The Mount

As I wrote in the introduction, my present altazimuth mount is a Skywatcher AZ-GTI, a quite compact GOTO mount with built-in WiFi that you can control through an app on your smartphone or tablet (Android or IOS) or on your Windows PC.

Some schematics and characteristics taken from the Skywatcher documentation:

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The sturdy interior of the mount, with all the moving and load bearing parts made in metal and not in plastic, can be seen in this image, originally posted by user “dr.who” on the Cloudy Nights forum:


The mount can be powered by 8 AA (shortlived) batteries or by a 12 V DC power supply: working in the comfort of my apartment I have the luxury of using the second option.

The best alignment option for the GOTO relies on just two stars and let us say that GOTO and tracking precision appear to depend on many not exactly known factors.

Some stars are better than others, but the same couple of stars can give different results from night to night, one night allowing 30s exposures at a focal length (FL) of 640 mm while on another night limiting you to 20s at the same FL and with the same telescope and camera.

Another very important factor influencing tracking precision is the weight and shape of the telescope: the lighter and the more compact it is, the better.

Drawing a parallel to the well known “500 rule” for untracked photography (i.e. the product of your exposure in seconds times your focal length in mm must be lower than 500), I may say that with the rather light Skywatcher ST80 I can always rely at least on a “12000 rule” (i.e. 30s at 400 mm), sometimes, with some periodical “adjustment” of the mount (more on that later) even a “18000 rule” (i.e. 45s at 400 mm).

My old 4 inch SCT, the glorious Meade 2045D OTA, weighs more or less like the ST80 but is more compact, and even with its 640 mm focal length (obtained through a F 6.3 focal reducer), I can usually reach 30s, with a product of little more than 18000.

But my extremely light and compact Skywatcher MAK-90, albeit awfully slow at almost F14, can achieve exposures as long as 20s at a focal length of 1250 mm, hence following a “25000 rule”.

On the other hand, the quite heavier Bresser AR 102-XS rarely allows exposures longer than 20s at 460 mm, behaving according to a rather disappointing “9200 rule”.

One must however reckon that this rules are not only approximate but that they also do not always scale up linearly.

For instance in my experience I had big trouble achieving decent 90s exposures (even in the dead center of the frame, as to exclude field rotation) with a 135 mm prime lens, which should have been possible according to the “12000 rule”, moreover with a quite light and compact setup.

Of course all these times depend on the the sensor you use, or better on the size of its pixels, and these ones in particular, are valid for the 4.3 µm pixels of my Canon 1300D DSLR, while you have basically to double them for the 9.6×7.5 µm pixels of my old faithful Meade DSI Pro, or if you bin 2×2 your sensor.

A more comprehensive analysis can be found here.

To align I always use the two-star alignment, taking the utmost care to center the star well in the center of the frame.
For this purpose I always perform the centering on the screen, with the instrument and the camera that I will use to shoot the pics.
With my DSLRs I use APT (Astro Photography Tool), with the ZWO cameras the ASICAP suite.
This far re alignment.
After having made the alignment, “I refine” it by centering other stars, possibly close to the targets that I intend to photograph.
I control the AZ-GTi from the APP on the iPad and it is VERY IMPORTANT to leave the APP always in the foreground with the screen on, even for hours: I have had direct proof that if you turn off the screen and / or the app goes in the background for more than a few minutes, tracking suffers dramatically.
Also, when you center the target with the goto in the sensor, let the goto finish moving by itself before doing centering operations. Only if the target is really leaving the field without stopping, give a sharp blow with the DOWN key (arrow down).

There are also pairs of alignment stars that ultimately provide better results for goto and, in my experience, also tracking … but those are things that hato be learnt empirically in the field.
Not to mention that the driving software itself evolves over time and therefore its behavior can change.

An aspect, or issue if you want, that has remained constant in this mount throughout its guiding software’s evolution so far is a different level of tracking precision depending on the azimuth angle.

While it is to be expected that altazimuth mount experience tracking problems close to the zenith (so much so that the AZ-GTi has a default maximum altitude angle of 75°), what I have constantly observed is a more “erratic” behaviour closer to the celestial pole.

More precisely, and in order to substantiate the case with some number, on the same night I had to discard 22 out of 102 45 seconds exposures taken at an alzimuth angle between 305° and 315°, while I discarded only one of 82 90 seconds exposure taken at an alzimuth angle between 260° and 280°.

In both cases the altitude angle was between 40° and 60°.

What happened closer to the pole was that from time to time the mount suddenly moved, “re-centering” the object or moving it slightly up and down the frame, thus spoiling the single exposure.

My guess is that that happens because closer to the pole the tracking software requires periodical “re-alignments” or anyway it deems it has to do some “position shift” in order to continue to track correctly.

This may be possibly related to the fact that the diurnal (apparent) speed of a celestial object is proportional to the cosine of its declination

v(arcsec/sec) = 15 x cos (declination)

and hence it decreases closer to the celestial pole.

If this is an issue for every altazimuth mount or just for the Skywatcher AZ-GTi because of some glitch in its software (even if that is the case it should affect all of the Skywatcher altazimuth mounts), I do not know because my experience is essentially limited to this mount.

My only previous experience was with the Meade ETX-80, but I did not take notes about this kind of experiences at the time. The only thing I can say from the few quantitative data I have, is that my single longest exposure with the ETX-80 was a 60 seconds image of M82, which has almost 70° of declination angle.

While it has been tested by a few people with loads even exceeding the manufacturer’s  specifications (and indeed even up to almost 7 kg), for DSO astrophotography the 2.8 kg of my Bresser AR 102-XS (which become 3.3 kg when you add the DSLR and the T2 adapter) are probably close to the maximum useful weight, even if, of course, also shape and balance of the telescope have to be considered.

On the the other hand one could use one of the new highly sensitive (80+ Q.E.) monochromatic cameras, which can make exposures in the 10s or less range rewarding, and then the use of OTA up to close to 5 kg weight, like a Celestron C6, profitable, an indeed that’s what I ultimately bought, weighing on the scales just 500 grams more than the Bresser and having a more favorable weight distribution.

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