SVBony 50mm guidescope focal length

[powerlord]

One fyi for others really.

I've been using this for 6 months or so, and took SVBony at their word, setting 240mm as the FL in asiair for guiding. tbh, it's worked fine, with a 0.6" RMS on my EQ6-r pro most of the time.

Last night though I thought I'd let asiair plate solve it.. and it says it's 175mm!

The info itself from svbony is not exactly clear:

https://www.amazon.co.uk/Svbony-Helical-Focuser-Astronomy-Telescope/dp/B07N1BNS4C

50mm..F4.. but 240mm FL.

Anyway, my assumption is that a plate solve cannot lie, so I set it to 175mm for guiding.

Whether that helped or it was just good seeing, my RMS last night was often 0.3" for many subs.

Possibly one for @vlaiv to explain ?

 

[vlaiv]

I'd say that is normal.

First - drop in measured RMS. By entering correct focal length - you changed unit of measure. Different unit of measure gives different numerical value.

I weigh 120 in Kg, but if I measure myself in lbs - that is 264.5. Does that mean that I'm suddenly heavier? No - I weigh the same, it just different units.

Guiding at 0.3" RMS, does that mean you have excellent mount? I guess not. At 175mm of guider FL - odds are that you have too coarse measuring device to precisely measure RMS value. Say you are using camera with 3.75µm pixel size to guide (I see both ASI244 and ASI120 in your signature).

That gives guide resolution of about 4.4". Centroid precision is 1/16 to 1/20 of a single pixel - so you can precisely measure star position in range 4.4" / 16 = 0.275 to 4.4" / 20 = 0.22"

Let's take value of 0.25" to be your guide star position precision measurement.

RMS measure is measure of how different / dispersed measured values are and say your real RMS is 0.6" (how much values "oscillate" around true value). However, you are trying to measure it with something that has precision of almost half of that. You simply can't precisely measure and express this value correctly.

Imagine that you have half meter stick and you are trying to measure mean height of a group of people and how much it varies over that group. Your measurements will be 1m, 1.5m 2m - not much variation in that, right?

 

[powerlord]

fair enough, understood - thanks. Yeh I was using asi120 last night.

though wouldn't I expect the guiding to jump around up and down with each measure as per your half meter stick example ? as that's not what I see in the phd2 graphs ?

stu

[vlaiv]

3 hours ago, powerlord said:

though wouldn't I expect the guiding to jump around up and down with each measure as per your half meter stick example ? as that's not what I see in the phd2 graphs ?

It does not actually mean it will be rounded to nearest value - it was just example to show that precision will have impact on final result.

Actual corrections will depend on Min mo parameter as well. Graph can look rather "calm" when you have low guiding resolution and high Min mo. Min mo parameter should be set at low value - say 0.1 at most.

Min mo parameter is only one related to pixel size and not to arc seconds. If you have it set to say 0.2 - that means PHD2 will make correction only if detected deviation is larger than 0.88" (which is 4.4"/px * 0.2px = 0.88").

 

 

[vlaiv]

@powerlord

I did some more research on the topic we discussed here, and I need to stand a bit corrected.

Several times I maintained that lower precision in star centroid will lead to (false) lower RMS - but that seems not to be true. I ran some simulations and lower precision actually increases RMS in star position.

Why does lower guide precision then tend to report smaller RMS then? I found answer in PHD user guide:

Quote

These values show the root-mean-square (RMS or standard deviation) of the motions in each axis along with the total for both axes. These are your best estimators of
guiding performance because they can be directly compared to star sizes and seeing conditions

It appears that RMS represents not error in star position but rather RMS of issued corrections (motion of mount in each axis).

In that case - lower RMS is directly related to Min Mo parameter. This one is, like I mentioned earlier, given in pixels rather than arc seconds. This means that same default value will have different meaning on guide scope with small FL and for example OAG with large focal length (or in another words - it's related to guide resolution).

0.2 of pixel at two different resolutions means that corrections won't be issued the same - with coarse guide resolution they will be issued much less than with fine guide resolution (where relative pixel size is larger).