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Topic 1: Alignment of the Mount - Orthogonality

Topic 2: Alignment of the Mount - Polar Alignment

 

 

1. Alignment of the Mount - Orthogonality

After finishing the building work, the mount was assembled and polar axis was aligned roughly using the AP Illuminated Polar Alignment Scope (PASIL). This screws into the lower end of the polar axis and has its own illuminated etched optics that will get you to within a 20 - 30 arcmins without much effort. The blanking cap where the PASIL fits can be seen in the photo below. The PASIL was removed after this initial setup and has not been used again - as I never move the mount. If I used a portable pier this would be an invaluable setup tool.

Once the mount is close to the celestial pole, accurate polar alignment was then achieved in steps.

Use the hand control to achieve orthogonality of the telescope with the Dec axis of the mount. This took several iterations of aligning with a star near the celestial equator and meridian and making adjustments to put the star at the centre of field with the scope on alternate sides of the pier. The orthogonality is then adjusted using thin shims inserted at the base of the mounting ring and the telescope tube assembly. There is a limit to how accurate final orthogonality can be for this reason. Packing under the scope has the effect of moving the star E-W only in the field of view . When only minor movement is discernable E - W the tube is orthogonal to the Declination axis of the mount. The procedure is well documented in the user guide to the mount. When close optically, this can be further improved using the CCD with the image cross-hairs option in use. However, most optical tube assemblies have fixed ring attachments to attach the scope rigidly to the mounting plate.

 
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2. Alignment of the Mount - Polar Align the Telescope

Either: - Use an alignment product such as TPoint or MaxPoint to determine the errors in alignment (and many other errors you never knew existed, never mind had in your system. These are very effective and will tell you by how much you need to move the Azimuth and Altitude axes to hit alignment.

Or: - Use a manual star drift method. This is the method I used to get within 4 - 5 arcminutes of the celestial pole. This was sufficient for my imaging needs (I think) as due to light pollution I was getting background sky saturation in about 10 minutes. I now take sub-exposures of up to 10 minutes and as a by-product this helps to minimise the impact of drift due to polar alignment error.

To achieve alignment using my CCD camera I used MaxIm in Focus mode - one second exposure continuous but without the focuser itself active. The mount RA drive is set to Sidereal with periodic error correction enabled if you have it.

To adjust for Azimuth alignment I followed this procedure. Select and centre a star that is both nearly on the meridian and at around 0° Declination. Take an an image on the screen I opened Telescope Control then used the screen (CCD frame) X-hairs to locate the centre point. By continuously imaging the star and adjusting ONLY the Azimuth adjuster on the mount I made adjustments until there was no discernable drift N or S from the centre of the Xhairs. This is a fast procedure if you are already well within the CCD frame width in accuracy of the Az. I continued until there was less than half the star diameter movement (approx 6arcsec) after 10 minutes of and this could have been due to moving past the Meridian as there was an equal amount of RA movement even with PEM running. This was the Azimuth alignment completed.

To complete alignment I continued by adjusting Alt. I put up Pulseguide with MaxIm as it has all the controls to switch meridian and used a star at about +15 Dec and on the Meridian. I used Focus to expose the frames in the MaxIm Focus window, centred the star on the X-hair using right click and selecting Move Telescope Here. Synch with TheSky and then Meridian flip with Pulseguide. The vertical displacement is twice the error in Alt and in my case this was so large that it was off the screen by about 8 arcmin. I took up approx half with the Alt adjustment knob and the rest by re-centreing the star with Move Telescope Here. I then re-synchronised with TheSky and did another Meridian flip. I continued three more flips, taking up half of the error with the Alt adjustment knob and half with the re-synch procedure.

c) Check Alignment using Planetarium Program

Align the telescope visually on a bright star and find the same star in the planetarium program. I use TheSky exclusively. Centre the star in TheSky and poke the Synch button (making sure that the scope is not past the meridian). Now choose another bright star at least 45° away and tell TheSky to slew to it. If the star is as near the centre of field as you need for your visual or imaging requirements then finish the alignment process and lock down all the adjustment knobs. Otherwise repeat the alignment method above. For CCD imaging it is important only that the target object appears somewhere in the field as you will nearly always want to re-frame the subject befor eyou image (ie change the position within the CCD field of view).

Some time after aligning optically I acquired the TPoint software to check the accuracy of my alignment. After two runs and making the TPoint calculated adjustment to Az and Alt, I was then approx 24 arcsecs out in Azimuth and under 18 arcsecs out in Altitude. I did not bother to try and improve from that result.

RESULTS IN USE

When imaging with the ST-2000XM, the field of view through the 300mm Newtonian is about 26x20 arcmin. The Newtonian has flexure in the mounting rings and between the focuser mount and telescope tube . Even so, when using TheSky to locate tagets, the target is always within the central 10% of the CCD field. When the AP155 is in use, the error is about half of this. In either case, the object is always near the centre of the CCD field of view. TPoint proved to be an excellent tool in refining the alignment.

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