Direct mount guider question


#1

I was trying direct mount guider to do my dithers the other night on a camera lens setup I am working on. I only
got a few frames before the clouds came in and it did dither fine but it seemed to be only moving at a 45 degree angle (lower left to upper right) instead of randomly.

Is this:

  1. A feature, not a bug.
  2. A bug (should be random directions).
  3. Coincidence since I just did not have enough images before it got cloudy.
  4. Some setting I missed.

Thanks!


#2

The dither direction is controlled by PHD2. SGP only sends a “dither now” command.

Kind regards,
Horia


#3

No - PhD is not even running since there is no guiding but the dither is happening, just not randomly as would be optimal.

Unless I am mistaken dither by mount sends a command to the mount from SGP. Has to be the case since PhD is not running.

Perhaps the devs can clarify.


#4

AIUI Direct Mount Guiding (DMG) is a feature of TheSkyX which takes an offset value and applies it rapidly rather than the usual pulse guide command that makes a move for a time at a rate that’s no more than the sidereal rate. The DMG offset is applied much faster than the equivalent pulse guide.

AIUI the only way to access it is through the TSX ASCOM interface by issuing PulseGuide commands. The driver converts the time and rate information to the offset that Direct Mount Guiding uses.

One feature of DMG is that the correction is made in a fraction of the time that a pulse guide would take and this can confuse some applications.

A lot of speculation here but maybe it will help.


#5

There is a “Direct Mount Guider” in SGP that nudges the mount directly vs using an actual Guider. This was implemented for highly precise mounts that don’t use autoguiding.

I can’t recall what the movement is supposed to he like but I would assume it’s supposed to he random. I’ll double check this evening.

Thanks,
Jared


#6

Sorry, confused the TSX Direct Guide with the SGP Direct Mount Guide.


#7

Thanks Jared! It could have been that I just did not have enough samples and that the samples I did have were just coincidentally in a line, but it would be good to know.


#8

It should be a random direction for North/South and East/West. So you’ll always get a dither in NW/NE/SW/SE directions and pulse amount will always be the same. We could probably make this more random but it should be random as written.

Thanks,
Jared


#9

OK Thanks I probably just needed more images to see that.


#10

hi
I have 20 frames where I use dither and SGP direct mount guider. .

  • John

https://youtu.be/Z-LvMLFesFk


#11

Does anyone know if using the Direct Mount Guider will help me eradicate ‘walking noise’ when I’m not autoguiding ? I’m getting streaky noise (‘walking noise’ ?) on my stacked images which I’m told is because I’m not dithering. I have a ZWO ASI 1600 single-shot colour cooled camera and have alot of success taking 20 sec subs without autoguiding apart from this ‘walking noise’ problem. If the Direct Mount Guider simulates dithering then this might get rid of the noise without autoguiding. If this is worth doing, how long should I pause between subs to let the mount settle ? I have a Celestron AVX mount.


#12

I’m not sure about dither amount and settle time with that mount, but yes that should help to randomize the noise so it will stack out.

Thanks,
Jared


#13

Just FYI, here is why I was asking. I was running prototyping tests for this system when I asked the OP question.

Just finished the final system today. It will take both the 200 (shown) and a 135. The Sti guider system was included because I had it laying around unused but the system does not really need it and dither by mount is way faster.

I removed part of the lens grip and glued a section of belt on upside down for a positive control non-slip grip w/o putting side pressure on the lens - that worked very nicely.


#14

Just found this thread and I have a question about what would happen using Direct Mount Guider on a mount that only guides in RA? I have an AstroTrac and have the same walking noise problem using a DSLR with this mount. I have a GPUSB lying around, and I could connect directly to the AstroTrac using that. What would happen then if the mount only guides in RA? Would the Dec guide commands be ignored and only the RA commands implemented? Just wondering if this would be feasible? Any ideas?


#15

I would assume this to be the case. But not having tried I can’t give you an accurate answer. Probably worth a try.

Thanks,
Jared


#16

Since AstroTrac is connected with the ST4 cable I’d expect this to work – the DEC pins on the cable are not connected to anything and the software doesn’t know anything about this. However, SGP’s dither only moves the mount a fixed amount to one of four possible directions. These are the four diagonals between RA and DEC axes. In the usual case this is not much of a problem since the possibility of visiting the same position many times is rather low (it happens but not often). But when the mount can guide only in RA each dither will effectively move the mount one fixed “step” forward or backward on the RA axis so the same positions are reused many times which is hardly optimal. Of course it is better than no dither, but here I see a possibility for some improvement on the SGP side.


#17

Practically, you may not need it. Unless you have a perfectly polar aligned astrotrac, you will get drift on both axes and no two images will perfectly overlap. I thought the issue with deliberate dithering was to overcome perfectly aligned images (which as most mounts have more than a few arcseconds of PE, could only occur as a result of autoguiding)


#18

Thanks for the responses. Very helpful. As is, I went looking for my GPUSB and remembered that I sold it a few years back with a guiding setup. So unless I pick up another one on the used market (come up fairly frequently) I won’t know for sure, but I expect that the responses are right and it will work in RA.

@buzz I’m sure that imprecise polar alignment and PE do not eliminate walking noise. In fact I’m positive as I’ve had pretty bad walking noise when using a 135mm lens with the AstroTrac. The hot/warm pixels still form a pattern across the frame because the motion isn’t random, as it would be (in RA anyway) with dithering.


#19

I wonder why the integration rejection settings do not rid the walking noise. It doesn’t care if the dots are randomly placed or regularly placed, only that they are not concurrent…


#20

I think the integration settings eliminate the statistical outliers, but I’m assuming that the walking noise comes from variation in the pixels that fall within the acceptable statistical range but nonetheless vary in pixel brightness. It is only seen when you stretch the image and then the walking noise appears in the background.


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