Guiding calibration, where to do it?

[LaurenceT]

After 2 years of avoiding guiding I'm finally starting to get to grips with it. I've read here and elsewhere that calibration is best at the target or close to the zenith yet my Asiair tells me that I'm "too far from the celestial equator".

My probably flawed and simplistic understanding of the celestial equator is that it is low in the sky.

I'd appreciate some help understanding this.

Edited September 20 by LaurenceT

[Tomatobro]

 

The RA movement rate is maximised at the celestial equator. If you calibrate close to the pole you get less movement of the RA axis so the calibration will be less precise

[AstroMuni]

Here is the guidance from PHD2 website....

Because this is a measurement process that is subject to various kinds of mount and atmospheric effects, the most accurate results will be gotten when the scope is pointing within 20 degrees of Dec = 0 (near the celestial equator) and at least 60 degrees above the nearest east/west horizon (i.e. within 2 hours of the celestial meridian).  Calibrations can be done in other pointing positions if required by conditions at your site but they will be subject to somewhat more measurement uncertainty.  You cannot do calibrations pointing very close to the north or south celestial poles, and the mount must be tracking at the sidereal rate.

[michael8554]

Hi Laurence.

If it's ST-4 guiding, you should Cal on target.

If it's ASCOM Pulse Guiding, best to Cal near Dec = 0 and near south.

Michael

[woldsman]

You'll wish you had tried it earlier - PHD2 is amazingly good and the learning curve not that steep. For pulse guiding, first look at where Stellarium says the ecliptic intersects with the meridian. You may have to turn on these lines in Stellarium. Second check whether your target has crossed the Meridian. Instruct the mount to go to a star in the vicinity of the intersection on the same side as your target.  You may have to redo calibration later in the evening if the mount performs a Meridian flip (there is a setting in PHD2 to adjust for the flip but in practice I've found it better to redo calibration). 

[michael8554]

With the most recent dev releases of PHD2 there is a Calibration Assistant that will point the mount to the correct position for you.

And take up any Dec Backlash before starting the Dec part of the Cal.

Michael

[Vroobel]

59 minutes ago, Tomatobro said:

 

The RA movement rate is maximised at the celestial equator. If you calibrate close to the pole you get less movement of the RA axis so the calibration will be less precise

The RA movement is the same across whole sky - in angular meaning. The important for us thing is that the linear movement is biggest at the celestial equator. Simply saying, there is no movement (other than rotation) at the NCP/SCP. 

Edited September 20 by Vroobel

[LaurenceT]

Thanks for the replies, the clearest way I can understand this is that for a given period of time a guide star will move a greater distance the closer it is to the celestial equator than it would do if it were closer to the NCP (or SCP) and that greater distance means greater guiding accuracy.

As far as using PHD2 goes, I shall have to continue using the cut down version used by the Asiair as my astro novice but elderly brain finds using the Asiair results in less brain ache!

 

[Elp]

I might be doing this wrong but since I started pointing the scope near South and an altitude of around 30-40 degrees and calibrating there (which I believe is near the celestial equator) guiding has generally been repeatable session to session. Before I used to calibrate near target (which is wrong, you'll get variation depending on where in the sky the target is, near zenith is also completely wrong).

[LaurenceT]

32 minutes ago, Elp said:

I might be doing this wrong but since I started pointing the scope near South and an altitude of around 30-40 degrees and calibrating there (which I believe is near the celestial equator) guiding has generally been repeatable session to session. Before I used to calibrate near target (which is wrong, you'll get variation depending on where in the sky the target is, near zenith is also completely wrong).

There are two things I don't quite understand about that, why would pointing the scope near the south matter as long as the declination is low enough to be closer to the celestial equator and also why would it matter pointing at the zenith as long as the observer is at a sufficient distance from the north pole?

[Elp]

Scope pointing south because the meridian is a line which runs from north, directly overhead at zenith to south, and the celestial equator that you can see above the horizon at the maximum altitude (northern hemisphere) is in the south, so its the prime point whereby calibration will see the most star movement, in order to get the best calibration this is where it's best to do it.

Edited September 20 by Elp

[LaurenceT]

58 minutes ago, Elp said:

Scope pointing south because the meridian is a line which runs from north, directly overhead at zenith to south, and the celestial equator that you can see above the horizon at the maximum altitude (northern hemisphere) is in the south, so its the prime point whereby calibration will see the most star movement, in order to get the best calibration this is where it's best to do it.

Thanks, that makes sense to me.

[imakebeer]

@LaurenceT

(Haven't forgotten BTW you were kind enough to let me come and check out your gear a couple of years back before I took the plunge and came over to the dark side👍🙏)

I can't tell you what you should do, but I can tell you I do (and it seems to work!), having got an ASIAIR Mini a few weeks ago (why oh why didn't I do this at the outset! 😫)

I can't quite see Polaris due to trees so I do an All Sky Polar Alignment - right now I use Altair which is lowish over my house.

