through support blocks inside the main truss (cut manually from a 16x16 aluminium rod... phew, 7 minutes per cut), to prevent truss's deformation:

 

Cutting a 44 mm hole with a hand-held drill was rather noisy and not too smooth:

...but after some filing and sanding they're fine:

 

I'm not using the high and heavy stock GSO Crayford at the moment, instead I opted for a T2 base plate, my trusty non-rotating helical Borg and some T2 extenders:

The secondary holder is attached to an aluminum mounting ring using the 8 holes normally used for spider vanes:

I made the support ring/“spider” (40 mm wide) out of a 155 mm stainless steel stove pipe. Mounting support blocks:

 

The contraption seats on a SW HEQ5 (the counterweight shaft had to be extended):

The main truss is 1.5 m long.

 

Rigidity

I still have some flexure in secondary, apparently it's the GSO holder itself. Not serious: laser spot on the primary moves at most a few mm while the scope dramatically changes orientation. For a 40-50° movements the spot stays in place, so after pointing to a new area of the sky, I only have to slightly readjust the secondary (and then the primary).

The rest of the structure is sufficiently rigid. A spot from a laser mounted at the end of the main truss and pointing towards the primary stays firmly in place (as clearly seen thanks to surrounding dust specks on the mirror).

Mass

• primary mirror + cell: 7.4 kg
• mirror cage: 1.7 kg
• main truss + secondary w. support + dovetail + screws + T2 base plate: 3.3 kg
• 9x50 finder w. bracket (not yet mounted): 0.35 kg

total: 12.75 kg (28.1 lbs)

Impressions after the first astrophotography session

Overall it was fine, some Moon images can be found here. The night was calm, so I couldn't asses vulnerability to wind buffeting yet. As you can imagine, manual focusing (with a camera) with such a long lever is rather inconvenient, but doable (after a few attempts). It's much easier visually (I popped in my 9 mm Antares Ortho for a few moments before packing up), even at 500x. Anyway, I've already got a SW focuser motor and will eventually couple it with the Borg using a belt.

A roll of sleeping mat (just 25 cm wide) stuffed in the cage around the primary sufficed in the role of dew shield; no trace of dew over the entire 4-hour session. The secondary, on the other hand, started dewing over pretty quickly. Luckily, it took only 10 s to remove the dew film with a small 12 V hairdryer, and then ~20 s before the image got back to normal. In the end, I had to do it every 5-6 minutes; I could definitely use a secondary heater.

Mounting/dismounting: quick and easy. As most of the mass is concentrated around the COG, putting the scope on the mount was not much harder than with a Mak 180.

[Stu]

Great work, looks fabulous.

Tempted to do something similar myself at some point for visual.

[Uplooker]

Excellent bit of telescope engineering!

would you increase the rigidity of the truss design by adding a strutt along the top?

 

well done.

[Peter Drew]

Very nice job!. I have also made  very similar 12" f8 and 18" F3.5 single truss Newtonians. Interestingly I also used a circular secondary support, I fitted my secondary inboard of the support such that the support formed the baffle opposite the eyepiece. As "Uplooker" suggested, if you fit a strut just below the focuser and connect it to the Dec shaft it will greatly improve the stability.   

[GreatAttractor]

Thanks, guys.

9 hours ago, Stu said:

Tempted to do something similar myself at some point for visual.

Visual is doable, but note that the eyepiece is in a convenient position only when pointing in the meridian vicinity.

7 hours ago, Peter Drew said:

if you fit a strut just below the focuser and connect it to the Dec shaft it will greatly improve the stability.

Hm, you mean another attachment point to the mount? Probably not necessary, as I'll use remote motofocusing anyway. As for vibration damping, in wessel's thread someone suggested filling the beam with foam. I'll think about it if wind turns out to be a problem.

[Peter Drew]

I woud stilll strongly recommend at least trying a strut as proposed originally by Hargreaves. My 12" version was transformed by using one. If fitted to the end of the Dec shaft it forms a good triangulation.   

[GreatAttractor]

Ahh, a big triangle; now I get it. How about attaching the strut to a nice sliding ring that can be locked on the counterweight shaft? I'll working something out before Jupiter season.

Peter, can I read more about your scope somewhere?