A new 16" F4 has first light

[Satchmo]

Just thought I would share this picture. Peter Bobroff in Canberra has sent an early picture of his 16" F4 all aluminium scope ( with Deep Sky Optics mirror ). This is the third in a series of similar design , the first two being 20" F5 ( Galaxy optics mirror ) and 16" F5 ( Deep Sky Optics mirror ) for Patrick Purcell. He reports that images are indistiguishable in quality from the F5.

He reports that his wife Judy, for whom the scope was built, can observe at the Zenith without steps. You'll notice also that the scope can be used from a seat by simply swinging the scope over the zenith. The eyepiece then angles down at 45 degrees for seated viewing :thumbsup: . The mirror is edge supported for either orientation. A shroud is in progress. If the scope moves anything like the F5 , it will be very stable and smooth. Hope it will be at the SPSP in April. Well done , Peter.

[Omaroo]

A fantastic effort Peter. Looks incredibly professional and sturdy. Mark - could Peter be a source of bino scope components?

Cheers
Chris

[paninaro]

Nice design, I like the full circle Altitude rings, looks more balanced than the semi-circles.

[iceman]

Wow what a piece of art. Brilliant.

[Satchmo]

G'day Chris. Peter obviously enjoys aluminium welding. He'll be at the SPSP, so have a word to him there ;)

While I'm at it , heres a picture Patrick Purcell with his 16" F5 , Peters previous all aluminium design. Note the eyepiece is in 45 degree downward mode. Patrick may well have had a big hand in building it too, I don't know the details. Canberra sure has a little pocket of innovative ATMing going on there.

[stringscope]

Having used both Peter's 20" and Patricks 16", I can say they are both delightful telescopes. Patrick's 16' is later generation and has particularly nice motions in both axis. Hopefully I will see the new one this weekend.

The down side of this type of design is that they are pretty bulky to transport.

BTW notice both 16"ers have curved spider vanes.

Hi Mark, I thought Patrick's was F5.5?? I guess at F4 a Paracor will live virtually permantly in the focuser.

Cheers,

[Satchmo]

I'm a big fan of curved spiders.

No Patricks is an F5. I believe Peter has a Paracor for the F4 but also that the 2X barlow works very well to sharpen up the field edges too.

[wavelandscott]

That sure is a pretty scope!

[Peter Bobroff]

The mirror cell is just glued with Bostik Rhodorsil V2 - an RTV silicone. There are no mechanical fastenings between the triangles and base. There are no edge supports at all although I plan to add three safety supports which won't touch the mirror.

It took a week to dry and firm up but all seems to be fine now. I haven't tried this before.

[Omaroo]

Very interesting technique Peter. What led you down this road rather than simply bolting it in place? I gather that your collimation adjustment comes from moving the entire assembly rather than just the mirror cell? I notice the three welded nuts on the periphery of the frame.

Will you be bringing this instrument to SPSP? I'm very keen to gather as many ideas as I can before I tackle the 12" bino, and I'm sure I'll learn a lot from what you've accomplished.

Cheers
Chris

[iceman]

Welcome Peter! Look forward to meeting you at SPSP!

[Peter Bobroff]

I wanted to fit the cell within a box made of 100mmx25mm rectangular section - so only 100mm thick. This would be easy and safe to handle with one hand with the weight close to the body.

The cell had to allow "over the top" viewing - so no lateral movement either way. I didn't want to confine the mirror between opposing edge supports.

Glueing offered the potential of no slack, backlash or movement at all, but with still the ability to cope with thermal expansion.

I wanted the collimation screws to be accessible from the top not the bottom.

Compared to mechanical fastenings, glueing was very easy. Just cut out the Plop trianges and glue it all together.

[Satchmo]

Well I've seen mirrors glued to cells but never cells glued together with silastic as well :).

Forgive the 'artistic licence' in my description of your telescope. I was going by the memory of having seen Patricks 16" F5.

Welcome to IIS .

[skies2clear]

Very nice work Peter (and Mark). It's always exciting to see new homebuilt scopes. And nice optics installed in it too by the way....only the best of course!

Clear skies

[rmcpb]

Peter,

The triangles are glued to eachother with silastic and not bolted at all and this allows enough movement for the floating needed? I think I will give this a try on my 13.1" as the bolting thing has been a pain to get right.

Did you set the thickness of the glue with a spacer? If so what size did you use?

Did you roughen the metal for a better bond or just clean the smooth surface?

Thanx

[Peter Bobroff]

The spacing is set with rubber O rings. I wanted about 2mm thick or a bit less but ended up using some only 1mm thick. There are 2mm holes in the trianges at the centre of glue points to allow the excess to escape.

The primary 27 glue points are about 15mm in diameter. The next 9 should be about 3 times larger in area eg about 25mm and the final three I made about 50mm which is the largest that would fit on my 25x50 base frame.

I think it is very important to use RTV silicone as it has the strength. Needed a week to cure.

I glued the O rings to the triangles with a little silicone a few days before the final assembly. The glue escape holes allow alignment rods (nails) to be temporarily inserted to get the triangles into alignment. I traced out the primary pattern on the mirror before I started.

[skies2clear]

I'm curious what type of welder Peter used for this (MIG?) Would be nice to get one that does aluminium and steel equally well, if such a thing exists? :shrug:

Clear skies,
Nick

[Peter Bobroff]

The welder is a TIG. It does aluminium, mild steel, stainless - almost anything including the occasional leg. It is slow and expensive compared to other welding methods but very forgiving. I just taught myself from a few books but I would recommend a TAFE course as the sensible approach.

[skies2clear]

Thanks for your advice Peter. Will look into it.

Enjoy your scope!

Cheers
Nick

[strongmanmike]

Another Bobroff - Suchting masterpiece, I can feel it :thumbsup:

Look forward to looking through it at SPSP Peter but this time with warm clothing on that fits me! :)

Mike Sidonio

[Peter Bobroff]

Here are some photos of the secondary holder and the filter slide.

A good microscope has a set of good quality eyepieces and a set of good quality objectives - why not a good telescope? Having bought a set of good quality eyepieces why should I only use them on a high power objective. This type of mounting and truss in particular is very suitable for mounting medium and low power objectives.

So I have mounted a 5.5" F3.5 Comet Catcher as the medium power objective, having modified the focuser to take 2" eyepieces. With the 22N This gives a field of 3.6 degrees at 23X. The 31N will also fit but gives an exit pupil of 8.6mm so the effective aperture is probably about 4.3" but the field is 5 degrees.

I am thinking of fitting low power objectives with focal lengths about 200mm and 50mm as well. The very low power objective would have a field of 40 degrees - Gum Nebula perhaps.

The filter slide is a dream. Just pull it out of the eyepiece box and plug it in - filters already loaded. Easy to compare filters and to examine an object in different bands.

The secondary holder is very light and rigid as the overhang is very small. I have bent the rules but not broken any.

See you at SPSP Mike - another long session coming up?

[rmcpb]

Peter,

What material did you use to make the spider vanes?

Brilliant scope BTW :)

Cheers

[Darkplague]

Going by the photos it looks like hes used 2 S/S rulers for the vanes.

[Peter Bobroff]

You are right - the vanes are made from 600mm stainless rules.

They are tempered enough to be very stiff but soft enough to drill easily.

[rmcpb]

That was what I was hoping you would say. Nice and easy to source such materials.

Thanx

[Peter Bobroff]

On further examination even the 6mm Radian benefits from the Paracorr.

The 12mm Nagler and Paracorr was better at the edges than a 2X Powermate with 22mm Nagler.