Jump to content


Photo

Alignment: Why will this not work?

  • Please log in to reply
24 replies to this topic

#1 dawziecat

dawziecat

    Vanguard

  • *****
  • Posts: 2425
  • Joined: 20 Oct 2010
  • Loc: Rural Nova Scotia

Posted 09 March 2011 - 09:14 AM

Please don't pillory me for this. It can't be this simple to align a GOTO mount or that's what we'd all be doing. So, it's wrong. It's GOT to be. But why is it wrong?

1/ Point the polar axis north within a few degrees in azimuth and at an altitude within a few degrees of the NCP.
Instruct the mount that it is now aligned, i.e., it now "thinks" it is perfectly on the NCP even though we know it's not.
2/ Instruct to mount to GOTO a bright star. Of course it will be off. Will it not be off the precise amount in alt/az as the initial position was off the NCP? Use the alt/az adjustment screws to bring the bright star to the CFOV in a high power EP.

Voila! Near perfect alignment!

No need for polar alignment scope. No need for a good bead on Polaris or even that Polaris be visible at all.

I know I'm gonn'a be embarrassed about this but what am I missing? :confused:

#2 EricJD

EricJD

    Vanguard

  • -----
  • Posts: 2148
  • Joined: 24 Jul 2007
  • Loc: Chesapeake VA

Posted 09 March 2011 - 09:42 AM

Will it not be off the precise amount in alt/az as the initial position was off the NCP?


Yes, and it will also be off in RA and DEC.

#3 jrcrilly

jrcrilly

    Refractor wienie no more

  • *****
  • Administrators
  • Posts: 33955
  • Joined: 30 Apr 2003
  • Loc: NE Ohio

Posted 09 March 2011 - 10:24 AM

what am I missing? :confused:


In the scenario you describe there is a presumption that no goto alignment is required; that it knows precisely how far to slew in both axes to reach the target star from the NCP. Most mounts aren't that precise, which is why they require goto alignment. This tells the mount how far it is really moving compared to how far it thinks it is moving - plus how far from orthogonal the optical axis is. When I used my EM-200 in the field with a refractor (optical axis in line with the tube) I could get away with those presumptions but it wouldn't have worked with an SCT (optical axis wherever it has to be to achieve collimation) or with a lesser mount.

#4 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 09 March 2011 - 10:44 AM

Hi-

Depending on the mount - this can actually work very well. I use it all the time with Celestron mounts (CGE or cge-pro, but most should work). The key is that the NexStar alignment include a calibration of the mount that corrects for cone and dec. errors - so that a "quick alignment" will point the mount in the right direction including the full calibration. If you use "last alignment" then it will end up being as far off polar alignment as it was when you did that last alignment, so "quick alignment" is preferred.

This works best with mounts that have index switches such as the CGE - so that it accurately recovers the previous pointing model.

I call this a "quick and dirty" polar alignment but it can work very well. If you just use the level to level the mount exactly as it was previously polar aligned, then all you need is to move the mount in azimuth and the object will appear in view - indicating the mount is polar aligned. You can use the sun or moon to do this before sunset - then refine later. This is one time where leveling can be useful and important. For greatest azimuth accuracy you want to use an object low - maybe 10 degrees up.

If your mount does not have a built in correction for cone and dec. offset errors - then this procedure may not work very well. I only have experience doing this with Celstron mounts.

Frank

#5 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 13174
  • Joined: 01 Feb 2006

Posted 09 March 2011 - 10:48 AM

This doesn't work well in practice.

There are several reasons.

The first is that for BEST result, to use the mount this way, you would have to first level it very carefully.

The second is that the alignment routine also deals with physical play in the gears or a bad emitter window (dust or debris).

The thrid is that the alignment routine deals with cone error that may be present in the mount.

HOWEVER...

Here is the alignment routine I sometimes use if I want a closer initial polar alignment for long period tracking.

I start by LEVELING the mount very carefully. This way, when I move the altitude aduster, I am moving the mount head strictly in a 100% vertical plane. This is very important because if the plane is not vertical, you will really be moving the axis in both vertical AND horizontal planes when you adjust EITHER screw.

So, start wit as level a mount as possible. If the mount doesn't have a bubble level on it, get one.

Now, point the mount north. When you slew to the first bright star, you can NOW use the Altitude and Azimuth controls and not the handset to position the star in the center of the field of view.

