quick and stupid drift alignment question

troypiggo (Troy) · #1 04-02-2009, 05:01 PM

Been reading several different pages on drift alignment and think I fried my brain. Kind of understand what they're trying to achieve, and while they all refer to the directions differently (some say up/down/left/right while others say N/S/E/W), they all agree that you don't worry about drift in the RA, only the DEC. Fine. Now for my dumb questions...

What do they mean by looking for drift in the dec direction? Does that mean look for the stars moving along the line of (ie parallel to) the dec axis? Or stars moving perpendicular to the dec axis (ie to my understanding their dec value is increasing or decreasing)?

By dec axis, I'm taking that to mean the line in my reticle that stars move along if I adjust the dec (up and down arrows on keypad) with Synscan keys.

I read the tutorials over and over, and I can read and interpret it both ways.

Gave it a go the other night, and when pointing east I adjusted the latitude up and down 10 degrees and the stars just kept on moving in the same direction, so I figured (afterwards) I must've been out by 90degrees with my understanding of "in the dec direction".

peter_4059 (Peter) · #2 04-02-2009, 05:51 PM

Troy,

If the alignment is out the star will appear to either drift north or south (ie in the same orientation as the scope moves in the Dec axis). If you are drift aligning at a star near the meridian and the star is appearing to drift north what is actually happening is the scope is tracking slightly south. To correct this you need to rotate the mount slightly clockwise using the jacking screws. For drift aligning in the east if the star appears to drift north - same issue - the scope is actually tracking slightly south - you therefore need to raise the altitude using the altitude jacking screws so the track of the scope is slightly more north.

Hope this helps but happy to demonstrate if you want (give me a call on the mobile). There is a good set of instructions that explains it well in the resources section of this site.

Peter

sheeny (Al) · #3 04-02-2009, 06:01 PM

Quote:

Originally Posted by troypiggo

Been reading several different pages on drift alignment and think I fried my brain. Kind of understand what they're trying to achieve, and while they all refer to the directions differently (some say up/down/left/right while others say N/S/E/W), they all agree that you don't worry about drift in the RA, only the DEC. Fine. Now for my dumb questions...

What do they mean by looking for drift in the dec direction? Does that mean look for the stars moving along the line of (ie parallel to) the dec axis? Or stars moving perpendicular to the dec axis (ie to my understanding their dec value is increasing or decreasing)?

They are talking about drift in declination, so it is an apparent change in declination i.e. drifting at right angles to the RA axis. I usually pick an alignment star, centre it, then move it to the edge of the FOV using the RA control only, then rotate the reticle of my guiding EP so the cross hairs are on the star. Run it back and forth in RA and the star should stay on the RA cross hair. Then adjust for any drift off the RA cross hair.

Quote:

Originally Posted by troypiggo

By dec axis, I'm taking that to mean the line in my reticle that stars move along if I adjust the dec (up and down arrows on keypad) with Synscan keys.

The reticle aligned with movement in the RA axis is the important one. Any drift away from that is what you're trying to correct.

Quote:

Originally Posted by troypiggo

I read the tutorials over and over, and I can read and interpret it both ways.

Gave it a go the other night, and when pointing east I adjusted the latitude up and down 10 degrees and the stars just kept on moving in the same direction, so I figured (afterwards) I must've been out by 90degrees with my understanding of "in the dec direction".

The most important thing to realise about drift aligning is to start of with BIG adjustments. When I start off drift aligning, I adjust until the star is just inside the field of view as far from the RA reticle as I can get (i.e. I adjust by half the FOV of my EP!). You will need to bring the star back into the FOV several times by movement in RA only to do this. If it drifts the same direction, I keep going by the same amount until I have gone too far.

After I have gone too far, I come back by 1/2 that amount. From there on I use a binary search method - each time I go too far (i.e. the direction of drift changes) I adjust the other way by half the previous adjustment.

Clear as mud?

Al.

troypiggo (Troy) · #4 04-02-2009, 06:45 PM

Quote:

Originally Posted by peter_4059

If the alignment is out the star will appear to either drift north or south (ie in the same orientation as the scope moves in the Dec axis). If you are drift aligning at a star near the meridian and the star is appearing to drift north what is actually happening is the scope is tracking slightly south. To correct this you need to rotate the mount slightly clockwise using the jacking screws. For drift aligning in the east if the star appears to drift north - same issue - the scope is actually tracking slightly south - you therefore need to raise the altitude using the altitude jacking screws so the track of the scope is slightly more north.

