German EQ5 GT mount...

Hi Folks,
Recently bought a new scope with above computerised mount. Invested in polarscope as well for more accurate alignment . On placing the polarscope into the RA port at the setting circles I proceeded to tighten the 3 small screws until I had centred the x as accurately as possible on my object (a pole). The theory is that when the scope is rotated in the RA direction +/- 180 degrees the centre of the x should stay in the same spot on the object (pole) if the polarscope is centred correctly . This is hard to explain :roll: but the central point of the x and indeed the whole small field of view through the polarscope appears to move when mount is rotated (in RA) approx. 180 degrees so in effect the actual central point cannot be accurately found. Its my understanding that while the RA axis is being rotated there is movement up /down and left to right throwing out the window that the RA axis of the mount rotates evenly and equally about a central point. Am I looking for perfection in my new mount (considering I splashed out quite alot) or is this a fault in the RA axis manufacture. I realise the RA axis is a moving part and may have some play in it. I know Dave Grennan you bought the same mount. It would be interesting to place the polarscope in your mount and see if the same result is found.

regards,

Eamonn.

Hi,

I do not have one of these polar finders but I assume that the three screws allow you to adjust the direction that the polar finder is pointing until it is parallel with the rotation axis of the mount. If so then I would start by pointing the mount so that you have a horozontal line on the cross hairs of the finder (like the top of a wall or a roof of a nearby house).
Then rotate your 180 degrees. Now adjust the three screws to get the cross hairs back to half way twoards the top of the wall. Try to move the finder only in the verticle direction.
Repeat this a couple of times and the wall top should be on the cross hair at the zero and 180 degree positions.
Then you have to adjust the horizontal direction of the finder.
For this try to pick a distant view.
Check again at the zero and 180 degree positions and adjust the finder in a horizontal direction only to point at the half way point and keep repeating.

This is taking longer to write than it would to do but have a go and let us know how you get on.

Michael Scully
KAC

Eamonn,

I cant say for certain I understand what you mean but let me tell you about my experience. I found that by polar aligning using the inbuilt polar alignment routine, i get much more accurate alignment than i could by using the polar scope. The thing about the inbuilt alignment is that you must be three star aligned first and you lose that alignment when you polar align. But i think its well worth the effort. I can three star align, polar align and re do the three star alignment in about 5 minutes.

Heres how it works.

1: just center polaris in the finder scope (you dont need to be too accurate).
2: do the three star alignment
3: goto utlilities polar align, you get a warning that this will lose you your three star alignment. Hit enter to proceed.
4: the scope slews to where it thinks polaris should be.
5: adjust the mount by using the altitude and azimuth knobs, DO NOT use the hand controller or move in ra or dec at all. Hit enter

and your done.

I have found this to be much more accurate than using the polar scope, and if you want to get super accurate (say for astrophotography) you can repeat the exercise again (or as many times as you like) to get super accurate alignment. I have found that by doing this twice you get close to the accuracy you would with the drift method (which is much harder and more time consuming to do)

This is a huge unexpected plus of the CG5 for me and has allowed me in the past to get two minutes or more of prime focus photography without a hint of field rotation (which you would get if you hadnt got good alignment).

You will also find that if you choose to three star align again after doing this the process is the much quicker because now that your more accurately polar aligned the first slews get much nearer the stars selected.

Just one thing to note, I found that when you do the polar alignment routine its best to turn off the mount and turn it back on again because sometimes if you go straight back into three star alignment mode the scope starts to slew like its possessed!!!

I highly recommend this method of polar alignment. The polar scope then just becomes an way to center polaris in the first place!!

I'll show you this all next time we get out.

Dave/Eamonn - I've continously struggled with polar alignment and the best way to do it. On my taxing journey through the various methods from using finders, polar scopes etc etc, its become clear that the best way to do a really good polar alignment is the drift method.

A polar scope will get you very close of course but the drift method will get you bang on within a few iterations. A couple of things I've learned that ease the pain somewhat are:

1. A polar scope will get you close and no closer.
2. Level your tripod/pier as much as possible
3. You can drift align under horrible seeing conditions (a great way to pass the time!)

Of course, how close you want to get to the true North celestial pole depends on what you want to do. If its casual observing, using the polarscope alone will be fine. If its short exposure photography, one or two short iterations of the drift method will do. If its long exposure photography (unguided), then a few iterations of long duration drift alignment will get you there.

Eventually, I'll personally look into software packages that help (PoleAlignMax etc).

One other point, if you use your OTA or a finder to polar align (including using GOTO routines), you will be relying on your OTA or finder being orthogonal to the mount - which it won't be. Even in high end scopes, orthogonality of the focal plane to the mount is very very difficult to obtain. If you drift align however or use a polar scope, you are taking OTA/Finder orthogonality out of the equation and working directly on the alignment of the mount.

Which drift routine is best? There one for every hour of the day - just type 'drift alignment' into a search engine and trawl through.

Cheers

Dave