How to balance short tube refractor for AP

[Geoff_L]

I want to use my Orion Short Tube 80-T for both prime focus and eyepiece projection AP. My camera is an unmodded EOS 450D. Moving the 'scope about as far as it'll go forward achieves balance for visual observation, but inot with a DSLR hanging off the focuser. For eyepiece projection, I have the focuser wound almost all the way out and then need two extensions (with an eyepiece in the one closest to the camera). There's no way this is going to balance without some form of counterweight.

For counterweight mass, I have a bunch of 40mm x M6 stainless steel penny washers and a 1/4"-20 machine screw, which make up just under a kilogram. The ST80-T has a permanently-attached vixen dovetail with three 1/4"-20 screw-holes for tripod attachment. Attaching the counterweight to the most forward hole isn't enough and I really need to have the counterweight under the dew sheild. About the best solution I can think of is a second (250mm long) dovetail, fastened onto the OEM's dovetail at the three 1/4"-20 tripod attachment points and then using a dovetail plate adapter to hold the counterweight washers and allow me to adjust how far along the dovetail to position the counterweight.

Can anyone see a reason why this wouldn't work or have ideas on a better way to balance short tube refractors?

Thanks and clear skies,

Geoff

[MikeP]

I may have misunderstood your problem but .....

I use ADM Dovetail Counterweight 

Mike  

[Skipper Billy]

Same issue but slightly bigger - cost less than a fiver!

https://sites.google.com/view/astro-imaging/equipment

Top two or three images - lump of ally bar with an Argos 'gym' weight fastened to it with a wing nut - easy to add or subtract more weights.

 

[Geoff_L]

Thanks both to MikeP and Skipper Billy. Unfortunately, the existing dovetail is too short (see attached piccy) both to provide enough leverage for the ADM kit and my mount covers two of the three tripod attachment holes, so the lump of ally would likely rotate under scope when the mount is pointing close to the meridian. What I'm trying to do is end up with a longer dovetail to which to attach a counterweight -- i.e. fix a longer dovetail under the existing one. The mount would then attach to the longer dovetail, of which there'd be space to attach a counterweight under the dew sheild That said, the 'proper' ADM counterweight kit seems better priced at FLO than at other sites I've seen, so it's worth considering -- but I still need a longer lever!

Thanks again, and clear skies.

[Thalestris24]

Hi Geoff

Yeah an ankle/wrist weight can be wrapped around the dew shield and lightly taped in place. Should do you.

Louise

[Geoff_L]

1 minute ago, Thalestris24 said:

Hi Geoff

Yeah an ankle/wrist weight can be wrapped around the dew shield and lightly taped in place. Should do you.

Louise

Thanks, I already had the same idea and found out the dew shield is only a push fit onto the front of the OTA when it started to come away!

[Davey-T]

I've had the same problem and made an aluminium plate bolted under the dovetail clamp and extending forwards a few inches drilled and tapped to take the dovetail clamp.

On a dual rig but similar idea.

Dave

 

 

[Grierson]

Very far from the most elegant solution but a wide strip of Velcro surrounding the tube with Gallant Wrist Weights attached is cheap and cheerful.  Weights between c. £6 to £16 from Amazon!  

[iapa]

If you can remove the existing dovetail:

something similar to your suggestion I think- or may be the same solution phrased differently ?

• acquire second, longer, dovetail
• remove existing dovetail
• fit a pair of ADM MDS Dovetail Plate Adapter in place of the dover tail.
• invert origin dover tail and attach to the longer one
• use clamps attached tot he to fit to the shorter dovetail
• longer dovetail goes in the mount.
• I do that sort of thing to fit my ST80 to the dovetail on top of my C8 and 200P-DS

It been torn down, but I can put it together if you want pictures?

[kilix]

Is the ST80 on an AZ-3?
Or what kind of mount do you have (I don't think that it was mentioned here)?

[happy-kat]

If on the az3 I've added a bar with weights to give balance when using a DSLR and I have the same ST80 T. I'll add links later.

[kilix]

with AZ3 I solved it like this:

I don't know which part is more effective, the counterweight, or the alt locknut upgrade? Locknut made by welding 3 things together - I welded a 11mm shaft into a M12 nut, then welded the M12 to the original M10 locknut . Shaft was then bored with a 6mm drill bit and M5 long screw put in the hole to serve as a handle. ST80+heavy Sony DSLR are held in place with no unwanted movement. My counterweight weighs too little and helps only partially, most of the job is done by the locknut.

(And now I see that I can't properly focus close objects )

[happy-kat]

If you get the balance right the nut only needs to be finger tight.

[Geoff_L]

4 hours ago, kilix said:

Is the ST80 on an AZ-3?
Or what kind of mount do you have (I don't think that it was mentioned here)?

I've got it on a Skywatcher EQ5 Pro.

My current Alt-Az mount is horribly wobbly thing that came bundled with the Bresser Skylux 70/700 that got us into Astronomy. I'm looking to upgrade that so that I have something reasonably steady to use as a 'grab-'n-go' at club practical sessions etc. but that's purely for visual observation, where there's no balancing issue, and the subject of another thread.

[kilix]

55 minutes ago, happy-kat said:

If you get the balance right the nut only needs to be finger tight.

then I borked it. No matter, it's very usable as it is now

[happy-kat]

I should elaborate if the balance is right from using a counter balance weight when using a camera then the nut shouldn't need to be so tight.

[ollypenrice]

I'd just bolt a longer dovetail bar beneath the present one so you could slide the scope further forward in the saddle plate. With a pillar drill it would be dead easy.

Olly

[Geoff_L]

1 minute ago, ollypenrice said:

I'd just bolt a longer dovetail bar beneath the present one so you could slide the scope further forward in the saddle plate. With a pillar drill it would be dead easy.

Olly

FWIW, with this 250mm dovetail bar (or the 340mm version) as you can attach the long dovetail to the original by screwing through the slots -- just need some "right length" 1/4-20 screws. I suspect that a combination of counterweighting and moving the scope forward (so the focuser is over the mount) might give the best solution.

Thanks to all for the help.

Clear skies, Geoff