Building an Imaging Newt - Optimal Config

[glend (Glen)]

I am in the process of putting together a spec sheet for an Imaging Newt that I hope to build later this year. After my 127mm refractor build I think I am ready to take on the newt project. I am after some helpful advice on component selection from people that are involved in imaging with newts.

This newt will sit on my guided NEQ6Pro, so it could be a fair size and weight - if that's required; however as an old guy I'd prefer the scope to be light and easy to handle (certainly weigh no more than the mount head unit), and my tube is likely to be an open truss design similiar to that used on my 127mm refractor, to keep the weight down.

I would welcome advice on what experienced newt imagers see as the best size in terms of performance, and if there are deminishing returns in larger sizes. Is there a sweet spot in terms of mirror size, f stop, and focal length that gives great return on investment?

My initial ideas are as follows:

Scope design: An 8-10" imaging newt with an open truss tube (with shroud when necessary). Sufficient focus travel to enable use as both a visual and imaging scope when required. Imaging camera is a Canon 450D DSLR for now. I am leaning towards an f5 design to make coma easier to manage, and a minimum focal length of 1000mm. My main imaging target objects are DSOs, nebs, etc.

Primary: Zambuto 8-10" , or other sizes considered on performance merits.
Mirror cell: undecided
Secondary: Antares Quartz
Spider vanes: Antares
Focuser: Moonlight with motor

All suggestions welcome.

[strongmanmike (Michael)]

Hi Glen, as you may be aware I use a fast imaging Newt but a high end commercial unit and it performs very well...for a homemade job however, I recon you can't go past Rolf Olsens two amazing OTA's he has built in recent years and used to great effect:

10 inch Rolfoscope

12 inch Rolfoscope

Rolo will have all the wisdom to impart to you but to me the stuffiness (NZ speak for stiffness) of the focuser to OTA connection is very important with imaging Newts and can not be underestimated for holding collimation across the sky as is a very rigid tube design You may not want to go down his quirky guitar string spider road though

Mike

[ZeroID (Brent)]

Second vote for the ROLFOSCOPE (!!!).
For lightness, stiffness and relative ease of manufacture. I 'copied' his 10" f5 for Skeletope II and can vouch for the design. Mine comes in at 11.5 kg for OTA, about 17-18 kg loaded with finder, guide etc. Quite within the EQ6 capabilities. I didn't attempt to emulate his guitar string secondary either, maybe one day for the heck of it.

[Satchmo]

I see from another thread that carbon truss Newt imaging trains will hit the market soon . I wouldn't see the need to scratch build a truss at the prices these things will likely come out at .

[glend (Glen)]

Mark the truss structure is the cheapest part of the whole thing, and the rumoured truss newts will not be carrying Zambuto mirrors. Yes I guess it could be argued that you could buy one to get the chassis and replace the mirrors; the point being that I intended to build it.

[SkyViking (Rolf)]

Originally Posted by strongmanmike

You may not want to go down his quirky guitar string spider road though

Excuse me Mr. Sidonio... as we all know, guitar string spiders are the ultimate when it comes to elegant telescope design
Actually in all seriousness, I wouldn't be without that spider for anything, it has performed superior in all aspects, and is way easier to adjust than anything else. But yes I agree it does look scary

Oh, and mine is no mere 12" scope, it is a 12.5", most importantly.

Cheers,
Rolf

[strongmanmike (Michael)]

Originally Posted by SkyViking

Excuse me Mr. Sidonio... as we all know, guitar string spiders are the ultimate when it comes to elegant telescope design
Actually in all seriousness, I wouldn't be without that spider for anything, it has performed superior in all aspects, and is way easier to adjust than anything else. But yes I agree it does look scary

Oh, and mine is no mere 12" scope, it is a 12.5", most importantly.

Cheers,
Rolf

Ooooh a touchy guitar player with a sense of inadequacy

I have a 6" which is apparently average ...for a Starfire APO

Hey I recon the guitar string idea is great but yes, probably looks a little daunting for people who like more standard conservative approaches

Mike

[glend (Glen)]

I am ok with guitar strings, my dob is string braced. Love your work Rolf!

[Satchmo]

Yes I was referring it as a cheap way to get a very nice professional chassis- anything with wood in it is going to move when it comes to imaging .

Originally Posted by glend

Mark the truss structure is the cheapest part of the whole thing, and the rumoured truss newts will not be carrying Zambuto mirrors. Yes I guess it could be argued that you could buy one to get the chassis and replace the mirrors; the point being that I intended to build it.

[SkyViking (Rolf)]

Thanks Glen, it's a pleasure building these things for sure.
If you do decide to build something similar there is a fair bit of info on those pages that Mike linked to, and I'm of course happy to answer any questions too.

The Moonlite focuser is awesome and works perfectly with FocusMax.
As for primary mirror you may want to also consider a Royce conical for the mounting simplicity and light weight.

