Hi Ludwig, Basically I use telescopes with German equatorial mount because we just need one drive/motor to follow the bodies, quite simply, of course that you can use computer systems also, but just one movement or drive is enough for that. It is not difficult the exact orientation about R.A. and DEC axes, may be I can just cite one disadvantage: the counterweight. Normaly I recomend to my students use that one and I have taught them how you can build it.

Hi Claudio, yes the counterweight is a problem in the German equatorial mount. The mount needs to carry both telescope and counterweight, so twice the weight of the telescope. I am mitigating this a bit by adding another telescope on my German equatorial mount where the counterweight is normally installed, but still need some means of balancing the mount, so some weight adjustment then needs to be added to the polar shaft (as the two telescopes are not exactly the same), and in addition some means to balance the two telescopes on the dec axis. This however complicates the way in how the system steers and points, as it can easily run into itself. I attach a photo of the telescope. Where you see the counterweights is of course where the 2nd telescope will go. The refractor telescope on the mount now is a 12.5 cm f12.5 and the second is a 10.8 cm f15.6. I am writing the software for control of equipment and steering of the telescope.

I suggest you take care about the mount because if you put to much weight (telescopes+ weights+camera) can damage mechanical parts, however you can control the exact weight using small balls (used for fishing) of lead or sand or increasing the length of support for weight (torque) if you have space the last one is better and easy to do. If you are thinking about pictures more weight will be necessary and good speed control. Good Luck and observations.

German EQ mounts don't suffer from field rotation which is a feature needed for photography.

A simple answer has to do with the structural analysis. In the case of alt-az mounts all structural deformation is a function of a single variable: the elevation. The azimuth axis is parallel to gravity. In a polar mount telescope structure (mount and mirror) deflect as a function of two variables (declination and hour angle). As the telescope tracks, the stress rotates relative to the structure. If the telescope is axially symmetric (simple tube and two lenses, like Galileo) then this is not a serious problem. However, with sub reflectors and spiders, off-axis optics, various focal configurations, and complex, built-up mirror-support structures, the simplification of having a single variable (elevation) to analyze makes the design and use very much easier. Deformations can cause both pointing and imaging problems.

Useful answers, thanks. I am working on a 1-m az-el telescope as well, to be used for lunar and interplanetary laser ranging. One of my ideas to characterize structural deformation is to have a real-time thermal deformation model of the telescope. This will be based on input received from a network of thermal transducers located on the tube assembly, main and secondary mirrors and the azimuth mount. We have also painted the complete telescope with a special heat reflective paint which contains minute glass globules to minimize differential heating of the structure. The picture which I add here is of the 1-m az-el telescope with the tube removed, parked inside a run-off enclosure. If you zoom in on the photo, there is a wooden board which we use to close the entrance which reads "Keep this door closed for reptiles from entering", the sign is not there for fun!

I really think that your 1m telescope and deformation model will greatly benefit of the Alt-Az mount. At that size the instrument will be much more compact and computers will easily do the rest (i.e. computing tracking speeds, derotation of the fov...). With increasing presence of computers, I suspect that soon most small/amateur telescopes will be soid in Alt-Az configuration.

Hi Paolo, this 1 m telescope actually comes from very close to Sophia Antipolis, it comes from the Observatoire de la Côte d’Azur (OCA) just outside Grasse. I add a photo here when this 1 m az-el was used for satellite laser ranging (SLR) at OCA. The French transportable SLR is left, the 1 m (now at HartRAO) is in the centre and to the right the 1.5 m still used for Lunar Laser Ranging (LLR) at OCA (France).

Ludwig, that's amazing!! I didn't know! In fact I work at OCA but on the Nice site - and I'm not specialized in laser ranging. Nice pic too, thanks!

Note that a small area at the zenith will be unavailable due to too rapid changes in azimuth.

Hi Igor, yes, that is an interesting problem and one may have to switch to some x-y coordinate at that stage for the az-el. With German Equatorial, there is also an unavailable area at zenith otherwise the tube runs into the pedestal.

Hello Ludwig! Sorry, but the German mount has no singular points. If your attachments rests on the column, you can always shift the position of the telescope from the west to the east of the column or pedestal.

Hi Igor, yes that would normally be the case, but my telescope has a box surrounding the pedestal, underneath which are two steel boxes which contains the power supplies and servo drives. This clutters the zenith position a bit. I think I can cut the to wooden box to a smaller size, which will allow unobstructed movement. See the attached photo.

Hi, and may be you move the attachment point of the tube to the declination axis closer to the oculars and add extra weight?

Hi Igor, yes, I still have to figure out some way to balance the tube, but as you say a lot of problems will disappear if I can move the tube attachment point. I can also increase the height of the pedestal. The mount unbolts just below the flange onto which the wedge is bolted. You can see this in the attached photo. If I can extend this tube section (with the 2 flanges on each side), and move the tube, the problem of an obscured zenith will be over.

The "zenith" problem is actually a "meridian" problem and afflicts all GEMs.

Depending on the various dimensions, you may be able to extend the range of hour angle movement a little in the "counterweight high" posture, but at some stage a "meridian flip" will be necessary and any exposure will have to be interrupted.