You STILL have to do the second star and the calibration star, becasue these will ensure that play and cone error are estimated and will ensure that the RA is adjusted for true siderial time.

As a final touch, AFTER I do this, I now use tape over the index marks for 90 degree dec buy using the handset to slew to +90. Most of the index marks that come on CG5s (and maybe LXDs) are off. STARTING from a good index point in the future means more realaible alignment.

I usually correct the RA index simply by sighting the counterweight shaft. Slight difference in the index for RA are not critical.

But you still have to do the Calibration star or if there is cone error, the pointing will be off on the opposite side that you did the alignment.

I Have done shimming to eliminate cone error on my 6" APO becaue the old LXD 750 did not have a built in routine for this.

I also did my C14 even though the CGE does do cone error.

If you start dead level though, and use the first star to do the Alt and Az pointing, I think that you DO see the result with better pointing and better long term tracking.

#6 RTLR 12

RTLR 12

    Aurora

  • *****
  • Posts: 4577
  • Joined: 04 Dec 2008
  • Loc: The Great Pacific NorthWest

Posted 09 March 2011 - 12:32 PM

I use that procedure on my SE mount all the time. If I'm just out for a quick Grab n Go it works well enough to find and track targets. Lazy...Yes, but it works.

Stan

#7 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3128
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 09 March 2011 - 01:05 PM

Terry,
Thanks for posting this question and I do not mean to thread jack as I think you may have some of the same questions just like I had this one! I assume you are working with your new G11 with Gemini II? I have been doing quite a bit of reading on the mount, as I am getting one in about 3 to 4 weeks, and it has raised a bunch of questions in my mind. The first being: is the Polar Scope for the Losmandy mounts any better than say the one for Celestron mounts??...opionion here generally seems to be that the C Polar scopes are unnecessay (because of the All Star Polar Alignment utility) and really need alot of adjusting. So if anyone can speak to the usefulness/accuracy of the Losmandy Polar scope out of the box (does it need lots of adjustments too?), that would be great. I see no mention of a level on the mount in any literature so to follow Eddgie's advice we will need to purchase a bubble level for our mounts. My thoughts for astrophotgraphy is that we will need to get the goto alignment setup, then do drift alignment to make sure we have the RA axis is pointed exactly at NCP to avoid field rotation issues. Next, probably check the goto alignment again to make sure it was still fairly accurate after fiddling with the altitude and azimuth knobs. If anyone feels this is not a good way to do it, please pipe in and correct me. I was a bit spoiled by the Celestron software on my CGEM, so the world of Gemini II should be a bit of work for me (maybe Terry too?) to understand at first. The irony has not escaped me that the software/firmware on some of the better mechanical quality mounts is not as good what Celestron is offering on their mass produced mounts! Again, this is my current point of view and if any Gemini or Gemini II users want to show me the errors of my thinking, please do! I think both Terry and myself could use some helpful advice on our new mounts.

Thanks,

#8 Duncan Kitchin

Duncan Kitchin

    Apollo

  • -----
  • Posts: 1278
  • Joined: 16 Dec 2004
  • Loc: Beaverton, OR, USA

Posted 09 March 2011 - 01:14 PM

I have a variant which I use that seems to work pretty well:

1) Start of with the mount well leveled (I use a torpedo level on the top of the tripod if setting up in the field)

2) Do a 1-star align on something with RA greater than 6

3) Slew to Polaris

4) Use the alt-az bolts to remove about 2/3 of the pointing error on Polaris

5) Repeat steps 2-4 several times. Each time the alignment will get a little more accurate, and after a few iterations the slew to Polaris will be dead on

6) Do a 3-star final align to sync the mount. After the first star, the next two should be pretty much dead on if the polar alignment is good.

#9 dawziecat

dawziecat

    Vanguard

  • *****
  • Posts: 2425
  • Joined: 20 Oct 2010
  • Loc: Rural Nova Scotia

Posted 09 March 2011 - 02:13 PM

Terry,
(...snip...) we will need to purchase a bubble level for our mounts.


Hi Kirk:
There are indeed two spirit levels at 90 degrees built into the G-11 head
directly below the Losmondy G-11 label.

#10 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 09 March 2011 - 02:45 PM

You STILL have to do the second star and the calibration star, becasue these will ensure that play and cone error are estimated and will ensure that the RA is adjusted for true siderial time.



