Mon, 8. September 2014
Meade LX-90 Telescope Equatorial Mount - 1
After using my 8" Meade LX-90 in Alt-Az mode since I bought it six years ago, I decided to take the plunge and try using it in equatorial mode. This is the first account of what I expect to be several, as I try to make sense of it all.
The Meade equatorial wedge mounted on the field tripod.
There are a number of basic criteria to learn to achieve a fairly reasonable alignment:
1. Setting up the wedge on the tripod before sunset and before attaching the telescope
Level it and aim it at the Celestial Pole. If the mount is level and the latitude is set correctly, the positioning is primarily dependent on finding magnetic South and offsetting by the correct angle to arrive at True South. The compass provided by Meade is not very responsive and it is mounted in a metal frame which could affect the pointer accuracy but it should provide an accuracy of two or three degrees. Altitude should be accurate, so that in theory, the Pole Star (Sigma Octans in the Southern Hemisphere) will be found after dark by sweeping slightly left or right.
The Meade equatorial wedge, with latitude adjuster.
2. Attaching the telescope to the mount and rotating it manually to "Polar Home".This seems to be a crucial operation for which there is no adequate documentation or nominal procedure to follow. How to align the tube with the axis of the telescope? This is a procedure with which I am still grappling - it seems a bit hit and miss - but at the moment it seems the telescope needs to be levelled and an inclinometer used to set the tube angle to the latitude, so the tube is parallel with the forks.
3. Fine adjusting the mount alignment after dark.
Once the tube is at " Polar Home", wait until well after sunset, when the magnitude 5 stars in Octans become visible in binoculars. Then, sit directly behind the telescope and note the left/right alignment of the telescope tube in relation to Sigma Octans. If it is off by just a small amount, use the Azimuth Control Adjuster to correct it. If it is off by a large amount, relocate the tripod and readjust the legs until the mount is again level.
The wedge, compass, bubble level and azimuth level control screws.
It all sounds quite straightforward but it's not that easy the first time out. I started by setting the scope up indoors with the wedge and testing it's behaviour. Then I had a field night with friends from the Society and spent much of the evening patiently attempting to align. I did achieve this but it was not nearly as accurate as I had hoped for; and I had difficulty recognising Sigma Octans in my new 9x50 finderscope, with a field of view of 5 degrees, slightly less than that of the standard finderscope which I had been using previously.
4. Aligning to Sigma Octans.Using the finderscope, confirm that Sigma Octans is in the field of view. For rough alignment, centre the star (which is 66 arc-minutes from the South Celestial Pole) using the Azimuth Control Adjuster and Latitude Control Adjuster and confirm through the eyepiece. For fine alignment, use the adjusters to centre the South Celestial Pole by star hopping.
5. Aligning the Autostar.
Using the Autostar handbox, set the alignment of the Pole and align with another bright star. It seems to me the procedure does not make it clear whether the alignment is to the Pole or to Sigma Octans.
LX-90 mounted on equatorial wedge.