Dob 300 rear counterweight system?

[kirkster501]

Hello, With my Telrad, 60mm RACI, 2" TV EP's and Dew Shield my Dob 300 is being weighed down at the front, upsetting the balance and causing it to "sag down" a bit and effect accuracy on Goto and alignment.

What I need is to put some weights at the back somehow. Anyone got any ideas how to do this please? I would prefer a permanent solution, maybe screw a bar along the bottom length of the OTA with weights on a slider and lockable sort of thing ideally. However, not averse to a few hand-weights being stuck to the back if need be. But how?

Appreciate some thoughts and pictures !!

Rgds, Steve

[Tinker1947]

Not sure if the base has 4 thread holes for a fan to fit, if so a metal plate could be placed here, with the balance adjusted by means of other thinner plates held in place with a magnet,,,,

[SkyGibbon]

If it is a steel tube, get a magnetic tool holder and just pop it on the bottom. Can put anything magnetic on it for extra weight. Can slide up and down tube. No drilling required. Comes off in a second. A 12" one wil weigh about 3 bs.

[Oblong]

Mine isn't really suitable for magnets - but I had some luck with diving weights and self-adhesive heavy duty velcro tape. Just make sure you attach to a sturdy part of the tube, and that it won't get in the way of the mount when you're near the zenith. Mine are on the edge of the mirror cell. The only mistake I made was first trying with plasticised weights. After a few days, the adhesive on the velcro tape melted the plastic, and I just got a gooey mess.

[crashtestdummy]

What i did was go to the local craft shop and purchased some adhesive velcro strips and some asda ankle weights with velcro ankle cuffs.total price £6.50

[kirkster501]

At the bottom of my mirror cell there are four threaded holes about four inches point to point. I plan to use a piece of paper to make a template and use the template to drill something a bit bigger and heavy to match. Perhaps a slab of lead or steel an inch thick. Will have to look where I could get such a slab. It would need to be five inches square and weight about 3 kg me thinks.

[Tinker1947]

At the bottom of my mirror cell there are four threaded holes about four inches point to point. I plan to use a piece of paper to make a template and use the template to drill something a bit bigger and heavy to match. Perhaps a slab of lead or steel an inch thick. Will have to look where I could get such a slab. It would need to be five inches square and weight about 3 kg me thinks.

The four holes are for attaching a Fan, i would think a 120mm type, if you have a old fan you also have a template....

There is another option i made some sash windows and the lead weights are now made square to different sizes and have a handy hole down the middle, so a suitable length of studding plus washer and nut will hold 4 pieces of lead,,,,,these people were good last time i made a order.... http://www.mightonproducts.com/products/sash-window-hardware/sash-weights

[earth titan]

Here's the solution fitted to the back if my dob (not by me - but by original owner).

Looks to be a magnetic knife rack IR similar. Works a treat.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[Moonshane]

the magnetic rails are a good idea and used successfully by many. I have OOUK scopes and they have aluminium tubes so I bought a spare tube ring and added a short dovetail. I then had a clamp in my bits box which was used to successfully add weight on when near the horizon with heavy eps. it's shown here not as cheap as other options but worked well.

[Andrew*]

Here's another solution. I would challenge anyone to do the job cheaper (and uglier)!!

http://stargazerslou...ilk#entry313413

[Moonshane]

have you seen the price of milk these days??!! good one Andrew

I think I can beat that, a damp towel thrown over the end? (I nicked this off John though his was a dry towel not a wet one - the wetting was my idea).

[Moonshane]

ps

I'd be cautious about adding much weight to the mirror cell directly. it might not affect anything but it would be a shame to create astigmatism in the primary after you have gone to all that trouble. this is possible depending on the quality of the cell.

[StarryBob]

Ankle weights and a long tie wrap works well, strap two of them around the tube.

Sent using iPad.

[kirkster501]

Well, just to "test out" the weight required, I have used a 2.5 Kg weight from my weight training kit and lashed it onto the bottom of the mirror cell. Seems to hold the scope much better. I reckon another kilo and it will be perfect. When I have fine tuned it I will drill and tap the 2.5kg weight and screw it into the fan holes at the bottom of the cell.

Anyone perceive any issues with this mechanism? The weight is central at the bottom of the cell (it does not look like it in the picture due to foreshortening effect of the photo.)

EDIT: Reading posts above here. Could this cause stresses in the mirror perhaps? Hmmm. Possibly. I may look into the magnetic system as well.

[Moonshane]

personally I wouldn't do it that way but maybe try it and see if it has any visible effect on the views. any effects will be corrected again when removed so it's not too scary to try.

[scarp15]

I have been able to keep the secondary cage on my 300p flextube fairly light and have no issues for adding weights fortunately. I replaced the heavy stock focuser with a moonlite CR1, and at 13oz also 3ounces lighter than a CR2. Removed the finder scope and fitted my telrad in its place. I did add a two inch riser though. I do not use a paracorr as at F4.9 it seems fine, or rarely fit a dew shield (also made out of lightish 8mm foam).

I do have some hefty 2" eps, yet changing from these to light weight 1.25" naglers, I feel that the tensioners on the flex tube base work well and I can retain the image in the field.