You sound like you are talking about the original handcontrol for the cge, with its single "cone" star. The newer firmware stores each calibration permanently so that when you start up in quickalign mode, it will reuse the cone and dec. offsets and just assume it is polar aligned - so you don't need to use any alignment or calibration stars to have the mount model active. The calibration may be a little out of date if the 'scope was taken apart and put back together - but for my C11 on cge it is pretty repeatable.

I find that the built in bubble level, used carefully, can be repeatable to a fraction of a degree. The switches are repeatable also to a very small fraction of a degree. This makes the whole process accurate to some fraction of a degree after you swing around in azimuth to polar align on a star. Later, you can refine the polar alignment by doing a fresh alignment and calibration with 3-4 stars and using AllStar align.

Note that on the CGE or CGE pro you should do the RA calibration procedure to calibrate the RA switch or else there may be an offset in RA. You also need the time to be correct. I think most of this stuff works with other Celestron mounts also, but the ones without switches would need careful matching of the alignment marks so that is also repeatable.

Frank

#11 Alph

Alph

    Surveyor 1

  • -----
  • Posts: 1769
  • Joined: 23 Nov 2006
  • Loc: Melmac

Posted 09 March 2011 - 03:12 PM

Note that on the CGE or CGE pro you should do the RA calibration procedure to calibrate the RA switch or else there may be an offset in RA.

Hmm? What does the RA calibration actually do? I always thought that it was only helpful in the situation where you have a permanent setup and the mount is not level. I don’t see how the RA calibration can be of any help when you have a portable setup. I always keep the RA switch value set to 90 and level the mount/tripod east/west using the RA home switch position and gravity.

#12 Alph

Alph

    Surveyor 1

  • -----
  • Posts: 1769
  • Joined: 23 Nov 2006
  • Loc: Melmac

Posted 09 March 2011 - 03:20 PM

No need for polar alignment scope. No need for a good bead on Polaris or even that Polaris be visible at all.


It is not that simple just google up "iterative polar alignment"

#13 gdd

gdd

    Surveyor 1

  • -----
  • Posts: 1707
  • Joined: 23 Nov 2005
  • Loc: Lynnwood, WA (N/O Seattle)

Posted 09 March 2011 - 03:23 PM

what am I missing?



I think this is what the mount's pointing model is all about (assuming your's has one, OK - I see you definitely have a G11). When you tell it how far off it was when pointing to a star, it knows what corrections it needs to make for your next request. With each request it will be a little more accurate.

I think the main purpose of the modelling is for GOTO pointing. But maybe it works with tracking also, I don't really know. Even if it does track properly you will still have field rotation (as was mentioned by others) which would be important for imaging, but not for visual observing. For imaging, you still want an accurate polar alignment, at least for long exposures.

Gale

#14 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 09 March 2011 - 03:34 PM

The RA calibration tells how far off the RA switch is from the meridian - effectively. Just like the Dec. switch calibration tells how far 0 degrees is from being truly perpendicular to polar axis - which is very important.

Normally the RA calibration doesn't matter once you align on a star - because an error in time or the RA switch are equivalent and irrelevant once a single star is aligned. But in order for QuickAlign to work well, which assumes the mount is aligned and level and calibrated and the Lat/Long and time are correct - then the RA switch calibration is needed to point right at the star without any error in RA. If the RA switch is a bit off and you do this "quick and dirty" polar alignment - then the mount will be a bit off polar alignment - just as it would be if the time or longitude were off. But if everything is calibrated and repeatable - then it should all work pretty well.

Frank

#15 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3128
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 09 March 2011 - 03:37 PM

Hi Kirk:
There are indeed two spirit levels at 90 degrees built into the G-11 head
directly below the Losmondy G-11 label.

Terry,
Thanks for that info!! :) Not clear in any of the literature on the mount.

#16 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 09 March 2011 - 05:27 PM

It is not that simple just google up "iterative polar alignment"



The quick and dirty alignment described here is extremely simple and fast - and quite effective. I use it almost every time I set up my cge remotely. Turn on cge. Find switches. Quick align. Goto star down low. Swing in azimuth until star or moon is centered in finder (or shadow of sun is minimized) and you are close to polar alignment. No iteration. Should have good enough alignment and tracking to find planets in daylight or twilight and track them.

