OTA Rotation and Balance

[TerraPassenger]

Hello:

 

I'm interested in purchasing a DDM60Pro.  I was wondering how sensitive the DDM60Pro is to balance.  If I have a guide scope on my OTA and I rotate the OTA within the tube rings to better frame an object within my CCD, would I need to rebalance the mount?

[Jay]

Most likely you have to. But why would you want to rotate the OTA and not the camera ?

[TerraPassenger]

Thanks Jay.

 

In fast systems (say f/3), orthogonality of the CCD is critical.  It may be a bit safer to rotate the entire OTA instead of just the CCD so that orthoganality is maintained.

[RamaSpaceShip]

Hi,

 

You can avoid the need to rebalance if you put additional weight to balance the focuser and the camera on the OTA itself. Then rotating the OTA does not change the position of its centre of gravity and so no rebalance is needed. But this means more weight and then you may reach the weight limit of the DDM60. It may add also some flexure.

 

Regards.

 

Bernard

[NorbertG]

Hi,

 

what about mounting the guide scope on the tube rings ?

 

Regards

Norbert

[GeorgeCarey]

^{Why do you need a guide scope?}

^{Once you have a good pointing model there is no need for a guide scope. Any object you want to aim at is very near the centre of the image.}

^{Rotating the OTA is not a good idea. The axis of the telescope will not be exactly parallel to the DEC axis.}

^{Your pointing model will be ruined if you rotate the telescope.}

^George

Edited January 20, 2014 by GeorgeCarey

[nakbrooks]

The one thing I've learned with DDM mounts is that they are very sensitive to balance so I try not to change anything once I've got a good balance and tune.  Also if you start rotating the camera you may have to re-do flats, etc.  I did have a rotator to enable adjustment to framing but removed it because my camera has a square CCD so it isn't really necessary.  Personally I prefer to keep everything locked solid - there are enough variables to contend with already without introducing new ones!

[NorbertG]

George,

 

you are correct, that the unguided Operation does not allow any changes. But your answer implies even a finder is not necessaary. But for mobile usage you need it., although it could be a very simple one. (I am also wondering if the thread opener is talking about guiding scope or  finder scope ? ).

Just the first sync´s require the finder, then you can take it off. I have put a light weight 90degree finder on a bar on top of the tube rings and I can move it along the bar to adjust the Balance in DEC.

 

Since every f/3 scope is newtonian style (except for  Schmidt cameras) so the change of the focuser/Image train  direction alone will not allow tube rotation without screwing up everything.

 

regards

Norbert

[GeorgeCarey]

Yes, I agree that for mobile use a finder may be necessary. However, when I have taken the scope off the mount for maintenance (cleaning the mirrors etc) and put it back, I have to find a star for the first sync. I do this in a few ways.

 

1 I hold a green laser pointer against the edge of the telescope mounting plate on the mount.

2 I sight along the line of the tube ring fasteners.

3 I sight along the edge of the telescope plate.

4 I look along the edge of the telescope and set it up so that a bright star can be seen near three sides of the tube.

 

Any one of these is good enought to get the star on the camera CCD after one or two tries.

 

I admit that it requires a certain skill to do this. Some of my friends at our astronomy club have tried it and failed. Others have learned to do it after a few tries.

[Jay]

 

Since every f/3 scope is newtonian style (except for  Schmidt cameras)...

 

 

 

Not quite true

http://www.officinastellare.com/products_scheda.php?idProd=15

http://www.officinastellare.com/products_scheda.php?idProd=16

Edited January 21, 2014 by Jay

[NorbertG]

nice... but we should wait with links to the competitors until the next autoslew is released... until that the ASA H type is superior and we consider it as links to interesting guider scopes, don´t we ?

[Jay]

Norbert,

 

No I don't agree with you. Links were just to show that your statement ”every f/3 scope is a Newtonian style…”  is not true. Besides Veloce series is well known, and anybody who’s looking for a fast high end scope will consider them as well as ASA alternatives for sure …so no great secrets have been revealed. In addition, OS is selling also ASA mounts, so I'm sure they are not such a dreadful competitor that their name shouldn't be mentioned here

 

P.S. It’s nice that you feel the need to let us know your opinion on what’s ok to post here and what’s not. However, I’m sure ASA moderators are very capable of making those decisions themselves and will delete those links if they for some reason see them improper. 

[NorbertG]

Jay,

 

I was just kidding.

I will never dictate anybody what to post or not, I apologize if that was the impression.

Maybe my english was not good enough to express that. but as non-digital native my usage of smileys wouldn´t be less ambiguous I am afraid.

 

 

Back to the original topic:

The sensitivity of the DDM to balancing is critical, but if one searches in astronomy forums it apperas as it should be necessary to adjust it to the last gramm (or put a coin with tape on the tube). For a mobile setup of my DDM60 I don´t need a caliper to bring the counterweights or tube in Position. If I remove the cover or change a cable it is rather unlikely that I need to rebalance. (of course the markers I use were done with the final configuration and the "imbalanced" operation is mainly limited to the setup procedures, like checking collimation with an eyepiece etc. So in general the need for balance is a little bit inconveniant, but there are many work arounds to live with it.   

The issue might be different for a permanent or remote setup due to the larger variety in mount positions.

[nakbrooks]

That's interesting.  I have a DDM85XL Premium with about 70kg (140kg including counterweights) mounted, which is well within the maximum of 100kg (excluding counterweights) for this mount.  I find it very sensitive to balance - I would have expected a DDM60 carrying much less weight to be more sensitive, not less.

[NorbertG]

Nigel,

 

yes, but it is a different Mount. I  can also confirm that it is much easier to run the 10inch Newton (appx 19 - 20 kg) compared to my C8 with less than 9kg total. 

Trying to run just a small refractor is nearly impossible without extra weight, but I think that is not only a balance problem, maybe also the preload of the roller bearings fits better to the higher weight. I cannot even balance the C8 manually, with the Newton it is no problem to get nearly the same if you do it manually or with the Software routine.  

[Jay]

Sorry Norbert, a bit bad morning

 

I think all ASA mounts are very sensitive to right balancing. However, if your setup is very simple e.g. refractor and camera with not heavy guider scopes etc., it's quite easy to do re-balancing (if needed) quite fast. I would imagine that a more complicated system e.g. Newton with heavy camera +filter wheels might require a lot more time. And of course the heavier the setup is (and closer to the limit), the more demanding is doing the balancing properly.

[Jay]

Nigel,

 

yes, but it is a different Mount. I  can also confirm that it is much easier to run the 10inch Newton (appx 19 - 20 kg) compared to my C8 with less than 9kg total. 

Trying to run just a small refractor is nearly impossible without extra weight, but I think that is not only a balance problem, maybe also the preload of the roller bearings fits better to the higher weight. I cannot even balance the C8 manually, with the Newton it is no problem to get nearly the same if you do it manually or with the Software routine.  

That is very interesting and quite the opposite from my experience, especially what comes to small refractors. When I'm using my 300mm camera lens and SBIG 8300 (with filter wheel) as a stand alone setup, it's VERY fast to tune and balance properly. (I have a DDM60 Pro)

Edited January 22, 2014 by Jay