[SkyViking (Rolf)]

Originally Posted by Satchmo

anything with wood in it is going to move when it comes to imaging .

Just wanted to add that based on my experience this is not the case. I have observed no focus shift at all with my truss scope which has plywood rings at each end.
I also use alu truss poles which everyone says are supposed to expand/contract with temperature changes (which I agree they should in theory), but again I have never observed that. Maybe the Serrurier truss design compensates for it to a degree, I don't know.
Bottom line is that you can certainly build a very high performance imaging OTA for little money using common basic materials if you want to.

[Satchmo]

I would imagine that problems with flexure in components may creep in with larger apertures as the mass goes up by the cube of aperture and `looks about right ' generally doesn't get you through without some engineering analysis. Good to know that its working for you up to 12" for you

[SkyViking (Rolf)]

Originally Posted by Satchmo

I would imagine that problems with flexure in components may creep in with larger apertures as the mass goes up by the cube of aperture and `looks about right ' generally doesn't get you through without some engineering analysis. Good to know that its working for you up to 12" for you

Absolutely, there may be problems with larger apertures when using these materials, but I don't have any experience with that, maybe others do.
What I can say for sure though is that FWHM is very consistent thoughout the entire night without any focus adjustment. So any shift must be extremely minimal since it wouldn't take many microns of movement to observe focus shift at f/4 with an image scale of 0.87"/pixel.
It's no engineering analysis of course, but probably a good deal better than 'looks about right'. I'd also be surprised if the same design in, say, 14" size would behave significantly different, but of course don't know where the practical limit lies when using such materials as I've only tested a 10" and 12.5".

However, I still haven't heard anyone saying "I built a truss tube OTA using plywood and alu tubes and have problems with focus shift", but interestingly have heard quite a few people arguing that it is not supposed to work.

[Shiraz (Ray)]

Glen, I don't think that you need a high spec mirror for DSO imaging - the atmosphere will limit you to about 1.5 arcsec FWHM at best and (almost) any commercial 8-10 inch mirror will do way better than that (I have a 200/f4 GSO, 300f5 GSO and 250f4 SW - they all provide seeing-limited DSO imaging). In any case, even if you decide to get a top spec mirror, the necessary coma corrector will pull it back to the pack - for example, the popular MPCC may well turn your 1/10 wave system into a 2/3 wave one and that is probably still good enough in most cases. If you want to use it a lot for visual or high res planetary though, a high spec mirror may be better.

I find that my EQ6 handles the weight and wind loading of the 200f4 better than the CF 250f4 and the smaller scope is much easier to lift (I am also an old guy). f4 is best for imaging time, but f5 is much more forgiving of misalignment and has much less coma to correct - on balance, I think that my vote would be for a 200f5 in a home built scope to put on an EQ6. That would provide a good pixel scale with the 450 or one of the 8300/694 cameras in future.

Have never used wood in a scope, but some types of wood have along-grain thermal expansion coefficients similar to carbon fibre composite - if sealed against moisture, wood should be a very useful material.

[ZeroID (Brent)]

Have to back up Rolf's comments here re stability of the Serrurier truss design and construction.
Wood if sealed correctly with several good coats of varnish works and looks nice too. Paint, oil based will obviously work as well but doen't look half as nice, black being the exception possibly.
I got my alum tube for free, chuck outs from work, marketing stands etc but even buying from good old Bunnings they would have come in at under $100. The ply was a half sheet, $30 which provided rings and the cage brace. All up I spent maybe $300 tops to build the OTA, nuts, bolts, rivets etc etc.
When I finished I stood it on end in the driveway THEN stood on top of it. That's how strong it was.

It has been modifed several times now, mainly to accommodate photography and still holds collimation. Due to it's open design it is easy to clean the mirror in place, adjust anything by just moving the lower light shroud and is a one handed carry when I use it in DOB mode.

I haven't made a guitar string spider but after Rolfs comment above I might just give it some thought. How much are machine heads btw ?

The hardest part in the whole project was ensuring build accuracy of the parts so it all came together. Which is also what made it a worthwhile project for me.

[TimberLand (Justin)]

I completely agree with the 6-10 inch range being well suited to timber construction when it comes to imaging. Correction 6-12.5 (Rolf has proved that very well).

My 16 f4.5 is almost all steel with some aluminium and very heavy to maintain enough stiffness with the truss tube design and maintain focus across the sky. (70Kg approx, I haven't weight it since last modifications). When I started it 8 years ago there wasn't a mount that could hold its estimated weight which is why I went to the horse shoe mount to support it.

Optical performance of the mirror is important but not critical given most nights the sky just doesn't play nice. My GSO is quiet dirty at the moment and is still performing well, and I only bought it to work out the bugs of the mount and scope before I put something more expensive in its place and now considering leaving it there.


Justin