Equatorial platform polar alignment help!

[Astro_Nic]

Hi everyone

Hopefully this is in the correct forum....so many to choose from!

So I have a 14 inch OO UK dobsonian on a TEPP equatorial platform.  I need a better way of getting great polar alignment for astrophotography.  Now I have a large aperture dob on an EQ platform, so I'm not expecting 300s exposures or anything and would be happy with 10s TBH.

I also have an OAG with ZWO ASI174MM mini, the platform has the ST4 guiding port, and I will be using a ZWO ASI2600MC with Antlia 5nm dual band filter with a view to using it for some DSOs.  Oh and also a Nexus Coma/ 0.75x reducer.

So far I have been playing around with placing a laser in a TEPP laser holder that I attach to the RA axis at one of the ends.  I just kinda guess using Stellarium etc where the actual celestial North is versus the North star.  I've been using a ZWO ASI678MC and 2.5x powermate for some moon and planetary action.  Realistically I'm getting about a minute at that mag before the image slowly moves enough that I re-centre and start again.  Probably fine for planetary/moon but not DSO I expect.  For visual on the moon I can have an Ethos 13mm in the eyepiece, wander off and 10 mins later there doesn't appear to be much movement.

I want the next level of polar alignment.

I was thinking of either the iOptron ipolar electronic polar scope or the QHY Pole Master.  Both have a flat base that I can attach to my RA axis like I do my laser.  I have no real idea how these devices work though and as mentioned, I'm not looking for massively long exposures as with 14 inches of F3.5 I don't think I need it.  I've seen reviews where long exposures with the iOptron is problematic, but hat probably wouldn't be an issue.

Will my idea work?  One better than the other?  Any other options?  I'm leaning towards the iOptron becase the setup of the polemaster talks about moving the mount 30 degrees then another 30 degrees to calibrate and I'm not sure this will be compatible with an EQ platform.

So my question (after all the waffle above), is will the iOptron ipolar work on an EQ platform?

Thanks you very much for your help.  Polar alignment is such a pain!

Nic 

Edited March 24, 2023 by Cornelius Varley
text colour removed

[Elp]

As far as I've experienced, all PA routines involve taking an image near Polaris, then the mount rotating in RA, then the routine taking another image to determine error distance from Polaris.

I've used PHD2, ipolar and asiair, they're all the same in how they work. You then make minor adjustments in altitude and azimuth of the mount head until you're aligned according to the routine feedback.

The asiair is the easiest I've used but requires the purchase of the unit and you're tied to using zwo cameras, but you have the option to use your phone or a tablet to connect to it so easier to use outdoors. If you've got a laptop handy the ipolar works with that once you've downloaded the software from ioptron. Again if a laptop is available there is free software you can use.

I'd assume the tepp works like an EQ mount, though looking at it I don't see how it tilts like one, it looks more like an alt az to me.

Edited March 24, 2023 by Elp

[alexbb]

Can't answer all your questions, but:

The platform doesn't have a true DEC compensation. I tried to do this to my DIY modified platform and it helps only over some sky areas.

iPolar also needs some calibration, at least that's what I remember since I set up for the fist time my iOptron mount.

True north is at ~40 arc minutes between Polaris and Kochab. The full moon is ~30 arc minutes. So the true north is at a distance of 1 1/3 moons away from Polaris, towards Kochab (Little Dipper).

[michael8554]

Hi Nic

I may be wrong, but doesn't the laser need to be mounted on a stationary part of the platform, so that the platform can be pointed accurately at the NCP  ?

Not "to the RA axis at one of the ends".

Michael

Edited March 25, 2023 by michael8554

[Astro_Nic]

The platform is a dual axis equatorial photographic platform.  The manufacturers show some great images using 180s subs for the full 110 minutes of each rotation.  So I think it is up to the job, although there are some DEC limitations as mentioned.

I attach two photos of how they suggest you polar align.  I was thinking that something like the iPolar would be significantly more accurate, but don't know if it would work.  They suggest it should.

The issue is that this is an EQ platform, not a GEM.  So using SHarpcap or other program isn't an option - It'sa real struggle to get the dob on the mount - limit of my strength, so I don't want to have to lift off the scope, move the platform 5mm, put it back, find out it should have been moved more/less etc and do that a hundred times.  I cannot move the platform with the scope on it - far too heavy.

So need something that attaches to the platform base before I add the scope and base.

Just need to know if an iPolar attached to the RA plate at the front will work, as these devices are designed to attach to a GEM in a specific way.

Thanks

Nic

 

Thanks

[Elp]

The ipolar attaches through the centre axis of the ra, so when the ra rotates it'll pick up on any discrepancy from the central axis when looking at Polaris.

As long as your mount rotates via a central axis, it should be similar. Note though, part of the PA routine involves initial Polaris image, then mount rotation and second Polaris image for it to figure it out, then a third if I recall. The asiair takes the image through your telescope camera (so not central to ra), then rotates the mount 60 degrees and takes a second image to work out how far it is off initially.

[Astro_Nic]

Trying to understand this!

Looking at the pics above, the ipolar will be attached to the RA axis, but in my example, I would have to rotate the camera only in situ - that way it is rotating around the RA axis.  I was thinking I would then move the mount in RA - but that would move the whole camera to a different position.  Is that right or have I got it wrong?

Thanks!

[Elp]

You could always test your theory with your current cameras just to test the positional shift. Theoretically, if Polaris is dead centre in the camera, by rotating it it shouldn't move at all so you'd know you are reasonably aligned. Note Polaris for proper PA isn't actually central, it rotates around a central pole depending on what year it is.

You don't necessarily need Polaris, people have been drift aligning for decades as an alternative and more accurate PA method.