Piggyback mounted rich field?

[The_bluester (Paul)]

OK, posting in here as I still regard myself as a beginner.

My main scope is a Celestron CPC925 GPS. Great scope and I am very happy with it. However due to it's nature I obviously have a fairly limited max FOV. Some reading recently left me wondering about piggy back mounting a rich field scope to it. All the benefits of ease of use of the computerised Alt/Az mount though I have found it a bit limiting lately, I wish the encoders were on the scope side of the drive clutches not on the motor side so I could change targets quickly by releasing the clutches and pointing manually without waiting for it to slew by itself.

Has anyone tried something like this, mounting a small, relatively cheap widefield refractor to something like the 925 on Alt/Az? Ideally I would like something suitable to a 2" EP as most of the ones I am using the most often are 2" format and I would like to be able to use them, pending exit pupil considerations and so on with the different focal ratio.

Balance I would not expect to be too great an issue, the only balance problems I have had was when I tried hanging a hulking great pro level DSLR off the visual back and trying to take exposures of objects that were well down the sky. I do have a dovetail rail mounted to the bottom of the OTA which would let me tweak balance with some weights.

A recent night started me thinking about this when I realised how much I had to motor around to take in the nebulosity of Eta Carinae with the limited TFOV of the 925 and my EPs and it is either something piggy back something with the convenience of the electronic tracking of my current scope or perhaps try to get a secondhand dob past the minister for finanace.

[ZeroID (Brent)]

Certainly can be done although most widefeild scopes are in the 80 -100 mm range and mostly take 1.25 EPs. My Lunt is 102mm f7 and takes 2" but would be rather heavy on a C925 I would imagine.
Hopefully someone with a similar setup will chime in

[The_bluester (Paul)]

I do have some 1.25" EPs as well but much prefer the use of the 2" ones. My biggest gripe is the Nagler 11mm is VERY inclined to fog internally, I really need a heater for it. Nice views but it is invariably the first one to go back in the box for the night when it all goes white! It has made me wonder about chasing up some ES sealed/purged ones.

I was wondering about one of the little short tube 80mm scopes as an option.

[ZeroID (Brent)]

My widefeild is just my 80mm f5 Skywatcher Achro which gets mounted above the Lunt and doubles as a guidescope sometimes. If you're picky an 80 or 76 APO would be the way to go but it seems a waste putting a 2" on it although it should be feasible.
Should be able to get a 2" diagonal with 1'25" nose I'd imagine. Then any 80 mm will do the job.

[The_bluester (Paul)]

I suppose another option could be to mount some astro suitable binoculars to it, that would leave the option open to take them off and use them hand held as well.

[OzEclipse (Joe Cali)]

Binoculars will be a pain to look through mounted on the scope.

If you only want a low power wide field scope, an achro short refractor is perfectly adequate. The 80mm f5 Shorty's give wonderful wide field views.

They only have a 1.25" eyepiece. The field probably isn't flat enough to properly use the field of a 2" eyepiece so I wouldn't look at changing to a 2" focusser. You don't need long FL eyepieces for wide fields.

I use my 80mmf5 with a 16mm UO Konig (65 degree) eyepiece I get 25x magnification and a 2.6 degree field of view and a 3mm exit pupil. The refractor only weighs about 1kg and a 1.25 diagonal and 1.25 eyepiece is a fraction of the weight of a 2" combo. For any such combo, you definitely don't want the eyepiece focal length to exceed (7 x f-ratio) or (6 x f-ratio) if you are older. than teens or 20's.

The GSO superview 20mm 68 deg eyepiece ($39) in an 80mm f5 shorty scope ($220) will yield : -

20x mag
4mm exit pupil
3.4 degree field of view

all in a lightweight package.

Cheers
Joe

[ZeroID (Brent)]

I have to go with Joe's post. I have the 20mm and 15mm GSO SV's and they are perfect in the 80mm scope.

[The_bluester (Paul)]

Time to start saving, there have just been heaps of times recently where I would have loved a wide FOV.

[MattT]

Gotta agree with Joe too. Mine is a 70mm f7 achro with a 20mm 70º 1.25" eyepiece that gives amazing 2.5º views with an under 3mm exit pupil, just right for the city and darker sites. All fast refractors are going to have Field Curvature, the 70 f7 has a bit, but it is easy to ignore. Have seen that Myastro shop sell 80mm f5 refractor OTA's for about $159.
Well worth mounting it on your scope IMO.
Matt