You can also find switches and do a two-star align - but for the first star you rotate the eq. head in az. until it is centered. Then you lock it down and align as usual and goto the second star. The key is that you are free to move the mount all you want before you actually tell the mount you are aligned on a star. Doing this before a 2-star alignment means you are not only aligned to the sky - but you are roughly polar aligned.

I assume something similar is available for other mounts - but they need to store the mount model and be able to use it "blind" with no stars - which QuickAlign does.

Frank

#17 Alph

Alph

    Surveyor 1

  • -----
  • Posts: 1769
  • Joined: 23 Nov 2006
  • Loc: Melmac

Posted 09 March 2011 - 07:05 PM

Quick align. Goto star down low.


I think you could also do one star alignment as it reuses the saved mount model and accurate time is not required.
The problem though with such a quick and dirty setup is the limit on the azimuth adjustment and a compass or Polaris is needed anyway

#18 lawrie

lawrie

    Vanguard

  • *****
  • Posts: 2212
  • Joined: 31 Jan 2006
  • Loc: Okanagan Valley

Posted 09 March 2011 - 09:27 PM

There is a couple functions in the Gemini that works like you describe,they are called polar axis correction assist and polar axis correction.

#19 dawziecat

dawziecat

    Vanguard

  • *****
  • Posts: 2425
  • Joined: 20 Oct 2010
  • Loc: Rural Nova Scotia

Posted 09 March 2011 - 10:15 PM

There is a couple functions in the Gemini that works like you describe,they are called polar axis correction assist and polar axis correction.


One of the real troubles I am having is that there seems not to be a manual for the Gemini 2 yet. I have downloaded the Gemini Level 4 manual but the HC for Gemini 2 is different and I don't know how to access many (most?) of these special features.

#20 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 09 March 2011 - 10:24 PM

For polar alignment - as I said, the first star of the two star alignment - and as you suggest the single star of the one star alignment - will both work - but in all cases you need accurate time and RA calibration for the resulting polar alignment to be good. Good pointing in the sky after a star calibration doesn't need good time or RA cal - but in order to point blind at the exact point in the sky at the right time - everything has to be correct. But it all still works since the mount model, time, and geographic info are known.

Compass? No compass here. Just the mount and a star, moon, planet, or sun. If you don't have enough az range you need to pick up the tripod no matter what and rotate it.

Frank

#21 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3128
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 09 March 2011 - 10:31 PM

Terry,
Have you looked at the Gemini II Quick Start Guide from here? It is kind of superficial right now, but it does cover some alignment topics.

#22 Sharkman

Sharkman

    Vostok 1

  • -----
  • Posts: 119
  • Joined: 12 Aug 2006
  • Loc: Nelson, New Zealand

Posted 10 March 2011 - 04:17 PM

One problem with the technique in the OP is that it doesn't accurately detect or resolve the problems caused by drift.

If you're doing imaging, and you're looking for an alternative to the drift method, the approach described on the following page is interesting:

http://www.minorplan...aralignment.htm

#23 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3128
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 10 March 2011 - 04:37 PM

Sharkman,
Thanks for the info. I had actually seen a reference to this method before and it looks like a great idea (book marked it!). I will have to give it a try if the weather ever clears here!

#24 gdd

gdd

    Surveyor 1

  • -----
  • Posts: 1707
  • Joined: 23 Nov 2005
  • Loc: Lynnwood, WA (N/O Seattle)

Posted 10 March 2011 - 07:33 PM

There is a couple functions in the Gemini that works like you describe,they are called polar axis correction assist and polar axis correction.



Hi Lawrie,

Are we saying the Polar Scope is a "nice to have" rather than "required" for quickly getting a good polar alignment with G11-Gemini? Just like the CGEM is supposed to make the polar scope optional?

Thanks,

Gale

#25 freestar8n

freestar8n

    Skylab

  • *****
  • Posts: 4048
  • Joined: 12 Oct 2007

Posted 10 March 2011 - 09:53 PM

One problem with the technique in the OP is that it doesn't accurately detect or resolve the problems caused by drift.



With the right mount and a little care, in about 10 seconds, and as part of the normal mount setup process, you can be polar aligned to within a fraction of a degree. It's not perfect and can be improved upon by later, much more time consuming methods - but the method is sound and limited only by how well your mount is calibrated and how repeatable things are when setup in the field.

But it only works well for certain mounts that store the mount model and can point blind at startup using that model.

Frank






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics