Alt-azimuth and Equatorial mounts both have their strengths and both have their weaknesses. Which is best depends on the use to which you currently want to put it.
Alt-azimuth mounts are the easiest and quickest to set up. The prime requirement is that the mount should be level. The provision of a level observing/imaging area is a great help for either kind of mount. Marks on the floor facilitate the placing of the mount legs for either kind of mount.
iOptron Alt-azimuth mount with a CaK solar telescope mounted for imaging
Some Alt-azimuth mounts such as the Meade ETX need to be level and the scope facing north. Some, such as the iOptron Cube Pro that we often use requires the mount to face south, but the scope to point to the zenith. A magnetic compass is normally sufficient to acheive adequate north or south alignment. Mounts such as the Skywatcher AZ mounts for example, the Star Discovery, can be set up level and facing north. When started in this configuration, they can be sent immediately to the required objects without further alignment. The accuracy depending on how good the level and north are set up.
GPS can either be built into a mount as it is with the iOptron Cube Pro, or it can be an optional accessory for an Equatorial mount. GPS can reduce the amount of time required to set up a mount as when the GPS has aquired enough satellites the system knows the date, time, Latitude, Longitude and altitude and these parameters don't need to be entered manually via the hand controller.
Equatorial mounts have a requirement for accurate polar alignment and this can be done in several ways. In the northern hemisphere, if polaris is visible and a polar scope is fitted and correctly collimated, the mount can be polar aligned using Polaris and the polar reticle. In the southern hemisphere other target stars are used for polar alignment with a polar scope.
Skywatcher HEQ5 mount with a Bresser 102mm refractor mounted
Celestron AVX Equatorial mount with a Skymax 127 mounted
It can be seen that the tripod has been placed on marks on the level concrete base. Moreover, it can also be seen that the building obscures polaris so, polar alignment cannot be done by means of the polar scope or polar alignment camera. In this location (the only location that this particular mount is used), Celestron's Any Star Polar alignment was used to adjust the altitude and the azimuth of the polar axis to achieve a good polar alignment. Using the marks on the ground, the mount can be placed and alignment using alignments stars and calibration stars can be acheived that gives excellent tracking. Using auto-guiding, long exposures of several minutes can easily be achieved.
In northern latitudes it can be quite uncomfortable bending, kneeling and peering throgh a polar scope. Fortunately there are alternative methods using cameras such as the QHYCCD PoleMaster which can get the polar alignment within 30 arc seconds of the NCP.
A QHYCCD PoleMaster fitted to an HEQ5 mount in our observatory
For visual observing, the Alt-azimuth system is the most covenient and comfortable system to use. We can set up the iOptron Cube Pro mount within about 5 minutes and an equal time to put away the equipment.
If we are capturing a lunar SER file or a couple of overlapping Solar SER files, the whole process of setting up, imaging and putting away can take as little as 20 minutes.
Setting up the Equatorial system takes about 30 minutes to set up and a similar amount of time to put the equipment away. Setting up takes comensurately longer if autoguiding is to be used.
For quick imaging of the Moon, Sun or a planet, the Altazimuth system is the quickest and easiest to set up. That being said, the AVX EQ mount is rock solid and there is very little shake if the scope is being manually focused, whereas with the iOptron AZ being on a less substantial tripod, experiences shake when manually focusing the scope resulting in longer focusing times. The issue disappears if motor-focusing is used.
The iOptron AZ system is not suitable for heavy equipment, but there are much more heavy duty AZ systems available that can work with heavier scopes.
Field and image Rotation
For visual observation this is not a problem, but it is a potential problem for imaging.
The Sky rotates through the night and with it, the objects that we observe and image. As a thought experiment, imagine a vertical rod with a ball on top of it rising in the east. By the time it reaches the meridian in the south, it will be horizontal and by the time it sets in the west, it will be vertical again but with the ball at the bottom.
Stellarium simulation of the Moon viewed through an Alt-azimuth system between the hours of 1am and 4am on January 21, 2022.
Actual view of the Moon captured at 0:20 by AstroDMx Capture for Windows and a ZWO ZWO ASI 178MC with a 66mm, f/5.9, APO, ED doublet refractor mounted on an iOptron Cube Pro AZ mount.
The image is a stack of the best 50% of the frames in a 2000-frame SER file, stacked in Autostakkert! wavelet processed in Registax 6 and post-processsed in the Gimp 2.10.
The thing to notice is that there was no problem capturing the image data on the Moon during this session. This is because during the capture of the data (a period of 1m 16s) there was no significant rotation of the Moon at this image scale.
However, if we wished to image a deep sky object such as the Orion nebula, we would need to collect a large number of sub-frames to stack. The exposures just have to be short enough to ensure that there is no significant image rotation WITHIN the capture of an individual sub-frame.
On a night with no significant interference from the Moon A William Optics Zenithstar, 66mm, f/5.9, APO, ED doublet refractor was mounted on an iOptron Cube Pro AZ GOTO mount. A ZWO ASI 178MC camera was placed at the focus.
AstroDMx Capture for Windows was used to capture 200 x 12s exposures of the Orion nebula with matching dark frames. The 200 frames were simply stacked in Registax 5.1.
An AZ mount tracks the sky in such a way that image rotation occurs over time. and this was evident in the stacked image. The image was post-processed in the Gimp 2.10.
Image of the Orion nebula showing rotation
The same data were stacked in Deep Sky Stacker which is able to derotate the images before they are stacked. The image was post processed in the Gimp 2.10.
Image of the Orion nebula derotated
The next image demonstrates that the stars in the derotated image are exactly coincident with the star trails in the rotated image.
The important point is that as long as the exposures are short enough so that rotation WITHIN an image is not significant, then an Alt-azimuth mount can be used for deep sky imaging as the data can be derotated when they are stacked.
Some Alt-azimuth systems have available mechanical derotators that remove the effect of rotation when collecting image data. Probably one of the best (but expensive) Alt-azimuth mounts is a Track The Stars TTS-160 PANTHER TELESCOPE MOUNT. For this system there is also available a telescope rOTAtor. With this unit installed on the mount head the telescope will track equatorially for long exposure Astrophotography. Yes, equatorial tracking with an Alt-azimuth system!
With an Equatorial mount, well polar-aligned, long exposure is easily possible. With auto-guiding we have achieved 20minute exposures with no star movement between exposures and 5minute exposures are routine. A recent imaging session with the same equipment on the AVX EQ mount can be seen HERE. That session was atypical for us as we were doing ST4 autoguiding to test the SV905nguide camera in that mode. Typically we would use pulse guiding but the ST4 auto-guiding worked well.
The message here is that whether you have an Equatorial mount or an Alt-azimuth mount, you will be able to do imaging of deep-sky objects. Whatever mount you have it is imperative that it is set up as well as possible in terms of level, orientation, polar alignment etc. Any deviation from the optimal situation will compromise the tracking required for the imaging. The shorter the focal length of the scope you are using, the more forgiving it will be.
Experimentation is required to find out for your system, the maximum realistic exposure you will be able to get results from. Based on this, you will be able to plan your imaging session with the best chance of success.
