Balancing a dob for use with 1kg+ eps's

[DAT]

Hello all, still waiting for my dob to arrive (8" f/5.9 Stellalyra).

I have got hold of some heavy eps (1kg+) and I have been advised that counterweights will be necessary.

I have researched various option (magnets, velchro, knife strip...) and in terms of cost, ease of use and preference, I think I would like to use some magnetic weights to attach to the tube (with some material to ease the strength off and protect the tube and make it easier to move). I understand these should be placed at the opposite end and side of the ota to the focuser/ep. 

My question is how much weight?

From a previous thread on SGL I found this link:

 Mutuactor 4PCS Magnetic Base Strong Neodymium Magnet Pot Rubber Coated With M6 Male Thread Stud, Vertical Magnetic Pull-force 9.0kg : Amazon.co.uk: Business, Industry & Science

I think each one weighs 160g.  In my case:

would these be sufficient and how many at a time would I have to use to balance ep's weighing 1 to 1.3kg?

If there are better or cheaper or more effective solutions, I am open to suggestions. I would like to avoid drilling or permanently sticking anything to the ota.

Thanks in advance

[Louis D]

It really depends on how much tension you have set in the altitude axis.  I'm pretty sure that mount has adjustable tension.  If you crank it up, you may be able to avoid any balance issues.

The bigger problem you're going to run into if you run with light tension for a smoother altitude motion is the tendency for the tube to swing upward when you remove the eyepiece for an eyepiece change no matter how well balanced your tube is.  There are no clutches on Dob mounts to lock it during eyepiece changes.  You'll need to become adept at keeping the scope from swinging upward during eyepiece changes if you aren't keeping a high level of tension in that axis.

[NGC 1502]

Previous post contains good advice, depends how much tension or friction is in the altitude bearings.  I made an altitude brake similar to the optional brakes on Orion Optics UK Dobs.  I can very easily add just the right tension to allow smooth movement and yet eyepiece changes from 1kg eyepieces to 250g eyepieces without any problems at all.

[bosun21]

I have always used these.

https://amzn.eu/d/gTojxHc..

I used one on my 8" and sometimes two on my 12". I just wrapped them in fabric to protect the tube. I don't like having to tighten the tensioners too much as it can cause a lot of stiction in the alt movement. I've just bought another two for my new 10" as I included the previously owned ones when I sold the scopes.

Edited December 9, 2023 by bosun21

[cajen2]

I had a range of magnets on my SL 8" dob which I would slide up and down as EPs were changed. You can change the point of balance on the alt bearings, but it's a pain, needing the OTA to be separated from the base and messing about with an Allen key. The magnets were much easier. Just make sure the scope is protected by plastic (etc) at the foot of the magnet.

[DAT]

Hi there, thanks for the replies. How much weight is needed at the bottom of the ota to support the eps question?

Edited December 9, 2023 by DAT

[cajen2]

An odd but effective ready-made magnet is an aquarium algae magnet: made for removing algae from the inside of the glass. These are obviously made not to scratch glass so won't harm your dob's paintwork. You'd need a fairly large one.

[DAT]

18 minutes ago, bosun21 said:

I have always used these.

https://amzn.eu/d/gTojxHc..

I used one on my 8" and sometimes two on my 12". I just wrapped them in fabric to protect the tube. I don't like having to tighten the tensioners too much as it can cause a lot of stiction in the alt movement. I've just bought another two for my new 10" as I included the previously owned ones when I sold the scopes.

Can’t seem to load that link…

 

[bosun21]

11 minutes ago, DAT said:

Can’t seem to load that link…

 

Sorry, there you go 👇.

 

Ferrite Pot Magnet for Arts, Crafts, Model Making, DIY, Hobbies, Office, and Home - 100mm dia x 22mm thick x 11mm hole - 68kg Pull https://amzn.eu/d/gTojxHc

Edited December 9, 2023 by bosun21

[John]

I used a sliding weight system on my old 12 inch dob. I had a couple of weights (.75kg and 1.5kg I seem to recall) and I could vary their positioning along a magnetic strip and combining the weights for the really heavy eyepieces eg: the 17mm ES 92 degree monster which is the heaviest that I've owned.

My current dob is a Skywatcher 200P classic and that manages to handle my Nagler 31mm and Ethos 21mm without needing a counterweight - the tension control is enough.

