Equatorial Platform - New Build

[Stub Mandrel]

Excellent!

[Rusted]

Given a suitable, ring form support, it would be quite possible to bolt a sturdy stud [all thread] to the centre of the upper moving platform surface.
A compact, lead weight on the stud, placed well below the platform, would lower the C of G quite nicely.
The platform base plate would need a slot to clear the counterweight bar. Some commercial equatorial platforms use open frames.
While the lead weight would make portability an issue it would greatly improve stability.

For larger instruments [Dobs] I could imagine the platform's baseplate or frame mounted on a concrete pipe buried in the ground.
With the pipe projecting from the ground only enough for clearance. To avoid lifting heavy DOBs to place them on the platform.
The diameter of the pipe would limit the allowable tilt of the upper platform when the lead weight reached the inside of the pipe. 
Which suggests a cylindrical lead weight for minimum diameter. 
 

[Rusted]

[wobblewing]

Ah yes, my version here is not very big.  It's mainly designed to be used on a small Skywatcher Heritage 150p .. there is plenty of 'margin' on the CofG.. I think now I've done it, it could be used for bigger dobs without much modification.

Edited February 22, 2021 by wobblewing

[wobblewing]

I had no issue at all with the error induced from pushing the platform following a circle profile by using a linear leadscrew.  As you can see the motion is as good as linear through its entire travel, so a fixed speed across it (without varying across its travel according to position) worked out fine.

 

I was going to replace the brass leadscrew 'nuts' with a split nut design so it's fast to reset.. but I found that it only took about 10seconds to spin it back by hand - so I didn't bother.

There are no limit stops with microswitches, I basically reset it to the start position and press a button to reset a counter.. once it reaches the count limit at the end of travel it stops and flashes the screen to indicate a reset is required (although it's pretty obvious of course!)

Edited February 22, 2021 by wobblewing

[AlexK]

You forgot to add a link to your build thread: 

 

[Stub Mandrel]

On 22/02/2021 at 13:29, wobblewing said:

I had no issue at all with the error induced from pushing the platform following a circle profile by using a linear leadscrew.  As you can see the motion is as good as linear through its entire travel, so a fixed speed across it (without varying across its travel according to position) worked out fine.

In principle, it out to be reasonably simple to implement RA-only guiding. With good polar alignment 60 second exposures with a ~60" fl should be achievable. That could be fun!

[wobblewing]

9 minutes ago, Stub Mandrel said:

In principle, it out to be reasonably simple to implement RA-only guiding. With good polar alignment 60 second exposures with a ~60" fl should be achievable. That could be fun!

I'm struggling to get spot on Polaris alignment throughout the whole travel, but this is because of me 🙂 and I'm getting used to it!  I'm thinking about making a little gadget that clips onto the back to help with alignment (or maybe something that clamps the OTA into a fixed spot for alignment with the red dot finder).  That said, rough alignment seems to work perfectly for visual.

Edited February 23, 2021 by wobblewing

[wobblewing]

One of the difficulties i have is that i cannot easily see polaris from my observation spot.   My garden is south facing but just around the other side of my house is a street lamp which causes some interference.   So I sit the 'scope out the way off the street lamp by hiding behind the house,  but then i cannot see polaris!

 

I waited for solar noon, projected the sun's position onto the ground with a broomstick and then put a small mark on my patio to line the platform against!

Edited February 23, 2021 by wobblewing

[AlexK]

That's still quite rough. Even Polaris itself is too rough for imaging.  Use this method: https://skyandtelescope.org/astronomy-resources/accurate-polar-alignment/

To skip that ordeal in the future, use the APM method "Artificial Pole Marker".

The easiest would be to use an RLP (red laser pointer) rigidly mounted on the platform's base (at least at the time of alignment) in a way to have its beam projected somewhere on the house wall where you can always precisely reproduce its position in the future. Just perfect the drift alignment above once, then mount the GLP and either align it with easy to remember/reach point on the house (the farther the better), or just go and mark that position with some marker/object. Just make sure you can see that target at night or have some light source to turn on nearby. I know folks sticking a retro reflective tape which is easy to spot when hit by the beam. Surely, the base location should be marked very well too. You can easily calculate the required accuracy of the PA, marker position, and EQP location depending on your camera setup parameters (there are some PA apps for that as well).

One of the RLP mounting methods (for those without a printer) is a dab of epoxy putty around its body in a paper sleeve (so it's not sticking to the RLP) and some mounting wedge roughly in the needed direction (i.e. to the corner of the top floor window frame). Then just plop the RLP in the putty blob on it, align, let polymerize without shifting. A green pointer would also work. But it might be too bright and known to attract LEOs  

Edited February 24, 2021 by AlexK

[wobblewing]

I just realised I put this in the wrong thread - I've copied this post over to there, but cannot seem to delete it from here! Sorry.

 

Well, clear night tonight finally - albeit with a full moon!

So, testing the tracking out.. here's a hyperlapse video.. bear in mind this is ~22mins squash into 22seconds!

I can juuuuust notice the non-linearity, which equates to about one third to a half millisecond timing on a step..  Will fiddle with the code, but it's not really necessary as it exceeds the exposure length I can do with my phone up against the lens.

Not quite sue what the up and down is all about 🙂  I think my PA needs work... But the hyperlapse really amplifies it... in real time it doesn't seem like it moves at all.

 

 

Edited February 26, 2021 by wobblewing

[AlexK]

Just delete it here. Put "wrong thread". And notify moderatord (3 dots menu).

Edited February 26, 2021 by AlexK

[wobblewing]

I'm running another timelapse video over the whole sweep, so it might help characterise the non-linearity (maybe)