Titan Equatorial mount alignment

Mount alignment is a requirement to proper tracking and guiding for astrophotography. It will not matter how good your mount is if your alignment is bad. To achieve proper alignment, there are several techniques at your disposal, such as:

  1. Using a polar axis scope
  2. Bigourdan method
  3. Polar align assist

Using a polar scope

Using a polar scope (which my Titan mount does no have by the way it’s an option) usually allows you to get a rough alignment at best. You can easily live with it especially with a mount such as a Gemini equipped mount, as you need to train it and it will compensate for alignment errors in its goto function, but not in tracking (meaning that it is not going to make any adjustment in declination while tracking). if your mount is misaligned and your guiding software still manages to keep up with it, you will get field rotation in the long run, centered on your guide star. What polar scopes are good for, however, is to get your mount roughly aligned so that you can use one of the drift methods below to get it spot on. That said, I know that some of you who practice astronomy in a nomadic fashion will tell me that using a polar scope to align their mount is all they do and I respect that.

Bigourdan method

This is the method I use. First and foremost, you will need a cross-haired eyepiece of fairly short focal length (usually is 12.5 mm). The method consist in aiming at 2 stars, one at the meridian to the south, to adjust the pointing of the mount in azimuth, and the other, 6 hours away from the meridian, either to the East or to the West, to adjust elevation of the mount. Corrections are made looking at drift direction of the star in relation to the cross-hair.

Adjustment in Azimuth

Point a star to the south meridian, as near to the celestrial equator as you can get on, and orientate the cross-hair in a East-West direction (horizontally). Keep in mind that in a Schmit-Cassegrainian telescope, up an down are reversed. place the star on the horizontal hair, and observe the way it drifts for a few minutes. The directions take into account the fact that the image is reversed.

  • if the star drift toward the horizon (hence to the south), move the top of the right ascension axis to the WEST.
  • if the star drifts toward the zenith (hence to the North), move the top of the right ascension axis to the EAST.

Once you are satisfied there is no up or down movement of the star for a few minutes, the orientation of the right ascension axis is good.

Adjustment in Elevation

point a star to the East or West, as near to  the celestrial equator as you can, and adjust the cross hair in a North-south direction (on a lign  polar star-south horizon). Again keep in mind that up and down drift direction are reversed. observe the drift of the star, in relation to the north-south hair. The directions take into account the fact that the image is reversed

  • if the star drifts down, under the north-south hair, lower the top of the right ascension axis until the star is back on the line.
  • if the star drifts up, rise the top of the right ascension axis, until the star is back on the line.

proceed in the same manner until there are no drifts and the star remains on the line. The picture below illustrates the Bigourdan method:

bigourdan

Gemini polar align assist & polar axis corrections

These are 2 gemini  “software” methods meant to help you setup the mount properly. I have only tried the “polar align assit”, which is an iterative method. It requires you to point to 2 stars, and do manual adjustment to the mount alignment. Unfortunately, for me, after the first iteration, the mount start pointing at stars with the eyepiece! Even though I have followed the manual instructions to the letter, I have never managed to get it to work, so if someone knows what I am doing wrong, plaese tell me?

Training the mount

Ok, so now the mount is properly aligned with Earth rotation axis it is time to “train” the mount. Why? because there are mechanical inaccuracies such as telescope optical axis perpendicularity in relation to declination axis and other geometric parameters which will affect mount pointing accuracy. At first, getting used to the Gemini Level 4 menus can be a bit of a hassle, because, lets face it, it is not very user friendly. We will assume at this stage, that you have already entered all the relevant information such as:

  • geographic location
  • time and date in the proper format
  • mount type

Position the telescope counter weight down (CWD) as best as you can. After the Gemini has finished initializing, it will display location, time and date, tells you GPS has timed out (unless you got one) and ask you wether you want to do a warm start or restart, or a cold start. Everytime you modify the mount alignment, you’ve got to do a cold start to overwrite the old settings. Then, select “go to bright star” and select a bright star overhead. the telescope will slew to that star give or take a few degrees (it depends on how accurate your initial CWD positioning was). If it’s really far out, double check the “setup” parameters you entered and start again.

Use the controls to center the star into the middle of the field of view and select and  press “synchronize”, to which the mount shall reply “aligned!” you then have to perform an “additional align” for the mount to buid a model of the mount. Now, logic would imply that you have to go to ” additional align”, “goto bright star”, and “synchronize” but that will give you a “rejected message”. what you have to do is a “goto bright star” select a new star far away from the first one, and only then, “additional align” and “synchronise”. This is one of the quirks of Gemini, we have to live with… You need to make several “additional” align” on all 4 corners of the sky for the gemini to have enough data to built a reliable model. Once that is done, the pointing accuracy really is very good. Aslos, after the “synchronise” on the third star, Gemini will display the pointing error in azimuth and elevetion, as it sees it. If both are under 5 minutes, I consider it done.

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