Alignment help needed

[Andrew C]

I need some advice.

I have a well used CG5 goto eq mount which I am using with a 6” Newtonian Celestron and Watec video camera for deep sky stuff. When taking reasonably long exposures (say over 1 minute) the images drift noticeably. Normally I would put this down to poor polar alignment, but I am reasonably confident that I am getting this right to within 5 arcmin or better each time. There is no facility on the mount for adjusting the sidereal tracking rate so I can only assume that that is OK. I have gone to some pains to check and shim the OTA to align it with the mount polar /RA axis.

One other area that I wonder about is the indexing when setting up the mount at the beginning of the session. Given that the index arrows are on a shaft of about 70mm diameter, a misalignment error of 0.5mm on indexing the dec axis (about as good as I can be sure of in the dark) would represent about a 50 arcmin wobble when rotating the scope on its RA axis around the SCP! One can polar align the scope very accurately in a single position, but this level of indexing error will overwhelm and negate that effort.

What I am wondering is: Does drift alignment cancel/overcome this error and should I be concentrating my efforts on practising that process, or will the dec indexing error remain to affect drift alignment as well (which would mean I will have to get the dec indexing right by repeated trial and error)?

Thanks, Andrew

[tempestwizz]

Andrew,
Drift alignment will allow you to get spot-on with your polar alignment, and with practice you can get reasonably close reasonably quick.
I suggest you can use your imaging setup to drift align. If you orient your camera so that RA is in one plane,eg horizontal, then your dec will be in vertical. For drift alignment you will only be interested in the dec shift.
I use my veiwing pogram to zoom in as much as I can when aligning, as this will produce the most change for the shortest time.
There are plenty of sites describing how to do it.
I suggest you start a note-book and write down which way you have to turn your knobs to produce what change. This will help you the next time(s), since when you notice a drift one way, you will know which knob to turn which way.

Also while doing this, you can keep an eye on your RA tracking rate over time. While sidereal is sidereal, you may be experiencing Periodic Error, which is a problem produced by small inaccuracies in manufacture of the worm gear that in short-term causes the RA to get ahead, and then drop back over the worm gear cycle time. This shows up as elongated stars stretched in the RA plane.
Many mounts have a Periodic Error Correction (PEC) capability which allows you to program in a negative PEC bias which cancels out the physical errors.
If this doesn't work, then there's guiding as an option, if the mount supports it, which is a whole other realm.

HTH,

BC

[[1ponders]]

Hi Andrew.

You can pretty much ignore any of the Altitude markings on the mount except to get you within a couple of degrees of the ball park. Its fairly common that they are out, even on fairly good mounts sometimes. Definitely practice your drift alignment.

[Andrew C]

Many thanks folks - I will work on the drift alignment.

It seems that what the equipment manuals don't explain is that the indexing process can only be a rough setting that may be good enough for visual work but can not be good enough on its own for photographic/video work (except on a rare lucky night where the indexing lobs the dec axis right on the SCP) without drift alignment.

[OneOfOne]

Basically, your mount is aligned when the centre line of the polar axis is pointing to the SCP. The drive will rotate the scope about this point whilst tracking. Even if the mount is not level when setup initially, drift aligning will essentially compensate for this error during the process. The closer the tripod is to level and closer to south it is at the start, the less manipulation the drift alignment will need to adjust for.

I used to have a CG5 mount, the index marks I think are to give you a similar amount of movement on either side and it basically "centres the gears" if you like. Ultimately it shouldn't affect your alignment, but you may run out of movement if you were trying to look at something very close to the horizon...not sure on this though...I never actually tried it.

[leinad]

Hi Guys,

A quick question which has confused me a little with initially pointing the mount to true south before drift aligning.

I think the confusion has come from a lot of people mentioning to point to 182deg. 180 deg, + 2 Deg for Magnetic Declination.(Perth)

I setup the other night as usual pointed the mount +2W of magnetic south on the compass; but after drift aligning I found the south pole to be more to the East.

After trying to read as many articles as I could last night; I confused myself in the process.

Should not all Magnetic Variations be worked out ' North through East' ?

So if Perth is +2W; this is from magnetic north. Meaning that my mount is going to roughly point 176-178Deg ; ie 2-3deg East of magnetic south.

:help::screwy:

[Dennis]

Brisbane has a Magnetic Declination of 11 degrees East. That is, Magnetic North is 11 degrees East of Grid North, where Grid North is a line of longitude that “points” to the NCP of the axis of rotation of the Earth.

So, in Brisbane, I need to “offset” the magnetic needle on my compass 11 degrees to the East, that is, 180-11=169 degrees as seen on the white, inner, rotating bezel. See attached file.

So, for Perth, if the Magnetic Declination is 2 degrees West, then Magnetic North lies 2 degrees West of grid North. So, using the same logic, in the attached (1st) diagram the inner white bezel would read 182 degrees.

Hope that helps and better still, hope that I’ve got it right!

Cheers

Dennis

PS - Link to Mag Dec maps here (http://www.geo-orbit.org/sizepgs/magmapsp.html)

[montewilson]

Are you using the compass a long way from the scope/mount/metal objects et al?

If not you will get goofy readings from the compass. I do the aligning at least 6 feet from the mount to be sure.

[leinad]

Hmm I'll have to double check when the sky clears again. Calcs are all correct, it was the drift aligning that put the azimuth way off where the mount was initially pointed.

Thanks for clarifying.

[Dennis]

Hi Monty

Good point re iron and magnetic compass. The Tak tripod head is aluminium and the only iron in the vicinity are the quick release catches, the tray and a few fasteners.

The compass is on a beam reasonably well away from these and of course, the Tak EM200 head is not yet fitted when I do the azimuth set up, as the azimuth beam sits on the tripod head.

So far, every time I have used this daylight setting up method, I have been very pleased to discover that I am within 1 to 1 ½ degrees of correct polar alignment, when I later on use the Polar Alignment ‘Scope after twilight has ended.

Cheers

Dennis

[bmitchell82]

I do have to say that is a mighty fine little set up with reasonably good accuracy if you say that its 1 - 1.5 deg of SCP.... Yay for beginners for finding sig octanis :) hehehe

[Dennis]

Hi Brendan,

The original design goal was to place Sigma Octans somewhere in the FOV of the Tak EM200 Polar Alignment ‘Scope (PAS) during daylight on the 1st day at astro camps, whilst waiting for nightfall to then get it spot on using the illuminated reticule.

The 4 stars that make the Octans Trapezium just fit inside the FOV of the Tak PAS, which is around 4 degrees.

Here is the link to the original post which describes the Altitude and Azimuth set up jigs. (http://www.iceinspace.com.au/forum/showthread.php?t=14036)

Cheers

Dennis