For long exposure photography of the deep sky, the most important thing to assess is the tracking accuracy of the mount. It is determined by evaluating its periodic error. Each mount produces a more or less pronounced periodic error. The periodic error is mainly due to the internal mechanics of the frame. The magnitude of the periodic error varies from mount to mount. Some very high quality frames have minimal periodic error. Obviously, the price of the mount increases with the tracking accuracy. You can invest more than $ 10 on a very high quality frame.
The periodic error of the mount is evaluated for the duration of the complete rotation of the worm in right ascension. It should be noted that the declination evaluation is much less important, because the mount must be in an equatorial position and precisely aligned with the Celestial North Pole (PNC), thus providing very little variation in declination.
First, we evaluate the periodic error of the mount in right ascension without PEC and autoguiding. Next, the improvement in the periodic error is evaluated with an active PEC. Finally, we test the tracking quality of the mount with a PEC and active autoguiding.
Here is the evaluation of the periodic error of the CGEM mount in the following situations:
- Periodic error without PEC and autoguiding
- Periodic error with an active PEC
- Periodic error with PEC and autoguiding active
The field tests were done by me because I own this mount. Here are the parameters that were used:
- The duration of rotation in right ascension of the endless screw of the CGEM mount is 8 minutes
- The following equipment has been fitted to the mount
- Celestron Edge HD 800 tube
- The Orion guide telescope with a diameter of 80 mm and a focal length of 400 mm, therefore open at f / 5, which is mounted in parallel on the Edge HD 800 telescope
- The DSI camera for autoguiding which is mounted on the guide scope
- The NightScape camera for imaging
- The total weight added to the mount is 10,27 kg out of a maximum load capacity of 18,1 kg (40 lbs), or 57% of the load capacity of the mount. I recommend never to exceed 70% of the load capacity of the mount in deep sky imaging
- Using the PHD guiding software and GPUSB module to send autoguiding commands to the mount
- The mount has been configured for autoguiding according to the following parameters:
- RA rate: 60%
- DEC rate: 99%
- The camera sampling in use with the guide scope is 4,92 ″ of arc. To adjust the camera sampling to 1 ″ of arc, I set the Fractional Pixel Autoguiding to 0,20 (minimum motion 0,20 in PHD Guiding, which is 4,92 x 0,20 = 0,984 ″ of arc). This makes it possible to send guidance commands to the mount from 2 ″ of arc, ie just above the air turbulence in very good conditions in Quebec.
- The alignment of the mount on the PNC was carried out with precision
- Autoguiding commands are performed every second
- I pointed a guide star in a southerly direction near the Meridian at a height of about 35 ° above the horizon
Periodic error without PEC and autoguiding
Here is the periodic error of the mount without PEC and autoguiding for the 8 minute worm rotation time:
The blue curve represents the periodic error in right ascension and the red curve that in declination. The red curve shows that the alignment of the mount on the PNC is precise, as it stays very close to the center line. The periodic frame error is represented by the blue curve. Each square on the height axis graph represents one pixel ie a right ascending displacement of approximately 5 ″ of arc (4,92 ″ of arc to be precise). The squares in the width axis represent time. 12 squares represents a duration of 8 minutes, or 40 seconds per square.
The values, of the blue curve, above the center line represent the displacements (the decentralization) of the guide star to the right of its center point. The values below the center line represent the movements of the guide star to the left of its center point. There is therefore a movement to the right of the guide star of 10 ″ of arc and to the left of 5 ″ of arc for a total periodic deviation of 15 ″ of arc.
Periodic error with an active PEC
Here is the periodic error of the mount with PEC active for the same duration of 8 minutes:
Note that the blue curve is inverted compared to that without PEC. The total periodic deviation remains at 15 ″ of arc. There is therefore no gain by activating the PEC. So for this mount, it is of no use to activate the PEC.
Periodic error with PEC and autoguiding active
Here is the periodic error of the mount with PEC and autoguiding active for a duration of just under 8 minutes (I miscalculated the total time to produce the graph):
We can see that the periodic error is reduced to +/- 2,5 ″ of arc for a total of 5 ″ of arc, or just a little higher than the average air turbulence in Quebec (between 2 ″ And 3,5 ″ arc). We can also appreciate that the periodic variation in declination is approximately the same, which will make it possible to obtain very round stars.
