How far along bar to mount SCT OTA

[BRUN]

ive never had an SCT before, how do you know how far along the dovetail to clamp it in place on your mount ?

i presume you need to make sure your diagonal and EP will clear the base of the mount if looking directly up ?

[Cosmic Geoff]

On my C8 I push the OTA almost as far forward (upward) as it will go. 

[BRUN]

Thanks Geoff, i assume its a question of the best balance and also clearance ?

[Stargazer McCabe]

8 minutes ago, BRUN said:

Thanks Geoff, i assume its a question of the best balance and also clearance ?

It's all about balance as you say... Whether EQ or Alt / Az I find the OTA has to be mounted, the finder, diagonal and eyepiece added and then find the point of balance by sliding it back and forth...

If I'm out with the heavy eyepieces it sometimes takes the dovetail being pushed forwards AND a kilo mount at the front of the tube / dovetail to get that point right...

trial and hopefully not too much error matey...

[BRUN]

my mount seems quite strong/powerful so i dont suppose it matters too much but always best to have it in the best position to reduce the load on the motor i guess

[2STAR]

35 minutes ago, BRUN said:

ive never had an SCT before, how do you know how far along the dovetail to clamp it in place on your mount ?

i presume you need to make sure your diagonal and EP will clear the base of the mount if looking directly up ?

Hi Brun, I presume you are using an Alt/Az mount with your SCT, if so the balance is not important at all, I push my Scope as far forward as possible in order to gain as much clearance for my FR, T ext, Camera, balance is only important if using GEQ.

Eric 

[SilverAstro]

54 minutes ago, BRUN said:

my mount seems quite strong/powerful

I was thinking the same about my Evo mount also I think I read somewhere that it uses sturdy brass worm & wheel whereas earlier mounts were spur gears ? but I may be wrong ! and other manufacturers might not even be metal gears,  it does pay to ask and gather infos. so I am also interested in this topic that you raise.

I think it is not a good idea to have it in exact balance as then backlash can slow things up as the drive hunts back/forward. So a bit of a pre-load in one direction or the other will be good I think.

Yep far enough forward at least to clear the eyepiece etc from disasters !

[Knighty2112]

I place my C8 SCT as far forward into my 8SE goto mount dovetail clamp as possible. Only had an issue with balance once, and not sure why really, as it has never repeated itself so far.

[SilverAstro]

If an OTA is so far forward that it heavily 'over balances' the drive chain might it cause a slipping clutch protection mechanism ?

This relates to the eyepiece scenario, are there any drive disengagement devices commonly in use in mounts to prevent the eyepiece, or even the drive train  suffering from the full force of a stalling motor ?

[Cosmic Geoff]

The Celestron SLT and SE have a slip clutch for the altitude axis. No need to worry overmuch about overloading the drive train. If the scope tail hits the mount it just ruins your GoTo alignment.

[SilverAstro]

1 hour ago, Cosmic Geoff said:

 it just ruins your GoTo alignment.

ah so, might this explain the many failures that cause so much trouble on fora

But seriously : I dont suppose you have measured the torque that this kicks in at ? (or out as the case may be!) and compared it with the torque of an overbalanced OTA position ?

Scientists and engineers everywhere need to know these things

[Cosmic Geoff]

The Celestron SLT altitude slip clutch is definitely adjustable (done it), and this may also apply to other mounts. It should not be hard to find out if it is about to slip through over-balance or lack of grip - just apply some hand pressure.

[SilverAstro]

I did ! and it didnt !! but I didnt want to apply too much, so I will consider it some more I will hunt around to see if the Evo also is, how, where and by how much.

 

[LukeSkywatcher]

I also mount my 8SE sct as far forward as possible. As said, balance isnt too important when the scope is motorized. You want clearance at the eyepiece end so that you can look as far up towards the zenith as possible.

[2STAR]

On 9/19/2017 at 11:50, SilverAstro said:

If an OTA is so far forward that it heavily 'over balances' the drive chain might it cause a slipping clutch protection mechanism ?

This relates to the eyepiece scenario, are there any drive disengagement devices commonly in use in mounts to prevent the eyepiece, or even the drive train  suffering from the full force of a stalling motor ?

This is no problem with the EVO worm gears, front heavy is fine, I push my OTA as far forward as is possible, no more backlash than when it is balanced, the motor handles the OTA wherever it is loaded, I have hit the base a couple of times and like said only makes a a mess your alignment.

eric 

[SilverAstro]

7 minutes ago, 2STAR said:

This is no problem with the EVO worm gears, front heavy is fine, I push my OTA as far forward as is possible, no more backlash than when it is balanced, the motor handles the OTA wherever it is loaded, I have hit the base a couple of times and like said only makes a a mess your alignment.

eric 

Thanks, that is all good to know.

I (or it maybe ) once made a mistake with a search and it headed off to the zenith unexpectedly, I stuck one fist in the narrowing gap whilst reaching for the power switch with the other, it was still resisting my pull with a fair urge by the time I got the switch off, so I have been concerned ever since that there might not have been a slip device. Will have to ask the dealer if it may be set too tight.  - -  Or just make sure that it is always further forward than I had set it then when I was new to it !! make more sense wouldnt it lol!

As for upsetting the GoTo, I find recently that I often dont bother with aligning it, just go to pointing it manually !

Thanks all.

 

[2STAR]

Could be difficult locating some DSO's manually, but a guy at our society does it with great accuracy the old fashioned Star Map route.