A piece of advice: in the control system, be sure to express the mount posture in terms of mechanical declination range rather than "side of pier". During the flip the mechanical declination passes through +90 deg (in the northern hemisphere) and can either be in the normal 90 range, while the relationship between mechanical HA and sky HA changes by 12 hours. Mechanical interference aside, any sky position can be seen in two mount postures, and it is important to distinguish between mechanical and sky coordinates. If you try and think of it all in terms of sky RA,Dec you may get into a tangle, especially with circumpolar fields.

To Patrick Wallace.

Hi, "meridian flip" helps to win a few hours of observations sometimes. It is not a little. And higher position of the tube relatively to the ground level helps to observe at big air masses, near the horizon. If you explore periodic processes this can be critical to eliminate 24-hours aliases. So the German mount has extra advantages.

Igor - I may not have been clear. Of course all normal GEMs require meridian flips to observe the whole sky. I simply meant that on some GEMs you don't have to do the meridian flip exactly on the meridian but can delay it until the telescope is about to hit something. And similarly you can get the flip done ahead of time by presetting east of the meridian. The size of the overlap band, where a given object can be observed in either flip state, depends on the design of the telescope and mount and on what instrumentation is present.

I am puzzled by your point about the higher position of the tube. This will give a visibility advantage only to the extent that parallax against nearby objects (say the wall of the enclosure) matters.

Hi Patrick and Igor, thanks for the information. Yes, meridian flip can be avoided just before the telescope runs into the mount. I think Igor was referring to my particular problem, which was causing a zenith no-go zone due to a box mounted on the telescope mount; see the photos. I attach another here, where the pillar is extended via a rack and pinion gear bolted to an extender. If I adopt Igor's suggestion of moving the tube, and re-balancing, the tube will clear the box and the tube will be able to point at zenith. So, where the extended piece is now, I will fabricate a flanged thick wall section, which will bolt (2 half sections) onto the top an bottom existing flanges. This means that I can reduce the size of the mount for transport purposes, yet have very good stability, and of course get rid of the zenith dead spot. I have yet to work into the software as to how far the telescope will move before an automated meridian flip will occur, and I will add a function for a forced (on demand) meridian flip.

Yes, I meant the surrounding walls taking conventional arrangement of the telescope in dome. Sometimes it takes up to an hour if you can not make a flip.

Hi Igor, very true. This refractor is being prepared for a light-bending (General Theory of Relativity test) experiment planned for the March 2015 total solar eclipse. The Sun will be quite low at that stage, ~10 deg elevation. A flip must of course be avoided as totality is about 2.3 minutes....any waste of time will be a disaster! The idea is to have a tent like enclosure surrounding the telescope, with an opening towards the Sun. So there will be no conventional dome wall obstructions. Most of those will probably not allow the telescope to access the sky at 10 degrees altitude.

Ludwig - For the GR test, consider comparing your astrometry with the software described here: www.iausofa.org/2013_1202_C/sofa/sofa_ast_c.pdf.

Hi Patrick, yes thanks, I am planning to incorporate the SOFA library as far as possible. It is an extremely useful library, especially now that there are some C code available. I will probably do some tests against DE421 to make sure that the code has been implemented properly.

Hi Patrick, were there ever write-ups as to the exact implementations of the references of the light deflection subroutines? Reference quoted are Urban, S. & Seidelmann, P. K. (eds), Explanatory Supplement to the Astronomical Almanac, 3rd ed., University Science Books (2013).

and

Klioner, Sergei A., "A practical relativistic model for micro-

arcsecond astrometry in space", Astr. J. 125, 1580-1597 (2003).

This was probably implemented to some level only.

The SOFA code, including inline documentation, is the definitive description of the algorithms used. In general those in Urban & Seidelmann (2013) are used, or close equivalents. The more complex treatment in Klioner (2003) is mainly about things that don't apply in the Sun-only case, such as quadrupole field. Some discussions of the light-deflection accuracy can be found in Section 4.3 of the document I mentioned.

An added benefit to an alt-az mount that is often under-rated has to do with operation in a protective dome: a benefit if wind-loading is an concern. The required dome radius is minimized with an alt-az configuration, potentially a significant cost-saving feature.

Hi Peter, very true. Wind loading will be of concern in most applications, in particular as many astronomical telescopes are located on top of high, exposed locations so as to benefit of improved astronomical seeing conditions. With the development of the LLR for South Africa, we have decided to use a run-off enclosure, and not to use a dome, to allow the telescope to stabilize optically and mechanically without being in its own dome micro-climate. Being an alt-az (1 m Cassegrain) it can easily move to 0 deg elevation (we have to go low for pre and post calibrations on fixed targets). We built a small brick wall to minimize ingression of dust (and snakes). Whether this approach is optimal is difficult to judge, but the general idea is to minimize any local distortion of the laser optical wavefront as it will severely affect the receive and transmission efficiency when ranging to the corner cube reflectors located on the Moon. As it is, with astronomical seeing conditions of 1-2 arcseconds, we expect a return rate of 4- 5 photons per minute, which as you can appreciate is not a whopping big signal. I attach a photo of the alt-az located in its run-off enclosure, the tube has been removed for renovation and mirror re-aluminizing.