Drift Alignment – better with a 45 degree erecting prism?

[g__day (Matthew)]

I am trying to drift align a motorised equatorial mount. A simple question – is this far easier to do with a 45 degree erecting prism than a 90 degree prism?

Many thanks!

[[1ponders] (Paul)]

Most drift aligning directions are given using either no diagonal (newtonian scopes) or with a 90 deg diagonal (refractors and sct). I would imagine that using a 45 deg erecting prism might get confusing particularly when drifting near the meridian for Azimuth using directions meant for the above scopes.....

You would be able to do it but I doubt if it would be any easier

[Merlin66 (Ken)]

Other than the comfort of the observer, won't make any difference!
A cross hair eyepiece, possibly a barlow ( at the end of the fine tuning) and patience is all that's required. Oh and a steady hand/ mounting to make the adjustments.

[g__day (Matthew)]

Sigh, from a total amateur trying to learn / decode the language... for instance I found this on the web, I'm sure its correct (for its hemisphere or maybe both hemispheres) but I am trying to turn it into simple English and looking up basic astronomical terminology as you go (things like celestial equator, meridan, declination, illuminated reticle ocular) basic terms I have to learn and am now googling... to answer where I have marked ***




Declination Drift

This method of polar alignment allows you to get the most accurate alignment on the celestial pole and is required if you want to do long exposure deep-sky astro-photography through the telescope. The declination drift method requires that you monitor the drift of selected guide stars. The drift of each guide star tells you how far away the polar axis is pointing from the true celestial pole and in what direction.



* so far so good - sounds like what I need!



To perform the declination drift method you need to choose two bright stars. One should be near the eastern horizon and one due south near the meridian.



* * * Okay East I can see and near the horizon makes sense, one due South - but where is the meridan is that the South Celestial Pole - a picture would have been great!)



Both stars should be near the celestial equator (0° declination).



* * * Celestial equator - what is this term?



You will monitor the drift of each star one at a time and in declination only.



* * * What does in delination only mean?



While monitoring a star on the meridian, any misalignment in the east-west direction will be revealed.



* This makes some sense - a meridan (whatever this is shows East-west - my horizontal fine tuning controls)



While monitoring a star near the east/west horizon, any misalignment in the north-south direction will be revealed.



* * * North - South - is this my latitude setting?



As for hardware, you will need an illuminated reticle ocular to help you recognize any drift. For very close alignment, a Barlow lens is also recommended since it increases the magnification and reveals any drift faster.



* * * So is an illuminated reticle ocular an eye piece with markings to precisely show you how far from centre and in what direction (North, South, East, West) you are out?



When looking due south with the scope on the side of the mount, insert the diagonal so it points straight up.



* * * What diagonal - a 90 degree that inverst and flips the image or a 45 that doesn't?



Insert the cross hair ocular and align cross hairs to be parallel to declination and right ascension motion Use ±16x guide setting to check parallel alignment.



First choose your star near where the celestial equator and the meridian meet.



* * * In English please?



The star should be approximately ± 1/2 hour of the meridian and ±5° of the celestial equator. Center the star in the field of your telescope and monitor the drift in declination.

• If the star drifts south, the polar axis is too far east.

• If the star drifts north, the polar axis is too far west.



* * * Is this a Southern Hemisphere + 90 degree prism correction?

* * * I presume you are looking due East near the horizon not the SCP?, and looking only at the star's horizontal error to see if a star moves left towards North - so shift your mount West or right towards South - so shift your mount East - on your equatorial's horizontal East/West fine tuning?



Make the appropriate adjustments to the polar axis to eliminate any drift. One you have managed to eliminate all drift, move to the star near the east horizon. The star should be 20° above the horizon and ± 5° of the celestial equator.



* * * Opps is this the point East near the horizontal. Is that 20 degrees above the horizon and +/- 5 degrees of due East?

• If the star drifts south, the polar axis is too low

• If the star drifts north, the polar axis is too high.



* * * Again is this correct for the Southern hemisphere



Once again, make the appropriate adjustments to the polar axis to eliminate any drift. Unfortunately, the latter adjustments interact with adjustments ever so slightly. Therefore, repeat the process again to improve the accuracy checking both axes for minimal drift. Once the drift has been eliminated, the telescope is very accurately aligned. You will be able to do prime focus deep-sky astro-photography for long periods. NOTE: If the eastern horizon is blocked, you may choose a star near the western horizon however, you will have to reverse the polar high/low error directions. If using this method in the southern hemisphere, the procedure is the same as described above. However, the direction of drift is reversed.

[rogerg (Roger)]

g__day,

I found the same confusions when starting a few years back. I finally worked it out and put it into my english, here:

http://www.rogergroom.com/rogergroom...em.jsp?Item=27

Hopefully that will help you at least a little.

Roger.

[[1ponders] (Paul)]

Hi g_day.

A couple of questions answered first.
The meridian is an imaginary line drawn through the sky from due south thru the zenith (highest point of the sky) to due north.

0deg declination is the celestial equator. Same as the earthly equator (and directly above it) but in the sky. Runs due east to due west. If you point your mount towards the south then the 0deg dec/celestial eq is the when you line your scope up with the 0 mark on the dec circle of the mount.

Your eq mount moves in two directions north/south, this is declination and east/west, this is Right ascension. So if you are measuring drift only in declination you ignore any movement in Right Ascension. To make it easy on your self when drifting when you put your reticle in and your mount is roughly polar aligned, se your dec control to move the scope and see which way a star moves in the reticle. rotat the reticle until the star moves parallel to one set of the cross hairs.

So to choose a star where the Celestial eq and the meridian meet, point your telescope due west ( 0 deg dec) and then lower the eyepiece end so that the telescope is pointing straight up. Your counterweight shaft should be horizonal. Now look for a star 1/2 hour (7 deg) either side of the meridian and +- 5 deg north/south of the 0 deg dec (preferably south. I find it does a better job. don't know why it just seems to)

North - South - is this my latitude setting?

Yes is it. When you select your eastern star (again at near 0 deg dec) try to keep around 15 - 20 deg above the horizon. Any lower and atmospheric refraction can effect it, plus the stars jump around too much.

For drift aligning your telescope doesn't point south. The RA axis does but not the telescope itself

[g__day (Matthew)]

Many thanks guys! One of those times when one needs to escape the jargon to achieve the end result!

[gregbradley]

Great site Roger. Good to see someone understands that if you want to communicate you have to define the words!

Greg.

[johnno]

Hi Roger,

I agree with GREG,
A great site you have,and very helpful to the less Experienced.
Many Thanks.

Regards.
John