Contents

Introduction
: I had been struggling to work out how to use the Vernier scales on the RA and DEC axes on my LX50 since I purchased it in October 2004 so I sent a plea out via the Meade LX50 owners' messageboard and started out this message thread -

"How to read Vernier scales on setting circles"

The responses were very helpful, from signposting me to sites which explain how vernier scales work in principle to exercises for practising reading Vernier scales (How to Read a Vernier Scale, Vernier Caliper Tutorial, University of Sheffield).

The RA clock is graduated in 5 min intervals with each division on the Vernier being 1 min. The DEC scale is graduated in 1° with each division on the Vernier an increment of 10 arcmin. Both the ranges, and especially the way you have to work out whether to add or subtract values to the RA reading if the main index mark is more or less than 3 min, did not feel very practical in the field (thanks to Nigel Burge for explaining how the original Vernier scales are calibrated and how to read them - note that I have revised the original article in light of his feedback).

I have designed my own Vernier scales for both RA and DEC. The RA now works in 30 sec increments right up to 300 sec (i.e. 5 min) so you only have to add the appropriate value to the main reading. Similarly my DEC scale is now sub-divided in 6 arcmin increments, in both N and S directions, with a range of 0 to 60 arcmin (i.e. 1°) in both directions.

I have tested them in the field and they are pretty good. I am very confident at finding anything using my wide-field 32mm Plossl then zooming in as required.

There is an excellent message thread in the Cloudy Nights Beginners Forum which describes how to set up a telescope and use setting circles in the field and I will guide you to it here rather than describe those procedures in this article.

!! Good luck and Happy Hunting !!

Making the new RA Vernier scale

My aim was to make the Vernier scales as practical and readable, and work to the best precision, as possible. I experimented with a few rough drawings and concluded that the ideal scale for the RA would be one which you only have to add to the value indicated by the index mark and that it can be read in increments of 30sec.

The final version is shown below. I made it by taking a digital photograph of the original scale and, using Adobe Photoshop, masked out the marks in between the index mark and the 3min (180sec) mark on the left side of the scale. I masked out everything on the right side of the scale. I then added four marks at evenly spaced intervals between these two marks and another five evenly spaced marks to give the desired range of 0 to 300sec (5min) in 30sec intervals.

You will be pleased to know that you don't have to go to that trouble. Simply print out the image below, laminate it in a plastic laminate sheet, and stick it on to your LX50, over the existing RA Vernier scale. The absolutely critical factor is printing out the scale to the exact correct width. The main Vernier index mark should be placed exactly on a 5 minute mark on the main RA scale and the 300sec mark on the Vernier should exactly correspond with a main scale mark 55 minutes to its left. The width I had to set my printer to is indicated under the image but it might be necessary to experiment with your own printer settings, and print several slightly different width scales, to get the right one. Note that "nearly right" is not good enough. This has to be exact or the Vernier scale just will not work.

RA Vernier scale

(set print width to 50.673 mm)

Making the new DEC Vernier scale
: Similarly, I experimented with a few rough drawings for the DEC Vernier scale and concluded that the ideal scale for it would be 0 to 60 arcmin in N and S (+ and -) directions with incremental marks of 6 arcmin instead of 10 arcmin. This results in a scale which is much easier to read and interpolate.; The final version is shown below. I made it by taking a digital photograph of the original scale and, using Adobe Photoshop, masked out the marks in between the index mark and the major 30 arcmin marks on either side of the scale. I then added four marks at evenly spaced intervals between these major marks to give the desired range of 0 to 60 arcmin (1°) in 6 arcmin intervals.; Once again, you don't have to go to that trouble. Simply print out the images below, laminate them in a plastic laminate sheet, and stick them on to your LX50, over the existing DEC Vernier scales. The absolutely critical factor is printing out the scale to the exact correct width. To check that the width of your scale is correct, the main Vernier index mark should be placed exactly on a degree mark on the main DEC scale and the 60 arcmin marks at both sides of the Vernier should exactly correspond with a main scale mark 9° to its left and right. The width I had to set my printer to is indicated under the image but it might be necessary to experiment with your own printer settings, and print several slightly different width scales, to get the right one. Note that "nearly right" is not good enough. This has to be exact or the Vernier scale just will not work.

DEC Vernier scale

(left & right fork)

(set print width to 33.570 mm)

How to read and use the RA scale

The Vernier scale is a standard method used in instruments to get higher precision readings. The method used in making a measurement determines the accuracy with which the numbers are reported. The method and scale I have used allows for a reading to 30sec for determining angular measurements.

The first image shows how the RA Vernier index mark (a) aligns exactly with the 21 hour mark on the RA clock and the 300sec aligns with the mark 55 minutes to its left as previously described. This shows that it is exactly 21h 0min 0sec in RA.

21h 00min 00sec
: The next image shows the RA Vernier index mark somewhere between 21 hours and the first 5 minute sub-division. If you look at which mark on the Vernier scale corresponds exactly with a mark on the RA clock you can see it is at 210 seconds. Therefore, the RA position is 21 hours plus 210sec which equals 21h 3min 30sec.

21h 03min 30sec

How to read and use the DEC scale
: The Vernier scale is a standard method used in instruments to get higher precision readings. The method used in making a measurement determines the accuracy with which the numbers are reported. The method and scale I have used allows for a reading to 6 arcmin (0.1°) for determining angular measurements.; The first image shows how the DEC Vernier index mark (a) aligns exactly with the 25° mark on the main DEC scale. The + and - 60 arcmin Vernier scale marks align exactly with positions 9° + and - this mark indicating that the DEC position is exactly 25°.

25.0° or 25°0arcmin

The next image shows the DEC Vernier index mark somewhere between the 25° and 26°marks on the main DEC scale (this position is north of the celestial equator therefore the + side of the Vernier scale needs to be used in this instance; use the - side for positions south of the celestial equator). Note that the 18 arcmin mark on the Vernier scale corresponds exactly with a mark on the main DEC scale indicating that the exact DEC position is 25° plus 18 arcmin, or 25.3°.

25.3° or 25°18arcmin

The final image shows the DEC Vernier index mark somewhere between the 25° and 26°marks on the main DEC scale (this position is north of the celestial equator therefore the + side of the Vernier scale needs to be used in this instance; use the - side for positions south of the celestial equator). Note that the 42 arcmin mark on the Vernier scale corresponds exactly with a mark on the main DEC scale indicating that the exact DEC position is 25° plus 42 arcmin, or 25.7°.

25.7° or 25°42arcmin