Guide camera recommendations for my set up

[Stuart1971]

Hello,

I image now with a Tak FSQ85 @ 450mm f5.3 with a QHY268c which has 3.6 micron pixels, so I am imaging at 1.72”/pixel

I use a Tak FS60cb for guiding @ 355mm f5.9 and currently use lodestar X2 with 8.3 x 8.6 micron pixels which guides at 4.76”/pixel, and I am finding this not to work very well and guiding is at best 0.7 - 1.0 RMS, I can’t help but think that smaller pixels would help….

So I am thinking of getting a ASI120 mini or similar with 3.75 micron pixels, which would give me 2.18”/pixel…

thought on what would work well for my set up….?

Thanks for looking….

 

[Skipper Billy]

This might help you.....

http://astronomy.tools/calculators/guidescope_suitability

[woldsman]

The calculator is good. Other considerations though are a) how much you wish to spend & b) how you want to polar align as the focal length of the guidescope has a bearing on this. 

[Stuart1971]

1 hour ago, woldsman said:

The calculator is good. Other considerations though are a) how much you wish to spend & b) how you want to polar align as the focal length of the guidescope has a bearing on this. 

Cost and All that aside, also i use a polemaster anyway for PA…

Edited February 16, 2022 by Stuart1971

[Stuart1971]

2 hours ago, Skipper Billy said:

This might help you.....

http://astronomy.tools/calculators/guidescope_suitability

Well not really as I already have the figures, but wanted peoples thoughts on good imaging v guiding pixel scales…based on the kit I listed above…

ie, what is a good guiding image scale, v my actual imaging image scale…?/

That calculator tells me nothing about that….👍🏼

Edited February 16, 2022 by Stuart1971

[Skipper Billy]

14 hours ago, Stuart1971 said:

Well not really as I already have the figures, but wanted peoples thoughts on good imaging v guiding pixel scales…based on the kit I listed above…

ie, what is a good guiding image scale, v my actual imaging image scale…?/

That calculator tells me nothing about that….

I am pretty sure that I read somewhere that the ratio between imaging and guiding needs to be in the range of 1:1 to 1:4 and your current setup is well within that range.

I have used various setups that gave less than 1:1 and as high a 1:5 and to be honest it didn't make much difference to the guiding accuracy - on a night of decent seeing it always hovered ~ 0.4 - 0.5" pixels Total RMS error.

If you are getting total RMS error of 0.7 to 1.0 it is well under your imaging scale so in theory improving your guiding wont improve your images.

 

[michael8554]

Your image scales aren't unusual, so I would look elsewhere for the cause of your guiding problems.

Is it RA or Dec that is suffering ?

Post a GuideLog that includes a Cal and a Guide Assistant run.

Michael

 

[Stuart1971]

3 hours ago, michael8554 said:

Your image scales aren't unusual, so I would look elsewhere for the cause of your guiding problems.

Is it RA or Dec that is suffering ?

Post a GuideLog that includes a Cal and a Guide Assistant run.

Michael

 

I guide in DEC with south moves only as with my EQ8 there is some backlash and this solves that issue, but my RA is not great, one thing I have yet to try is changing the guide rate from 0.9x (which was recommended on NINA discord) to 0.5x, as the issue seems to be over correcting all the time, and the only way I have dealt with it in the past is to set aggressiveness right down to 15, that seemed to help…

It’s just the pixel scale with the Lodestar means that any guide star is all on one pixel, which makes centriod calculation for PHD2 a lot harder and hence guiding is not as good…

‘Now I could double the focal length of my guidescope with a 2x extender which would then half the pixel scale….🤔 or another option, which would be cheaper, is to get a different guide cam with much smaller pixels…🤔🤔

Edited February 17, 2022 by Stuart1971

[Skipper Billy]

This is sounding more like a backlash issue and/or PHD2 settings rather than an equipment issue.

As Michael said - lets see a Guidelog then we can see some facts rather than guessing.

[Stuart1971]

37 minutes ago, Skipper Billy said:

This is sounding more like a backlash issue and/or PHD2 settings rather than an equipment issue.

As Michael said - lets see a Guidelog then we can see some facts rather than guessing.

