Sky Watcher AZ GTI Mount ?

[shropshire lad]

@Space Oddities , still no reply from AZ-GTI User Group, but I have joined DSLR Astronomy Imaging Group without any problem, so I think the AZ one have a problem with their Joining routine as I have tried with a different FB account (my wife's) and no joy on that one either, so may be you could ask or open a thread on there to ask if all is ok.

 

Paul

 

[AstroNebulee]

On 18/11/2020 at 19:25, vlaiv said:

Well, I did say that I added a proper wedge in the mean time since ball head is a bit of a pain to use, really.

I'm going to explain the principle of polar alignment for both ball head and proper wedge because they differ only in mechanics of motion - not the principle of operation. Proper wedge has separate controls for alt and az adjustment, while ball head, at least the model I have - has azimuth separately, but altitude is combined with "spin". Spin is of no consequence for polar alignment, so one only needs to be careful when handling ball head not to make large moves - otherwise it will be the same as proper wedge.

Here is the image of my setup for polar alignment:

It is so simple - even my dog can do it in daylight - as this image documents

You don't need dedicated polar scope - simple finder scope will do. One I'm using is Skywatcher 50mm x8 (or is it x9 - does not really matter).

First thing to know is - how precise you want your polar alignment to be. Most people blame polar alignment for trailing in the images, but I've found that even mediocre polar alignment will not cause as much trailing as periodic error. In most cases periodic error will be limiting factor on your exposure and not polar alignment.

For example, if I'm imaging with 85mm lens and using 4.8µm pixel camera, I'm imaging at 11.65"/px

(use this calculator for above: http://www.wilmslowastro.com/software/formulae.htm#ARCSEC_PIXEL)

I know that this might seem scary - math and all those strange terms, but don't worry about details now. I just want to put some perspective on how important polar alignment is or is not for wide field use (but if you follow the math - you'll be able to apply it for your use case as well).

This means that if I don't want any trailing due to polar alignment error, drift rate due to polar alignment error needs to be about 1px or less. Let's say I want to do 60 second exposure. My drift rate must be less than 1px / 60s = 11.65" / 60s = ~ 0.194"/s (or 11.65" per minute).

Using this tool: http://celestialwonders.com/tools/driftRateCalc.html

We can calculate drift rate based on how far away we are from NCP (North celestial pole).

For example if I'm 44 arc minutes away from NCP (you'll see later why is this important), I get this result in worst case scenario:

My drift rate is 11.5" per minute - very nice, I'll have just shy 1px of drift per exposure in worst case scenario.

Why did I choose 44 arc minutes, you may wonder? Well, it is because of this:

Polaris is roughly 44 arc minutes away from NCP. If you polar align to Polaris or closer - you'll get above drift rate or better. Btw, 44 arc minutes is 50% larger angular size than the full Moon!

Both the Moon and the Sun roughly have half of degree of angular diameter - that is 30 arc minutes. So you can be as much as full Moon width away from true north and still have no trailing in wide field shots.

This just shows level of importance of precise polar alignment - with wide field - it is not as important as often believed.

Ok, now on to polar alignment procedure (same for ball head and for wedge).

1. Align finder scope to RA axis.

In this part - you only need to touch alignment bolts on finder scope - and not ball head / wedge (you will move them but not for alignment purposes).

- Turn finder scope to one side of pier - let's say east (make finder scope lie on its "side" - much like in image above).

- Using ball head / wedge  - train cross hairs of finder scope onto Polaris

- Now turn finder scope on other side of pier - west (if first one was east) by rotating mount 180 around RA axis.

- Look thru finder scope - if Polaris is still under cross hairs in the center - you are done. If Polaris is off center - use finder scope adjustment screws to move finder cross hairs half way between where it is and Polaris.

Here is diagram for better understanding:

After that, return to step one of this part and repeat (move finder scope east of pier, center on Polaris, .....)

It usually takes only one to two rounds to align finder scope with RA axis if you are careful and Polaris stays under cross hairs on both sides of pier. Btw - you move east / west of pier by releasing RA clutch and rotating whole mount - not by wedge / ball head nor by adjusting polar scope.

2. Part two - polar alignment.

- If you are happy with 44 arc minutes of polar alignment error - you are done, just leave cross hairs on Polaris

- If you want more precise polar alignment, then you need to either use same finder as I'm having or get Stellarium and Oculars plugin. It allows you to simulate what you'll see thru your finder scope.

This is roughly the view you'll see thru 8x50 finder. There are three important stars. First is obviously Polaris - it will be by far the strongest one. There are two additional stars - around magnitude 6. They will be visible and brighter than other stars so you should not be having trouble identifying them - but they will not be as bright as Polaris. You find them by finding nearest bright star to Polaris and then finding next star by distance from Polaris - having same brightness as that first one.

Procedure is fairly simple - imagine line connecting those two other stars. Find middle of that line and mentally make line perpendicular to it. Take two lengths of distance between first line and Polaris. That is the place you should put your finder cross hairs at. Do it by moving wedge / ball head in az and alt.

Here is what it should look like without "helper" lines and markings:

Note that rotation of stars will be different each time you do this, but relative distances will be the same if you use same finder scope.

With this method you'll get within 5-10 arc minutes of NCP which is good 2.6" / minute of drift. Good for one minute exposure under highest resolutions I would recommend being imaged on this mount (3"/px), or longer exposure if you use lower resolution.

In fact, like I said - periodic error is more likely to cause issues than polar alignment. I think that worm period for AzGTI is around 10 minutes. With common periodic error for small mount of around 60" - this gives us something like 10" / minute of drift or worse (depends on actual PE curve). This is at best on level of Polar alignment error drift - and probably worse.

Here is animation from my recent session with AzGti. This is one hour of exposure - 60 x 60s exposures - not aligned and animated to show star drift:

Left to right motion is due to periodic error of the mount and slight downward drift is due to polar alignment error. You can see drastic difference between the magnitude of the two errors, and it also nicely shows how "periodic" - periodic error is (hence the name obviously).

In order to combat periodic error - one really needs to guide (or use periodic error correction  - but that is whole another story).

 

 

Love this summary and description, as a new az gti owner I'm following this thread with much interest, thank you for this. Clear skies 

[shropshire lad]

@LeeHore7, Hi ... I'm a new user also ..... if you get the chance see if you can join the AZ-GTI user group .... https://www.facebook.com/groups/402745617192175.

I have been trying to join for a few weeks now but with no joy ... 

[AstroNebulee]

3 hours ago, shropshire lad said:

@LeeHore7, Hi ... I'm a new user also ..... if you get the chance see if you can join the AZ-GTI user group .... https://www.facebook.com/groups/402745617192175.

I have been trying to join for a few weeks now but with no joy ... 

Hi, thank you but I don't do Facebook, I'll keep following any az gti content on here though, clear skies 

[shropshire lad]

On 15/01/2021 at 07:43, Space Oddities said:

Hmm strange, there are new posts every day... If you want, PM me your username, maybe I can add you from inside?

I have now been excepted into that AZ-GTI User Group.