Hope this helps but happy to demonstrate if you want (give me a call on the mobile). There is a good set of instructions that explains it well in the resources section of this site.

Thanks Peter. Ok, so let me get this straight:

I'm facing south. I'm using my illuminated reticle through a diagonal on my refractor, so one mirror[1]. Say the dec axis is almost horizontal, established by using the top and bottom directional keys on keypad and seeing the stars move along that line. South is up, north is down, east is right and west is left? (Got this from the drift article here) Place a star on the dec axis and it moves up/south. I need to rotate the mount counterclockwise?

Now I'm facing east. The dec axis is tilted low left to high right. I don't understand the definitions of NSEW here any more, because depending on what article I read, they seem to be different. I would think that east is upper left, west is down right, north is lower left and south is top right?
Place a star on the dec axis and it moves up/top left. Uh oh, that's east, not north or south as described in the article linked above. What have I assumed incorrectly?

[1] Does it matter which way the diagonal is rotated? Does it matter if I rotate it at different angles for south and east orientations?

troypiggo (Troy) · #5 04-02-2009, 06:48 PM

Quote:

Originally Posted by sheeny

They are talking about drift in declination, so it is an apparent change in declination i.e. drifting at right angles to the RA axis. I usually pick an alignment star, centre it, then move it to the edge of the FOV using the RA control only, then rotate the reticle of my guiding EP so the cross hairs are on the star. Run it back and forth in RA and the star should stay on the RA cross hair. Then adjust for any drift off the RA cross hair.

The reticle aligned with movement in the RA axis is the important one. Any drift away from that is what you're trying to correct.

The most important thing to realise about drift aligning is to start of with BIG adjustments. When I start off drift aligning, I adjust until the star is just inside the field of view as far from the RA reticle as I can get (i.e. I adjust by half the FOV of my EP!). You will need to bring the star back into the FOV several times by movement in RA only to do this. If it drifts the same direction, I keep going by the same amount until I have gone too far.

After I have gone too far, I come back by 1/2 that amount. From there on I use a binary search method - each time I go too far (i.e. the direction of drift changes) I adjust the other way by half the previous adjustment.

Clear as mud?

Al.

Bloody hell. I think I've got my axis definitions wrong by 90degrees

troypiggo (Troy) · #6 04-02-2009, 06:58 PM

Quote:

Originally Posted by sheeny

I usually pick an alignment star, centre it, then move it to the edge of the FOV using the RA control only, then rotate the reticle of my guiding EP so the cross hairs are on the star. Run it back and forth in RA and the star should stay on the RA cross hair. Then adjust for any drift off the RA cross hair.

I think that is a most eloquent way of describing it. It just clicked for me when I read that. Thanks.

Peter has also been kind enough to offer showing me tonight. You guys rock. Thanks.

Quark (Trevor) · #7 04-02-2009, 07:27 PM

Hi Troy,

Polar align your mount as best you can with a compass for azimuth, don't forget to apply the magnetic deviation for your area.

Align the altitude of your mount to be whatever your local latitude is using a spirit level and protractor.

Now star drift align.
Aim your scope at a star close to the meridian and celestial equator, with your drive turned off note the direction the star moves when you manually move the scope in RA , it will be from the east to the west. That is RA, so 90 degrees to that motion will be north and south.

I use an eyepiece with an illuminated grid there are several around with illuminated reticle's. I will assume you are doing this with such an eyepiece.

Align the star at the meridian such that with the drive turned off and manually moving the scope in RA the star moves parallel to the central line of the reticle. Turn on the drive, after about a minute the star will have drifted to the north or south, this means you must adjust the azimuth of your mount to the west or east. Make an adjustment of a couple of degrees east or west, doesn't matter, but record the direction that the star drifted ie north or south and record the direction of your adjustment ie east or west.

Realign the star, if it now drifts even faster the same direction then you have established that your adjustment needs to be in the opposite direction. If, on the other hand the star now drifts in the opposite direction then the initial adjustment was to great.