Backlash on RA….🤔🤔 no that’s not an issue…

[Skipper Billy]

1 hour ago, Stuart1971 said:

Backlash on RA….🤔🤔 no that’s not an issue…

Ok - I dont seem able to provide anything useful - Skipper Billy out.....

[Stuart1971]

18 minutes ago, Skipper Billy said:

Ok - I dont seem able to provide anything useful - Skipper Billy out.....

Backlash in RA is moot, as it’s always traveling in the same direction, same in DEC when guiding in one direction opposite to drift….👍🏼

[michael8554]

16 hours ago, Stuart1971 said:

any guide star is all on one pixel, which makes centriod calculation for PHD2 a lot harder

Good point.

15 hours ago, Stuart1971 said:

I could double the focal length of my guidescope with a 2x extender which would then half the pixel scale

That would make your guidescope f/5.9 x 2 = f/11.8 !!

I know the LodeStar is sensitive, but that's pushing it.

An OAG on the FSQ85 would have the same problem, so ASI 120MM mini it is.

Michael

[vlaiv]

+1 on mechanical issues - change of guide setup won't help much.

[Stuart1971]

20 minutes ago, vlaiv said:

+1 on mechanical issues - change of guide setup won't help much.

Can you explain on what mechanical issues, the mount is in excellent working order, with albeit backlash on the DEC axis, which doesn’t seem to be an issue with guiding in just one direction, and I was using 0.9x guide speeds, which apparently is too high, so am changing to 0.5x to try….

Also you think my guide set up with 8.6 micron pixels could not be made better…..🤔🤔

So what other mechanical issues am I missing here….??

Edited February 18, 2022 by Stuart1971

[vlaiv]

6 minutes ago, Stuart1971 said:

Can you explain on what mechanical issues, the mount is in excellent working order, with albeit backlash on the DEC axis, which doesn’t seem to be an issue with guiding in just one direction, and I was using 0.9x guide speeds, which apparently is too high, so am changing to 0.5x to try….

Also you think my guide set up with 8.6 micron pixels could not be made better…..🤔🤔

So what other mechanical issues am I missing here….??

System seems to be quite capable of measuring motions down to 0.02-0.04px which is less than 0.2".

How can it then be responsible for 1" RMS guide performance?

Maybe post a guide log so any actual issues can be seen?

[Stuart1971]

4 minutes ago, vlaiv said:

System seems to be quite capable of measuring motions down to 0.02-0.04px which is less than 0.2".

How can it then be responsible for 1" RMS guide performance?

Maybe post a guide log so any actual issues can be seen?

Yes I get that, and I will post a log as soon as I can get out

and my biggest issue it RA correction going back and fourth, this is with 0.9x guide speed and have now been told by a few EQ8 users that this is way to high and too aggressive, hence going to try 0.5x, to try and stop the oscillation….

‘I did adjust the aggressiveness right down to 15, and with this I saw figures of 0.65 RMS…

[vlaiv]

2 minutes ago, Stuart1971 said:

Yes I get that, and I will post a log as soon as I can get out

and my biggest issue it RA correction going back and fourth, this is with 0.9x guide speed and have now been told by a few EQ8 users that this is way to high and too aggressive, hence going to try 0.5x, to try and stop the oscillation….

‘I did adjust the aggressiveness right down to 15, and with this I saw figures of 0.65 RMS…

There is another problem with having high guide speed - that is minimum pulse duration as parameter. EQMod sets this at 50ms or something like that.

It is some archaic setting from the time when computers had issues of measuring time to higher precision than that so it was set at 50ms to avoid imprecision of measurement. What it means is that how ever small pulse needs to be issued - it will be overridden to 50ms.

If you have guide speed set to x0.9 sidereal - that is about 15"/s. 50ms is 1/20 of a second so minimum mount correction can be 15 / 20 = 0.75".

My recommendation to everyone using EQmod is to lower guide speed significantly - to x0.25 of sidereal or change those 50ms to lower setting - like 20ms or less.

 

[Stuart1971]

5 minutes ago, vlaiv said:

There is another problem with having high guide speed - that is minimum pulse duration as parameter. EQMod sets this at 50ms or something like that.

It is some archaic setting from the time when computers had issues of measuring time to higher precision than that so it was set at 50ms to avoid imprecision of measurement. What it means is that how ever small pulse needs to be issued - it will be overridden to 50ms.