Here is a recent graph with a duration of 13,33 minutes that represents the maximum display time in PHD Guiding:
The periodic error is a little less than 5 ″ of arc (here, each square in height is 4,10 ″ of arc). I adjusted the field Mm mo to 0,25 to take into account the sampling of the guide camera with my new Orion 80 ED f / 6 guide scope. Sampling always remains within 1 ″ of an arc as a fraction of a pixel.
To evaluate this performance, here is an evaluation grid for the quality of monitoring of a self-guided mount:
| Excellent | 2-3 ″ arc tracking |
| Very good | 5-6 ″ arc tracking |
| Good | 8-12 ″ arc tracking |
| Minimum | 15-20 ″ arc tracking |
source: The New CCD Astronomy, Ron Wodaski, New Astronomy Press, page 218
According to this evaluation grid, we can see that the CGEM mount produces very good tracking in autoguiding, which is a very interesting performance for a mount in this price range (approximately $ 1 Canadian as of May 500 ). In my opinion, it would take several thousand additional dollars to purchase a frame with better tracking. Note that I have been using this setup for over a year and always get guidance accuracy to 2013 ″ of arc or less no matter where I point the telescope in the sky. With this stability and consistency, it is possible to envisage exposure times per photo of 5 minutes and more without problems with practically no loss of images.
Autoguiding dated 2016/07/06
Version 2 of PHD Guiding allows to display the periodic error on the graph directly in arc second. Here is the graph displayed using the software PHD2 Log Viewer :
Click on the image to display it full screen
The displayed time of 8 minutes represents one complete rotation of the CGEM mount's worm gear. Here is the material used for this test:
- The following equipment has been fitted to the mount
- The Celestron Edge HD 800 tube with f / 6,3 focal reducer. 1mm focal length
- The optical divider Orion Thin Off-Axis Guide
- The ZWO ASI 120MM camera (monochrome) for autoguiding
- Using the PHD guiding version 2 software and the GPUSB module to send autoguiding commands to the mount using the mount's autoguiding port
- The mount has been configured for autoguiding according to the following parameters:
- RA rate: 99%
- DEC rate: 99%
- The ASI camera sampling when used with the Celestron Edge HD 800 tube is 0,60 ″ of arc. To adjust the camera sampling to 1 ″ of arc, I set the Fractional Pixel Autoguiding to 1,67 (minimum motion 1,67 in PHD Guiding, which is 0,60 x 1,67 = 1,0 ″ of arc). This allows you to self-guide at +/- 1 ″ of arc (2 ″ of arc in total), or just above the air turbulence in good conditions in Quebec (2 ″ of arc)
- The alignment of the mount on the PNC was carried out with precision
- Autoguiding commands are performed every two seconds
- I pointed a guide star in a southerly direction near the Meridian (east side) at a height of about 40 ° above the horizon
- Dither was not enabled for this test. This makes it easier to visualize the graph
- PEC has not been activated
- The other configuration details appear in the graph
To view the assembly of this configuration, click on this lien.
Air turbulence and sky transparency were average (3/5), as is often the case in Quebec!
The yellow line represents the quality of the guide star which is poor to medium (the guide star is never very good with an optical divider). The blue line displays the periodic error in right ascension (AD) and the red in declination (DEC). Looking at the graph, the maximum deviations in AD and DEC are approximately +/- 2,5 ″ of arc (5 ″ of total arc). The mean values (RMS) being 1,84 ″ of arc for AD and DEC (3,68 ″ of arc in total). See RMS data at the bottom of the graph. These are the average values (RMS) for the entire autoguiding duration which is 25 min 52 s. Also, in the graph, we can see the target object (top right) which shows the movement of the guide star every 2 seconds. The majority of the displacements are in the red circle which has a diameter of 1 ″ of arc, which is excellent.
This test therefore represents a constant behavior of the quality of monitoring of the frame in autoguiding since the first evaluations in 2013, that is to say for three years. The quality of follow-up remains very good.
Richard Beauregard
The Sky Astro-CCD
Revised 2021/03/30