F/4 with bino & powermate or f/5

I'm building a 16 inch GEM mount and was intending to build it with an f/4.2 mirror & use a bino for DSO. My thoughts were to use a 1.5 or 2.0 powermate for planets (and of course the bino). Am I on track or just NUTS? Any other advice is welcome. I'm attempting to build it as light as possible.

Thanks Paul

For a 16 inch GEM Newt you're going to need a massive mount similar to the AP1200 so design heavy and strong. The binoviewer is a problem because it will add a lot of weight that will be leveraged way up there. So you're either going to need real good clutches to keep the scope from moving under the weight, or you may need to re-balance the OTA every time you switch between binos and the eyepiece.

It's do-able, I would think, but I will comment that a scope of this length (I'm estimating 1700-1800mm) on a really strong equatorial mount is going to have some elevation (it will have to for the mirror side to clear the ground) and so you'll be using a ladder. This raises the question of why bother when you can put the same scope on a dob mount and get tracking for it.

You may or may not need a barlow. AP makes a nice one that is very strong. The Denks have a built in power switch which would allow you to change magnifications without affecting scope balance. Also on a long Newt you are more sensitive to wind, and while one can never exclude the possibility that a mount will be strong enough to hold against a moderate breeze, there is a tendency not to build/buy enough mount for the OTA, scrimping where one should not. Dobs are also vulnerable to wind but I think it's probably easier and more cost effective to overbuild the dob mount than the Newtonian. A member of my club uses a 16" Newt on an AP1200 and he says he considers the setup just barely adequate.

Having grown up in the era of Newtonians on GEMs I like them, but truth be told I think there is a certain engineering logic to the fact that these days 98% of the scopes on German equatorial mounts are either refractors or compound scopes like the SCTs.

good luck, it sounds like an ambitious and fascinating project.

Greg N

Paul Neesam said:

I'm building a 16 inch GEM mount and was intending to build it with an f/4.2 mirror & use a bino for DSO. My thoughts were to use a 1.5 or 2.0 powermate for planets (and of course the bino). Am I on track or just NUTS? Any other advice is welcome. I'm attempting to build it as light as possible.

Thanks Paul

Paul: Have you ever spent much time with a big Newtonian on a GEM? I am not really in a position to help with the bino and Powermate question but I do have a few thoughts on the big Newt on a gem...

- It will almost certainly require a ladder to use...

- You will most definitely want rotating rings or a rotating upper cage assembly if you are building a truss style scope. At F/4.2 a rotating cage would probably require collimation every time you rotated it. Without some reasonably easy method of positioning the focuser, many eyepiece positions will be either awkward or impossible. The BinoViewer though may need a counterweight to keep the tube from rotating if using rings.

- Large Newtonians are best suited for a pier mount so the tube and/or the ladder doesn't tangle with the tripod legs.

- F/4.2 is definitely Paracorr time.

- The added weight of the binoviewer and eyepieces (plus the Paracorr too) present some problems. As you move a Gem across the sky, the tube rotates, if you are moving the scope from one target to another, the that whole bundle may endup pointed down instead of up and if it is not properly secured it may hit the ground...

If you have spend much time with a large Gem mounted newt, you know these things so take it for what it is worth...

But I think Greg's point is a good one, it takes a big GEM to handle a 16 inch Newtonian and it will still be a PITA to use. A DOB mount with drives is much more manageable as well as less expensive...

I own a 12.5 inch with a 1905mm Focal length on a GEM. The OTA weighs around 100lbs. They are neat to look at and to look through but my 16 inch F/4.42 and 12.5 inch F/4.06 DOBs get much more use because they are much easier to setup, much easier to transport and much easier to view through.



Jon

I think that an f/4 SN-10 is about as big a Newt on a GEM that I could tolerate. With my tripod legs all the way retracted, I sometimes have a hard time seeing through the EP of my SN-8. If you are using a camera for viewing, then it makes no difference and the fast focal ratio of a Newt is superior.

The Baader MPCC will probably be more versatile than a ParaCorr, and a lot less costly. You will need a ParaCorr or MPCC with big wideangle EP's, or for decent photos, so figure that into the cost while you are at it.