Here is my 12 inch dob system:

 

 

Edited December 9, 2023 by John

[bosun21]

Just now, John said:

I used a sliding weight system on my old 12 inch dob. I had a couple of weights (.75kg and 1.5kg I seem to recall) and by varying their positioning along a magnetic strip and combining the weights for the really heavy eyepieces eg: the 17mm ES 92 degree monster which is the heaviest that I've owned.

My current dob is a Skywatcher 200P classic and that manages to handle my Nagler 31mm and Ethos 21mm without needing a counterweight - the tension control is enough.

Here is my 12 inch dob system:

 

 

Nice little mod there John. I just used two strips of black adhesive draught excluder on the bottom of my 12". Worked well. Not as nice as yours though.

[DAT]

2 minutes ago, John said:

I used a sliding weight system on my old 12 inch dob. I had a couple of weights (.75kg and 1.5kg I seem to recall) and I could vary their positioning along a magnetic strip and combining the weights for the really heavy eyepieces eg: the 17mm ES 92 degree monster which is the heaviest that I've owned.

My current dob is a Skywatcher 200P classic and that manages to handle my Nagler 31mm and Ethos 21mm without needing a counterweight - the tension control is enough.

Here is my 12 inch dob system:

 

 

Did that strip have to be screwed on? Looks nice!

[DAT]

Am I right in thinking all strips would need to be fixed in one way or another? I am assuming the strips in question are the sort used for knives….

[bosun21]

7 minutes ago, DAT said:

Hi there, thanks for the replies. How much weight is needed at the bottom of the ota to support the eps question?

The magnets I linked to are approximately 900g.

[John]

3 minutes ago, DAT said:

Did that strip have to be screwed on? Looks nice!

It was a magnetic knife rack and held on to the scope tube using very strong double sided sticky pads. I could have bolted it on but didn't need to.

The tube of the scope (an Orion Optics) is aluminium so I could not stick magnetic weights straight on to it.

[bosun21]

1 minute ago, DAT said:

Am I right in thinking all strips would need to be fixed in one way or another? I am assuming the strips in question are the sort used for knives….

You don't really need strips if the magnet has a strong enough pull allowing you to wrap it in fabric and it will stick solidly to the tube of the scope. The ones I linked to work great. One would be enough for an 8".

[cajen2]

If you balance the dob with no EP in it (by adjusting the alt pivot position), logically the magnet needs to be no heavier than your biggest EP.

[John]

Dobs get increasingly nose heavy as the altitude they are pointed at gets lower. So a system that is easily adjustable is needed to compensate for that. 

[bosun21]

I had both my SL dobsonian scopes adjusted on the fulcrum positioner to maximise the rear weight and I still required the magnet(s). I also had a Starsense unit fitted though. One without the Starsense and two with it and my phone.

Edited December 9, 2023 by bosun21

[John]

Apparently, the ideal position for a lower tube counterweight is on the opposite side of the tube to the focuser. That is not easy to apply though in real-world dob design.

I'm surprised that the rather simple Skywatcher tension handles approach is as effective as it is. It does seem to do the job though. I thought the Stellalyra dobs had a tension system on the alt axis ?

Edited December 9, 2023 by John

[bosun21]

1 minute ago, John said:

Apparently, the ideal position for a lower tube counterweight is on the opposite side of the tube to the focuser. That is not easy to apply though in real-world dob design.

That makes a lot of sense. You've planted a seed now 🤔.

[bosun21]

5 minutes ago, John said:

Apparently, the ideal position for a lower tube counterweight is on the opposite side of the tube to the focuser. That is not easy to apply though in real-world dob design.

I'm surprised that the rather simple Skywatcher tension handles approach is as effective as it is. It does seem to do the job though. I thought the Stellalyra dobs had a tension system on the alt axis ?

They do. I found that tightening them to achieve balance when front loaded caused stiction on the alt axis.

[bosun21]

11 minutes ago, John said:

tension handles approach is as effective as it is. It does seem to do the job though. I thought the Stellalyra dobs had a tension system on the alt axis ?

I think it’s due to the SW tensioners having a small surface area whereas the SL are about 3.5” in diameter causing more stiction.

[Dave scutt]

I went for the cheap option, some velcro and a 1kg ankle weight . Worked well with the 21mm ethos

 

[DAT]

Thanks you all for your replies, definitely some good options to work with. I like the option of a strip that you can move weights up and down on as that would appear to make it quicker to adjust balance when swapping between average, medium and heavy eps. I think the easiest seems to be a simple (strong) weight wrapped in  something that I can attach straight to the ota. I will most likely start with that and see how I get on