Just bought a new 60mm guide scope for my 8” f5 newt located south coast NSW and will keep my old 50mm guide scope for my 6” f6 newt located in Sydney. When I added the equipment profile for the new 60mm guide scope in PHD2 I noticed in the Calibration Step Calculator that I had previously used a guide speed of 1.0 x sidereal. My mount settings in EQMOD were 0.5 x sidereal
I’ve been guiding ok with these settings in PHD2 and EQMOD for over a year now
Is there any reason or advantage to change the guide speed in PHD2 to 0.5 x sidereal ( the same as EQMOD )
Any advice or personal experience greatly appreciated
This is an extract from the PHD1 user manual so I assume PHD2 will be the same.
The guide rate should be selected such that typical guiding commands are in the range of 10 to 300 milliseconds. The image scale of the guide system will affect this choice.
In most cases the image scale will be 2 to 5 arc-seconds per pixel and a guide rate of 50% is recommended. If the image scale is greater than 5 (such as when using a very short guide scope) a higher guide rate is probably appropriate – as much as 100%.
If the image scale is less than 2 you might want to go down to 25%. This would often be the case when using an off-axis guider through a long 'scope such as an SCT. In any of these cases, the characteristics of the mount might also influence the choice of guide rate.
A very well behaved mount should use a slower rate to get the best precision in pulse timing, but one that moves erratically might need a faster guide rate to keep the pulses reasonably short.
As for me I have always used 0.5 with no problems.
Bill,
Thanks for replying
After reading the PHD2 and EQMOD manuals , EQMOD dictates the mounts speed which in my case is 0.5 x sidereal
The guide speed setting in PHD2 under the Calibration step calculator is only for PHD2 to calibrate the guiding for the equipment used by moving the guide star appropriately
In the Calibration Step Calculator window , the box where you fill in the guide speed is a multiple of the mounts guide speed , which in my case is set by EQMOD to 0.5 x sidereal
Therefore I should just leave the box filled in with 1.00
If I reduce my setting in EQMOD to say 0.2 x sidereal, then PHD2 may struggle to calibrate as the guide star may not move enough to complete calibration
I will leave EQMOD as 0.5 x sidereal but may try 0.5 guide speed in PHD2 and see if guiding improves, otherwise I will go back to my original setting of 1.00 in PHD2
Thanks
The guide speed is set by the EQMod settings not PHD2.
PHD2 just sends pulses and direction info to the ST4 port. The speed that these pulses are implemented is that set in EQMod.
See attached extract from the PHD2 manual.
Thanks Ken
I think PHD and PHD2 May differ a bit in the Calibration Calculator
The way I read it in the PHD2 manual, in PHD2 the guide speed box where you fill in your guide speed is a “multiple of the mounts guide speed” not the actual guide speed
So if your EQMOD guide speed is set to 0.5 x sidereal and you enter 1.00 in the PHD2 Calibration Calculator you are in fact setting a 0.5 x sidereal in PHD2 for your Calibration
If you enter 0.5 x sidereal in PHD2 Calibration Calculator you are in fact setting 0.25 x sidereal in PHD2 for your Calibration
Hmmmmm...
From my reading the guide rate in PHD2 is not used for any physical guiding of the mount, it seems to be an “internal “ factor used within PHD2.
PHD2 only sends a signal and a direction to the ST-4 socket, EQMod then applies this signal to the mount using the EQMod set guiding rate.
Think of it in terms of ...PHD2 presses the direction button for a period of time and EQMod connects it to the mount at the pre-set guide rate.
Yes your right, the guide rate in PHD2 is used as part of the calculation to enable good calibration of your guide star based on your guide equipment and the resultant image scale
The image scale of my guide set up ( 50mm Scope 162mm focal length)for my 6” f6 newt is 4.77 arc sec per pixel which quite high
The image scale of my guide set up ( 60mm Scope 240mm focal length ) for my 8” f5 newt is 3.22 arc sec per pixel which is I assume is better suited for good calibration and better guiding
I found an interesting post in Cloudy Nights which basically sums ups PHD2 , EQMOD , guide speed settings and their affect on guiding with Ascom mounts
“Guide rate is the fraction of the sidereal rate that is used for guiding moves. For DEC, this is simple. If your guide rate is 0.5x, then the guide pulses will be at half of the sidereal rate. A rate of 0.9x would guide at 90% of the sidereal rate. For RA it is a bit more complicated since the RA axis is already moving at sidereal speed. Here, if your guide rate is 0.5x, it means that the RA guide rate will change by half of the sidereal rate. So a pulse to the west will move the mount at 1.5x the sidereal rate, and a pulse east will move it at 0.5x the sidereal rate. Similarly, if your guide rate is 0.9x, a west pulse would be at 1.9x sidereal, and a pulse east would be at 0.1x sidereal.