If you have guide speed set to x0.9 sidereal - that is about 15"/s. 50ms is 1/20 of a second so minimum mount correction can be 15 / 20 = 0.75".

My recommendation to everyone using EQmod is to lower guide speed significantly - to x0.25 of sidereal or change those 50ms to lower setting - like 20ms or less.

 

So the fact that I moved the aggressiveness setting down to 15 from the default of 70, this would have only sent 15% of the computed pulse and so compensated a bit, because my guide rate and minimum pulse is too high ..?

[Stuart1971]

11 minutes ago, vlaiv said:

There is another problem with having high guide speed - that is minimum pulse duration as parameter. EQMod sets this at 50ms or something like that.

It is some archaic setting from the time when computers had issues of measuring time to higher precision than that so it was set at 50ms to avoid imprecision of measurement. What it means is that how ever small pulse needs to be issued - it will be overridden to 50ms.

If you have guide speed set to x0.9 sidereal - that is about 15"/s. 50ms is 1/20 of a second so minimum mount correction can be 15 / 20 = 0.75".

My recommendation to everyone using EQmod is to lower guide speed significantly - to x0.25 of sidereal or change those 50ms to lower setting - like 20ms or less.

 

You say..

 

If you have guide speed set to x0.9 sidereal - that is about 15"/s. 50ms is 1/20 of a second so minimum mount correction can be 15 / 20 = 0.75".

so are you saying that 0.75” is too much for a minimum, and why I get overshoot and oscillation on the RA..??

and to “either” lower the guide rate or reduce the minimum pulse, but not both…??

[vlaiv]

35 minutes ago, Stuart1971 said:

So the fact that I moved the aggressiveness setting down to 15 from the default of 70, this would have only sent 15% of the computed pulse and so compensated a bit, because my guide rate and minimum pulse is too high ..?

Yes, aggressiveness is percentage of calculated pulse used.

Lower aggressiveness - shorter pulse duration is used than needed.

31 minutes ago, Stuart1971 said:

You say..

 

If you have guide speed set to x0.9 sidereal - that is about 15"/s. 50ms is 1/20 of a second so minimum mount correction can be 15 / 20 = 0.75".

so are you saying that 0.75” is too much for a minimum, and why I get overshoot and oscillation on the RA..??

and to “either” lower the guide rate or reduce the minimum pulse, but not both…??

I would lower the both.

Set min pulse to 20ms and use 0.25.

There is nothing wrong with using slower guide rate. People are afraid that correction will not be made sufficiently fast - but we are talking milliseconds here and guide cycles of 3-4s.

For example - something weird happens and you need to correct for 1" of error at once. How long will it take?

At x0.25 guide speed - that is quarter of sidereal so about 3.75"/s - correction will last for about 300ms - or 1/3 of a second. That is still pretty fast, even for large correction.

Lowering guide speed also helps with stabilizing the mount. Sudden large change in speed introduces jerk (change in acceleration) and that this impacts things if your mount is not rigid (it may feel rigid under hand but we are talking micro things here in gear train) - which leads to oscillations if there is sudden jerk on system.

Think of it in terms of driving car. Is it better to slowly break or slam on the breaks. If you slam on the breaks everyone in the car gets tugged forward. And if you slam on the accelerator peddle - everyone gets pulled in their seats.

Same thing happens with the scope if you there is drastic increase in speed.

You need to match three things in your guiding setup:

- guide speed

- min pulse duration

and

- min mo parameter

Set guide speed to as low value as possible for smooth corrections without introducing too much vibration in the mount - x0.25 is good value (but not so low that it takes seconds to recover from serious error)

Lower min pulse duration to say 20ms.

From the two - calculate what is minimal correction that your mount  will make.

For example with x0.25 speed is 3.75"/s and for 20ms (1/50th of a second) that will give movement of 0.075" - that is very fine movement of the mount.

Now you have to adjust min mo parameter to match this value. Min mo is minimum movement in camera pixels. In another words - if there is need for correction that is larger than "so and so pixels" correction will be issued - otherwise, nothing will happen.

Guide assistant advised you that high frequency motion is around 0.04px - so I would set min mo to a bit higher than that - say 0.08. This will ignore 95% (two standard deviations) of high frequency motion due to seeing and will still issue corrections for errors larger than 0.4"  (4.76"/px * 0.08 = 0.38").

