Mounting an 8" dobsonian on an EQ mount

[nikanj]

I was wondering whether anyone has any experience with mounting a 8" dobsonian on an eq mount to use for astrophotography? At 8kg for the tube it's well within the payload capacity of a mount like the NEQ6/EQ6R.


The long length may present some issues though. Firstly it might prevent you from imaging near the meridian which isn't really too big a deal. Also because the weight is distributed so far from the axis (camera and mirror) the intertia may make it more difficult to make guiding corrections in DEC.


But it's still a very tempting idea. A 1200mm f5.9 would give you a very similar focal lenght and focal ratio to a C8 with f6.3 reducer but for less than half the price.

Is this a good idea or not?

[sharkbite]

Hi Andy...

If you mean mounting an 8" newtonian on a GEM...

you can do it - in fact you can buy one factory:
https://www.celestron.com.au/products/advanced-vx-8-newtonian-telescope

(dobsonian is a type of mount - newtonian optical tubes are generally used for that)

[Startrek]

I can’t see any reason why you would want to put a longer tube dob on a mount when there are more suitable options

1/ Dobs usually come with single speed focusers , you really need dual speed focusers to do any serious AP either planetary or DSO

2/ The longer tube of the Dob on that EQ6-R is going to place unnecessary torque and backlash whilst tracking and guiding than a standard imaging Newt

3/ You can buy an 8” f5 Bintel GSO newt for only $700 purpose built for AP , has a dual speed focuser , is only 820mm long , 300mm shorter than the Dob , will track and guide beautifully ( I’ve used one on an EQ6-R for years )

4/ The Altitude support brackets on the Dob may interfere with the tube rings when time comes to balance the damn thing ( if you remove them you have to block all the holes etc…

5/ Balancing in Dec will be a pain in rear end

6/ An bonafide 8” f5 imaging Newt is a faster imaging system than the dob at f6 therefore collect more signal for the same duration ( only slightly narrower FOV )

To answer you question yes it could be done , but not a viable long term solution for long exposure Astrophotography

I would leave the Dob as a Dob for observing, that’s what they were originally designed for ( not on an EQ mount )

My 2 cents …..

Hope the above is helpful

Cheers
Martin

[nikanj]

Thanks for the inputs. Yes I meant a Newtonian, but specifically the Newtonian that Sky-Watcher use for their 8" dobs due to the additional focal length over a typical 8" f5. The Sky-Watcher 250mm f5 is has the same 1200mm focal length but is much heavier than than either ther 200mm f5 or 200mm f5.9 (from the dobsonian). But I guess considering all the potential issues it's not really worth it for a small gain in focal length.

[Startrek]

I have both Skywatcher and GSO Dobs and imaging newts
I favour GSO Bintel over Skywatcher newts for imaging
An 8” f5 imaging newt at 1000mm focal length has plenty of reach and is not a small scope
Just remember the longer the focal length to less forgiving in regards to tracking and guiding
Personally I wouldn’t go any bigger than an 8” f5 Newt on the EQ6-R unless it’s on a pier or in a Dome on a pier

Good luck with your set up

[Boozlefoot]

[QUOTE
Personally I wouldn’t go any bigger than an 8” f5 Newt on the EQ6-R unless it’s on a pier or in a Dome on a pier.
Good luck with your set up[/QUOTE]

Absolutely! I find anything bigger than an 8" Newt a bit of a wrestle nowadays.

A quick pic of a 12" Dobber in a GEM with original built ring/adaptor proprietary rings, and proprietary GSO rings. (The purpose built adaptor was nearly a 2 man lift on its own!)

The balance point of the dob bearings, (if a GSO) are fairly close as can be seen with the proprietary rings (for visual), but may not give enough leeway as Martin says if loading up the focuser for AP work. We used studio sandbags on the rear of the tube to balance "the beast" dependant on the EP used.

Definitely suggest the purpose built newt as they quite often come with rings and dovetail. The added cost to adapt a Dob can be close to the cost of another new OTA.

[doppler]

I already had a skywatcher 10" dob when I scored a cheap heq5 pro mount, and thought why not try the two together. I got some tube rings and mounted them to a long alloy bar keeping the rings spread apart to make it easier to balance, I then bolted the 12" dovetail bar to the balance point. The heq5 handles the weight without effort and is usable on the tripod legs but for long exposure work you cant beat the pier mount. I can still use the scope as a dob when not imaging, but if I was starting from scratch an imaging newt would be the preferred choice.
Rick