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There is no relationship between guide rate and guiding aggressiveness. The aggressiveness setting determines the amount of correction to send to the mount. The guide rate lets PHD2 translate that amount of correction into a pulse length. Guide rate will not affect the amount of correction needed, only the amount of time the correction takes. For example, if PHD2 determines that a 0.5 arc-second move is needed to the north, it calculates the pulse length needed based on the guide rate. If you are guiding at 0.5x, that translates into 7.5 arc-seconds per second. To make a 0.5 arc-second correction, PHD2 would need to send a pulse that is 7.5/0.5 = 67ms in duration. If your guide rate is 0.9x, PHD2 would calculate a pulse length of 37ms*to make the same correction.
If you're using ASCOM guiding (if you're not, you should be for this very reason) don't worry to much about guide rate. You typically can set it as high as you want up to the point of inducing mechanical bounce. When people ask me about this, I typically tell them to start at 0.5x and work your was up until you notice a degradation in guiding, or until you reach your mounts guide rate range limit - whichever comes first. If you don't want to do that, just start at 0.5x and leave it there. You are not likely to see mechanical bounce at that rate.
*
Finally, when using ASCOM guiding, the guide rate should have little to no effect on guiding performance provided you aren't seeing bounce. Even long pulses will almost always be completed before the next guide image is taken since even long pulses are still only a fraction of a second long. As a result, the correction is almost always in place before the next guide image is taken. So, no harm, no foul regarding guiding performance. However, a higher guide rate can produce better images even when the guiding is unchanged. That's because the corrections happen faster so the imaging sensor is collecting less time with the tracking deviated. However, this effect will likely be tiny and not noticeable. If you are having guiding problems, there are at least half a dozen other genuine gremlins you should be chasing before you even consider changing the guide rate”
Sort of got my head around the guide speed settings issue now, so in the end not super critical for guiding performance, there are many other factors more important relating to good or bad guiding
Imaged M8 last night in all the damn wind and just before running PHD2 I did something I’ve never done before when everything’s connected, I checked the guiding tab in the brain, opened up the Calibration Step Calculator and found that the guide speed had changed from 1.00 (which I previously manually set inside the house without the gear connected ) to 0.5 ( which is my EQMOD setting )
Also the Calibration declination degrees was automatically calculated with a value
The penny dropped , after nearly 2 years running PHD2 I’ve never set all the parameters with the mount and camera connected to the laptop , I’ve just set them inside on my desk
So therefore PHD2 has calculated the guide speed and therefore the Calibration data from my EQMOD setting via Ascom when both camera and mount are connected
Stupid me !!
Anyway another query solved
PHD2 and my HEQ5 were doing a superb job last night in all that wind ( 25 to 30 km/hr )
Guiding was around 1.60 to 1.90 arc sec error with dithering ,perfect round stars to edge of field
Out of 70 subs , only discarded 5 and 2 were satellite streaks
If you are using pulse guiding with an EQdirect cable instead of ST4, PHD2 will pull the guide rate out of EQMod. If you change the guide rate in EQMod then PHD2 will ask you to either change it back or recalibrate.
I always run at 0.9X rate in EQMod, I did not see any particular difference when running it lower, In RA it is not important as per info above, a guide east pulse will run the mount at 10% of siderial rate, so you don't really move the scope east, you just slow it down and wait for the sky to catch up, tat means it never reverses. Unless you have very little dec backlash I have always considered the highest guide rate to be best so that backlash pulses if the direction reverses are shorter. If you have a 1 second backlash pulse at 0.9 rate then at 0.5 it will be nearly 1.8 seconds just to take up the backlash. I am not sure if PHD2 deals with that by waiting for another exposure before moving again or not.