 

[Stuart1971]

2 hours ago, vlaiv said:

Yes, aggressiveness is percentage of calculated pulse used.

Lower aggressiveness - shorter pulse duration is used than needed.

I would lower the both.

Set min pulse to 20ms and use 0.25.

There is nothing wrong with using slower guide rate. People are afraid that correction will not be made sufficiently fast - but we are talking milliseconds here and guide cycles of 3-4s.

For example - something weird happens and you need to correct for 1" of error at once. How long will it take?

At x0.25 guide speed - that is quarter of sidereal so about 3.75"/s - correction will last for about 300ms - or 1/3 of a second. That is still pretty fast, even for large correction.

Lowering guide speed also helps with stabilizing the mount. Sudden large change in speed introduces jerk (change in acceleration) and that this impacts things if your mount is not rigid (it may feel rigid under hand but we are talking micro things here in gear train) - which leads to oscillations if there is sudden jerk on system.

Think of it in terms of driving car. Is it better to slowly break or slam on the breaks. If you slam on the breaks everyone in the car gets tugged forward. And if you slam on the accelerator peddle - everyone gets pulled in their seats.

Same thing happens with the scope if you there is drastic increase in speed.

You need to match three things in your guiding setup:

- guide speed

- min pulse duration

and

- min mo parameter

Set guide speed to as low value as possible for smooth corrections without introducing too much vibration in the mount - x0.25 is good value (but not so low that it takes seconds to recover from serious error)

Lower min pulse duration to say 20ms.

From the two - calculate what is minimal correction that your mount  will make.

For example with x0.25 speed is 3.75"/s and for 20ms (1/50th of a second) that will give movement of 0.075" - that is very fine movement of the mount.

Now you have to adjust min mo parameter to match this value. Min mo is minimum movement in camera pixels. In another words - if there is need for correction that is larger than "so and so pixels" correction will be issued - otherwise, nothing will happen.

Guide assistant advised you that high frequency motion is around 0.04px - so I would set min mo to a bit higher than that - say 0.08. This will ignore 95% (two standard deviations) of high frequency motion due to seeing and will still issue corrections for errors larger than 0.4"  (4.76"/px * 0.08 = 0.38").

 

Thanks for this, this is what I thought was going to be the issue, 0.9x is just far too fast for the guide rate on this mount…the guiding assistant recommended 0.1 for min move…I did try that but still had to really lower the AG setting…

I also read in the paperwork for EQMOD that a setting less than 50ms would not work properly…and not to bother adjusting, why does it say this, as what you say makes a lot of sense…🤔

[vlaiv]

1 minute ago, Stuart1971 said:

I also read in the paperwork for EQMOD that a setting less than 50ms would not work properly…and not to bother adjusting, why does it say this, as what you say makes a lot of sense…🤔

That part also confuses me in EQMOD as well. Why the limit and why default value of 50ms?

It literally says:

Quote

Minimum Pulse Width
The Minimum Pulse Width slider specifies the minimum length of time a correction will be applied and
overrides any request by the autoguider for a shorter duration. EQASCOM always imposes a minimum pulse
width of 50ms as it is not practical to accurately measure shorter periods.

Being computer programmer I know that we can measure time with much grater precision. In fact even very simple controllers can accurately measure micro second intervals let alone personal computers with multi gigahertz clocks that can measure down to nanoseconds.

[Stuart1971]

16 minutes ago, vlaiv said:

That part also confuses me in EQMOD as well. Why the limit and why default value of 50ms?

It literally says:

Being computer programmer I know that we can measure time with much grater precision. In fact even very simple controllers can accurately measure micro second intervals let alone personal computers with multi gigahertz clocks that can measure down to nanoseconds.

Ok, so one last question, 

would all this still be applicable for a guide camera with 3.75 micron pixels, and a guiding scale of 2.18”/pixel…🤔

[vlaiv]

5 minutes ago, Stuart1971 said:

Ok, so one last question, 

would all this still be applicable for a guide camera with 3.75 micron pixels, and a guiding scale of 2.18”/pixel…🤔

Still the same. It only impacts min mo parameters in sense that it changes its value as value is pixel related and not in arc second.

0.1 of 8.6um pixel is 0.86 microns and that would be something like 0.25 of 3.75um pixel size.