It really is quite logical and ounce you get into it is no big deal.

Having adjusted the azimuth so that it is about 5 minutes before the star drifts north or south, lock the azimuth and move to a star low in the east but close to the celestial equator.

Do the same thing with your eyepiece and drive turned off to get the star drifting parallel with the central line of the reticle. Turn on the drive, this time the movement of the star to the north or south indicates the need for an adjustment of the altitude of your mount. Don't be afraid to make a reasonable adjustment.

Again record the direction the star initially drifted, north or south and record the direction of your adjustment increase or decrease in altitude.
Realign on the star, if it now drifts even faster in the same direction then your adjustment was in the wrong direction. If the drift is in the opposite direction then your adjustment was too great.

You will have to repeat this sequence several times, each time the adjustment of azimuth and altitude will be finer, honestly, ounce you have done this a few times you will find it a quite simple procedure and because you recorded the results of your adjustments you will know next time what direction those adjustments need to be.

Regards
Trevor

sheeny (Al) · #8 04-02-2009, 07:48 PM

Quote:

Originally Posted by troypiggo

Bloody hell. I think I've got my axis definitions wrong by 90degrees

Well that's not impossible!

Quote:

Originally Posted by troypiggo

I think that is a most eloquent way of describing it. It just clicked for me when I read that. Thanks.

Peter has also been kind enough to offer showing me tonight. You guys rock. Thanks.

Thanks.

Glad you've got someone handy who can show you. Onya Peter! You'll be drift aligning in no time!

Al.

troypiggo (Troy) · #9 04-02-2009, 11:32 PM

Thanks for the detailed explanation Trevor.

I just got back from Peter's place, and having seen it being done with the telescope makes so much more sense that reading about it. I was all out with me equators and axes.

Met Allan Gould there also. Good to put faces to names, and I sure did glean heaps of information from the 2 of them. Thanks guys.

peter_4059 (Peter) · #10 05-02-2009, 07:52 AM

No problem Troy - good to meet you also. As you would expect it came over all cloudy just as Saturn was appearing from behind the trees so I packed it in. By the time I was done packing up it was all clear again.

troypiggo (Troy) · #11 05-02-2009, 08:48 AM

Bugger. Perhaps I can return the favour some time and host you. Similar light pollution I guess, but not as many trees, more open so more opportunities.

Colsmith · #12 05-02-2009, 09:01 AM

Been following this thread with interest! I have an EQ6, can't do a real polar alignment with the polar scope coz I can barely see Octans, let alone the asterism!

So, I do a quick polar alignment with compass/protractor/level. Do I then do a drift alignment followed by a three star alignment, or the other way around?

Quark (Trevor) · #13 05-02-2009, 10:07 AM

Hi Colsmith,
You don't need to be able to see the pole to polar align.

As you say, just align as well as you can with a compass, protractor and spirit level, then star drift align as explained in this thread.

I assume the three star alignment you mention is for a telescope computer system. That three star alignment will probably allow you to find objects, even when your mount is not polar aligned. However, to then track that or any other object your mount needs to be polar aligned.

Regards
Trevor

troypiggo (Troy) · #14 05-02-2009, 10:47 AM

Quote:

Originally Posted by Colsmith

So, I do a quick polar alignment with compass/protractor/level. Do I then do a drift alignment followed by a three star alignment, or the other way around?

I think that's right. Drift alignment should be thought of as part of the polar alignment process and best done first after balancing. This helps get your mount axes all correct for the spin of the earth.

The 3 star alignment isn't really part of the polar alignment. It's aligning your computer to the star positions so your goto can find them. But you need the polar alignment correct for this to work best.

Hope that's all correct and haven't let me new teachers down.

bmitchell82 (Brendan) · #15 05-02-2009, 11:02 AM

hehehe i am used to Fork mounted meades that have LTN technology.. thats bloody genious! turn it on it finds the tilt of the mount the level of the mount and true "north" hehehe bloody norther hemisphererereressss.... but it works a treat. now all they need to do is get the gems to do the same thing!

but yes if you don't have your gem closely polar aligned you will find the object but not track it properly, it will "drift" out of the ep after a while.