Alignment - No EAST/WEST aspect. Would this work?

[DIYman (Doug)]

In my case I am trying to align an equatorial platform and I have really racked my brains on this one and cannot identify a solution. Alignment in the azimuth is no problem as it is a matter of drift alignment near the meridian and celestial equator – but getting the right angle in respect to the CP (altitude) is another ball game without the benefit of an EAST or WEST horizon. In my case trees block up to 60 degrees in both directions. (It’s not like a GEM where just pointing the scope at the CP with the tube positioned directly above the RA axis and then adjusting the height of the axis until the CP is in the middle of the field.)

I have wasted too much viewing time trying to resolve this issue. Tried using spirit levels to level the platform for my latitude but none are sufficiently accurate. My goal is to get at least half a minute accuracy to make it less work for the auto guider when imaging.

Would this solution work?

Place the reticule of a guiding eyepiece on a star (near the celestial equator) which is say 30 minutes from transit Let the scope track for 30 minutes and at the time of transit note the position of the star in the eyepiece. Let the scope track for another 30 minutes and then again note the position of the star. My reasoning is that if the altitude of the platform is too high in relation to the SCP, then the star will appear to move north at transit but then move back to the starting position (centre of the field) after 30 minutes. The opposite will occur if the altitude is too low in respect to the SCP. (One would have to be careful that any drift is not caused by poor azimuth alignment which needs to be very close for the proposed solution to work.)

Sorry for the length of this post but it has been good therapy for me.

Regards
Doug

[ZeroID (Brent)]

You can do a similar thing with a DSLR and 1 minute exposures. Pick a high zenith star. Stop tracking for 30 secs to get a short star trail then double speed west to get and angle. Look at the exposure an correct on this.
I've done it by stop tracking and then just moving the mount along the RA axis. A swing E & W with the handpiece will show the RA alignment track, stop tarcking with show the difference as the star moves S or N

[TR (Terry)]

Doug,

I can't see the east or west from my backyard. Only 50 degrees and up. Given this, I generally get polar aligned in about 20 min using Pem Pro. There are several posting referencing Pem Pro in http://www.iceinspace.com.au/forum/s...d.php?t=117607

I have an AP mount with known calibration marks on the Az and Alt. I know exactly how far the mount moves with each mark on the registration scales. Makes Polar Alignment fairly quick. Now, I tried this with my Vixen Mount without any calibration marks and found this almost impossible. I just don't know how far to turn the knobs.

Pem Pro requires you to point straight up, 5 degrees west of zenith to determine image scale and camera rotation and other calibration stuff. Then you start adjustments until your Az is correct. This is done through your main scope. Then slew 30 degrees to the west of zenith to adjust your altitude. Given I can't see west from about 45 degrees down - I still have no problems aligning on pole. I believe there is a 60 day trial of the software at CCDWare.

With my Vixen, I will have to align it one day, then turn the Azimuth adjustments one full turn to determine what that translates to in the sky and make some calibration scales for that mount. Once I have that worked out, I can use Pem Pro on that mount as well.

Terry

[Barrykgerdes]

Doing a polar alignment is a big problem for most with a portable equatorial mount mount in the field. However a lot of time can be saved if you start off in a good manner.

Drift aligning using a star at the zenith will get your azimuth correct relatively easy but getting the elevation can be very time consuming as the calibration for elevation is usually difficult to read out with any precision or accuracy from the built in scales and drift aligning at the horizon is often impossible due to visibility.

When doing a polar alignment the most import first procedure is to get the elevation in the "ball park". 30 arc minutes will be good enough for astro photography up to about 3 hours. Measuring the elevation was quite a task in the past but now we can get digital inclinometers that can set the elevation quickly to about 20 arc minutes. The azimuth can then be set within a degree or two using a compass.

With this sort of elevation accuracy the drift alignment at the zenith can be quickly completed with programs like PHD and have you ready to take photos in about 15 minutes from initial set up ignoring any further elevation corrections.

If your mount is to be permanent getting the alignment much closer can be achieved using more precise techniques to get the alignment spot on but cone error may also need to be corrected. this will also improve your gotos.

Barry

[DIYman (Doug)]

Barry, Terry and Brent

All good replies - thanks for the advice.

Using an equatorial platform (EP) with a dob mount does pose special problems for alignment compared to GEM, fork mounts etc. I thought about using PHD, Pro Tem etc but at the moment I am using a DSLR and believe you need a CCD to allow these programs to read pixel positions. I will look into this. However in the interim, I am going to give Brent's idea a run this weekend and see how I go. (Clouds willing)

I suppose I should find a star on the celestial equator rather than the zenith (-32 degrees) as it will be a right angle to the virtual axis of the EP or does it not matter?

Doug