Equatorial Platform question

[Bucky1379]

I have a really dumb question but I just need to confirm something.

I am designing and building an equatorial platform for my GS-680 and think (hope) I understand most of the issues. The main problem I'm having is sourcing and designing around a drive motor (ie getting the speed and drive ratios correct) but I have a different, REALLY basic question to ask,

In the southern hemisphere, GEM's need to move in the opposite RA direction to 'normal' and the same is obviously true for Equatorial Platforms but the more I try to think it out to confirm this, the more my poor tired, old brain gets confused.

Can someone please give me the gospel truth on this before I buy and mount the motor as I don't want to have to design and build it again because of such a silly mistake. I'd guess that anyone who has ordered one from overseas would know if this (and the latitude) are the things that the design needs to be altered for.

Thanks for any help.

[netwolf]

You are pointing at a different pole so the Anti clockwise (E-W) movement in the north translates to the same AC movement when turned 180deg and pointed south. The design does not have to accommodate for this. Latitude is the same if you design for -33 it will work in +33 also.

If you have not already, then join the Eqplatforms yahoo group.
http://tech.groups.yahoo.com/group/eqplatforms/

Regards
Fahim

[bojan]

In other words, if your platform moves in the wrong direction, just dreverse the direction of your motor rotation. How hard or easy this is to achieve, that depends on the type of the motor you are using.
If it is a stepper motor, you have to re-arrange the cable (or reverse it on the driver circuit). If it is syncron motor, you will probably have to open it and place the plastic ancor the other way (you will know what I am talking about after you open the motor). If it is cage-type AC motor (like the one used in turn-tables), you will have to dismantle it and turn the coil the other way around.
And... if it has the shaft on both sides, just use the other side of the shaft :-)

[Gargoyle_Steve]

I mean no disrespect at all Fahim but is that correct??

Wouldn't the "anticlock" tracking movement required in the Northern Hemisphere cause a mount to track AGAINST the stars, not with them, in the Southern Hemisphere. If I stand in the yard and look south to the SCP the stars are rotating clockwise.

I have to admit I don't own or use a EQ mount or platform at all, but I've had thoughts of getting a Equatorial platform for my dob.

Can someone confirm or otherwise clarify this?

[netwolf]

Steve, looks like i have blundered and made a mistake, i thank you for the correction. The stars would move Anti clockwise in the north and in the south they move clockwise. As per you backyard experiment if you faced north you will not stars move anti clockwise.

[Kokatha man]

Good grief fellas - you mean I've been dudded with both my EQP and my HEQ5 SynScan as well as totally deluded with what I thought I was witnessing in the scope(s)?......they weren't actually tracking.....that I shoulda specified one of those "special Southern Hemisphere EQ's" for each of them?.....an now you tell me I gotta rip apart the motors if just reversing their polarity don't work?.....an how's this Bojan - "steada rippin the cage out how's about I just turn the motor upside down 'n shove it back in this ways?"

Will report back after I've made the mods this thread implies and let you all know!!!

Cheers (and the best of a stirrer's humour) Darryl.

(ps netwolf - I see you didn't follow my parting hints in that (in)famous thread of mine some time back!)

[bojan]

OK, I am originally from North so I should know :-)
If you face North celestial pole, the stars (Big Bear etc) rotate around Polaris Anticlockwise......

Now, here on the Southern hemisphere, if you face South pole, stars rotate around sigma Octans (well, not quite but close enough) Clockwise.

Of course, Earth (and the sky) still rotates in the same direction as it was since I have been on the north side, but it is about where our backyard is ( and we with it) , hence the difference (and sometimes confusion) .

As far as motors are concerned... same principle applies :-)
Here, on your polar mount, the lower side of the shaft points to the North pole, but over there, it is the top side of the shaft.. so the motor must rotate in opposite direction (if the mount is of the same design)

[Kokatha man]

No Bojan, I'm sorry (and I hope noone took any offence at my "humour" - if so I apologise)..... but the reality is there are no such animals as different EQs for Nthn Hemi and for Sthn.....

It's all relative, as you said (virtually) when you are facing north your left hand is pointing west and vice versa for when you're facing south.

The EQ's polar axis is set to point to the SCP "dowunder" and when/if you take your mount north of the equator it's flipped over the vertical position such that it points to the NSP (roughly Polaris) relative to one's latitude.

The direction of rotation of the polar axis isn't altered - it's just that relative to you it's now rotating anti-clockwise and still following the astral motion.

This is why in one thread I rather facetiously proposed we call Right Ascension Left Ascension: it's only a relative thing.

Best wishes, Darryl.

[Kokatha man]

Got out my surplus EQ and now I'm confused......?!

[Kokatha man]

Well, serves me right for not only mouthing off but also being "funny" too! Mea culpa and everything - on further thinking (some could well say just "on thinking") you must be right - though I'd presume(!!??!!) that the drive motors are essentially ac-dc units (usually running on dc) and that polarity reversal would reverse directional movement.....

That's the best I can come up with; I'd best apologise and let others continue....

Regards, Darryl.

[bojan]

No worries Darryl :-) ... I was also a bit humorous :-)