First light for new 20" compact dob

[PhilW]

This scope had its genesis at Snake Valley last year, when Stephen Sharpley & I were discussing the trend to faster optical systems made possible by the Paracorr. These have various practical advantages, the main one being that they enable large scopes that can be used without ladders. Not wishing to waste a good idea, the following week we ordered a 20" F/3.7 mirror from Mark Suchting, & here we are.

A few key details:
- It is built from 12mm hardwood ply, with a stainless steel cell
- Scope weight is 25kg, excluding rocker box
- It has a 4" secondary from Antares Optics
- There's a built-in filter slide with an NPB filter permanently mounted
- Mirror has front collimation for ease of adjustment
- Components can be handled by one person, & it fits in a compact car.

Phil (self-confessed ladderphobe )

[DavidU (Dave)]

That's awesome Phil. Well done mate!

[StarGazing (Alex)]

Wow Phil,

What a great job you have done, what suprises me the most is its height . Makes good sense to have the primary knobs at the front,...... great idea. Cant wait to see it.

Well done mate, great effort. Many clear skies to you. Alex.
:thum bsup:

[mental4astro (Alexander)]

Mate, it's a ripper, .

Well done.

Did you so the welding of the aluminium yourself? Why did you go that route? Do the same at the secondary's end?

Mental.

[Paddy (Patrick)]

Oh, Phil, it looks just fantastic. My biggest question is why wasn't it at SV on the weekend for all of us to drool over. Maybe you should take it to VicSouth!

And 25kg!!!!!!Amazing!

[gary]

Originally Posted by PhilW

These have various practical advantages, the main one being that they enable large scopes that can be used without ladders. Not wishing to waste a good idea, the following week we ordered a 20" F/3.7 mirror from Mark Suchting, & here we are.

Hi Phil,

Mark was telling me about your mirror at Lostock last weekend and it is great to
see that such a carefully figured piece of glass has found its way into such a
beautifully crafted telescope.

F/3.7 is the perfect prescription for those of us who are vertically challenged.

May many nights of enjoyable observing be your reward!

[toryglen-boy (Duncan)]

any idea how much something like this would cost to make?

[PhilW]

Mental, the only welding in the structure is the stainless steel cell. I gave the pattern to a custom bicycle builder & he welded it up for me. Everything else is riveted.

The mirror was certainly a labour of love for Mark.

[tnott]

Well one Phil!

Notice much difference from your 14"?

Will you be making it to Vicsouth?

[PhilW]

We're finding everything a lot brighter, but we need to do lots more testing to really get the measure of it.

It won't be at VicSouth, but definitely at the ASV's big December event.
____
I am now in a much better position to answer your question Tim.

It has had its first trip to a dark sky site, and performance was outstanding. Globulars resolved into pinpoint stars, and the nebulosity in the LMC was very bright & contrasty, especially with the NPB filter. What is perhaps more surprising is that it is also a wonderful lunar and planetary scope. It is pulling more detail out of Jupiter than I've ever seen. So it truly is an all-purpose scope.

One caveat: the mirror has to be properly cooled before it will really perform on high-resolution targets, like any big newt. I run the fans for a minimum of half an hour before observing, and an hour is better.

[Satchmo]

Originally Posted by PhilW

____
I am now in a much better position to answer your question Tim.

It has had its first trip to a dark sky site, and performance was outstanding. Globulars resolved into pinpoint stars, and the nebulosity in the LMC was very bright & contrasty, especially with the NPB filter. What is perhaps more surprising is that it is also a wonderful lunar and planetary scope. It is pulling more detail out of Jupiter than I've ever seen. So it truly is an all-purpose scope.

Great news, Phil

[circumpolar (Matt)]

Q: Is F/3.7 the limit for using a paracorr as part of the design concept?, or is this just the best combo for the focal length you wanted?

[CoombellKid]

Phil,

Fantastic looking scope well done!

I've been thinking of building a filter slide for my dob, similar to yours. I
dont suppose you have had enough time to trial it. But I would like to
hear if you have any dewing problems with it.

btw, if you dont mind me asking... what height are you? it would give me
and idea of the size of your scope.

Great to see Mark making mirrors faster than F/4, I wasn't aware he was
doing that.

Cheers,CS

[PhilW]

I have trialled the filter slide, & have had no dew problems as yet. I suppose I could put a heating strip around it if need be.

That's my son in the photo (he's 170cm, although he will have grown a centimetre by the time I post this ). I am 185cm, and eyepiece height at zenith is 180cm. One rarely observes at the zenith, so it is possible to use an observing stool most of the time.

As for the question about going even faster: Nagler has a paracorr in development for F/3 - F/3.5 mirrors. But making decent mirrors that fast is very difficult.

[telecasterguru (Frank)]

Looks absolutely beautiful. Drool.

Frank

[Starkler (Geoff)]

Great stuff Phil, and nice to see you used a proper focuser this time
Hope I get to see it up at the Briars some time.