6 replies to this topic

[CreAtivSpelErr]

I mostly do astrophotography and polar align by plate solving so I don't have a polar alignment scope on the rig.  But in this case I am setting up in a visual configuration for a star party.

Option 1: Set up the mount in GEM configuration, get Polaris roughly centered in the field of view.  Do a multi-star alignment.  Mechanically the GEM axis will be off of the pole by a bit and the software model would have to compensate for that. 

Option 2: Set up the mount in Alt/Az configuration, do a multi-star alignment.

Which option do you think will produce the more accurate alignment? 

My guess is that although GEM has a higher "accuracy ceiling", I think having the software model and the mechanical axis conflicting with each other will produce a worse result than the alt/az where it is not relying on the mechanical alignment in the first place.

 

[Astrojensen]

 

My guess is that although GEM has a higher "accuracy ceiling", I think having the software model and the mechanical axis conflicting with each other will produce a worse result than the alt/az where it is not relying on the mechanical alignment in the first place.

What makes you think the GEM has a higher "accuracy ceiling"? What makes you think the software uses different models for EQ or ALTAZ operation? 

 

Fundamentally, they could be the same, as an ALTAZ mount is just an EQ mount with really poor alignment. 

 

For visual use, I think you're overthinking the situation by an order of magnitude. Use whatever mounting configuration you find most practical and you'll be fine. 

 

 

Clear skies!

Thomas, Denmark

[UnityLover]

I mostly do astrophotography and polar align by plate solving so I don't have a polar alignment scope on the rig.  But in this case I am setting up in a visual configuration for a star party.

Option 1: Set up the mount in GEM configuration, get Polaris roughly centered in the field of view.  Do a multi-star alignment.  Mechanically the GEM axis will be off of the pole by a bit and the software model would have to compensate for that. 

Option 2: Set up the mount in Alt/Az configuration, do a multi-star alignment.

Which option do you think will produce the more accurate alignment? 

My guess is that although GEM has a higher "accuracy ceiling", I think having the software model and the mechanical axis conflicting with each other will produce a worse result than the alt/az where it is not relying on the mechanical alignment in the first place.

For visual use, option 1 is fine.

[CreAtivSpelErr]

Question: "What makes you think the GEM has a higher "accuracy ceiling"?

Answer: "an ALTAZ mount is just an EQ mount with really poor alignment." 

 

Also, I've seen videos saying, "Yes you actually can do astrophotography with an alt/az!" but then they go on to say "with shorter exposure times" and this or that procedure to compensate for the tracking... which is basically admitting that while it is possible, a GEM really would be more accurate in the first place.  Some of that is about field rotation and guiding accuracy and in this context those factors don't matter.  I just want to be able to slew between targets and have it dead center.  Since I only do a little visual, I'm not that fast at finding targets, and it's for an outreach event so I want to spend as little time hunting as possible.  

 

But perhaps the comment "an ALTAZ mount is just an EQ mount with really poor alignment" was meant to indicate that if the software map can handle alt/az, then it can also handle an arbitrary level of misalignment in EQ mode.  How the software model deals with mechanical misalignment is essentially what I am asking about, but I would not assume that it can handle arbitrary misalignment nor that the models are the same.  Case in point: in EQ mode, manually slewing to Polaris is tricky - I don't mean using the adjustment screws, I mean using the motors and trying to get Polaris centered could involve meridian flips just to get the star to move a tiny bit in the field of view.  The software would need to address the same challenge within its code.  But in Alt/az it's a piece of cake.  Also, in EQ mode I have sometimes tried doing a 3 star alignment without spending much time on mechanically aligning the axis and the alignment fails. 

 

Theory aside, I suppose that is the experience that is driving the question.  On my old mount I had to put a fair amount of time into the polar alignment or risk having the 3 star alignment fail.

[Phil Sherman]

I can set up my mount in either alt-az or GEM mode. If I'm using it for visual use, especially for star parties, alt-az is the only way to go. Alt-az keeps the height of the refractor's eyepiece in a relatively small range of heights above the ground. This makes it simpler to let everyone from small kids to full sized adults to look through the eyepiece. 

 

My biggest gripe is that the Skywatcher hand controller doesn't appear to support the simplest method of aligning an alt-az mount. This is the scope tube level and pointing North. My older lightweight single arm alt-az mounts' were easily aligned using this technique by pointing at Polaris, moving the tube to a horizontal position using a level then power cycling the mount.

[SeattleScott]

Either approach should work fine for visual. I mean, you can just point the scope North. It just has to be in the right zip code of Polaris.

For visual, field rotation doesn’t matter.

I would do whatever is simplist.

[Echolight]

Mostly depends on making sure that the alignment stars are the alignment stars, and how accurately the user centers the alignment stars.