(At the start I tried using (I think) Vega - I can still hear the gears grinding as the scope was driven into the mount! 😱 At least now I understand the warning the app was giving me beforehand! Altair seems a more reliable starting point for now).

I make sure to get the alignment spot on, error no bigger than 0°05'00" - the app rewards you with a smiley face for this 🙂

Then I point to my target - lately I've done The Veil and Crescent Nebulas, both in Cygnus. Go into guiding in the app, start looping then start guiding - magically it calibrates itself and shortly after starts guiding. First time round I just followed the prompts in the app, job jobbed, simple as - I was like "wait, that's it? I'm guiding now? Well that was a breeze!" 😊

Thus far I'm routinely getting sharp stars at 2 & 3 min exposures (I've done a single 5 min image which was sharp but haven't yet dared commit to a full run at 5 mins)

Only issues I've had:

- Guiding went wonky after meridian flip - solved by enabling "recalibrate after meridian flip" option 

- Guiding went wonky after I installed a new guide camera - solved by deleting the old calibration and thereby forcing a recalibration (all done in a couple of taps in the app).

Hope you can get it sorted, it seems to me it should be really easy with an ASIAIR and the app 🤞🔭

[LaurenceT]

39 minutes ago, imakebeer said:

@LaurenceT

(Haven't forgotten BTW you were kind enough to let me come and check out your gear a couple of years back before I took the plunge and came over to the dark side👍🙏)

I can't tell you what you should do, but I can tell you I do (and it seems to work!), having got an ASIAIR Mini a few weeks ago (why oh why didn't I do this at the outset! 😫)

I can't quite see Polaris due to trees so I do an All Sky Polar Alignment - right now I use Altair which is lowish over my house.

(At the start I tried using (I think) Vega - I can still hear the gears grinding as the scope was driven into the mount! 😱 At least now I understand the warning the app was giving me beforehand! Altair seems a more reliable starting point for now).

I make sure to get the alignment spot on, error no bigger than 0°05'00" - the app rewards you with a smiley face for this 🙂

Then I point to my target - lately I've done The Veil and Crescent Nebulas, both in Cygnus. Go into guiding in the app, start looping then start guiding - magically it calibrates itself and shortly after starts guiding. First time round I just followed the prompts in the app, job jobbed, simple as - I was like "wait, that's it? I'm guiding now? Well that was a breeze!" 😊

Thus far I'm routinely getting sharp stars at 2 & 3 min exposures (I've done a single 5 min image which was sharp but haven't yet dared commit to a full run at 5 mins)

Only issues I've had:

- Guiding went wonky after meridian flip - solved by enabling "recalibrate after meridian flip" option 

- Guiding went wonky after I installed a new guide camera - solved by deleting the old calibration and thereby forcing a recalibration (all done in a couple of taps in the app).

Hope you can get it sorted, it seems to me it should be really easy with an ASIAIR and the app 🤞🔭

Of course I remember you, I enjoyed our chat.

Glad you're getting on well with everything. The reason I originally asked the question was the error message I was getting from the Asiair but I'm sorting of having to ignore that for the time being as it's very difficult to get close enough to the celestial equator from my garden because of the large numbers of tall bushes and trees surrounding us.

Last night I began guiding in earnest but in my excitement forgot to change the default Asiair settings to those generally recommended for my size guide scope. Amazingly I was getting sub 1" but after 5 or 6 subs the clouds interfered. I was using my Askar FMA180 scope.

This evening I've switched to my WO ZS61 scope, I had more problems finding a suitable guide star because the seeing is so bad and I had difficulty in finding a star that moved sufficiently. In the end, despite the Asiair error message I calibrated OK, changed the calibration settings and started imaging the Western Veil using 4 minute subs.

The guiding varied quite widely, from about 1.3" to 1.8" but mostly around 1.4". I've managed to get 24 subs before the clouds have rolled in so I'm going to get some flats and call it a night.

[imakebeer]

See above - ultimately I'd have thought what's important is are the stars sharp?

(Also reading the rest of that thread it seems there are other factors can affect the guiding graph, e.g. guide scope focal length)

[LaurenceT]

31 minutes ago, imakebeer said:

See above - ultimately I'd have thought what's important is are the stars sharp?

(Also reading the rest of that thread it seems there are other factors can affect the guiding graph, e.g. guide scope focal length)

That was an interesting thread, it raised a smile as well! There's one website that I've had a look at that seemed very helpful, I think I'll go back and re-visit it

 East Wind Astrophotography: February 2021

I'm going to have to decide whether it's actually worth the bother of me guiding, I can get 2 minutes unguided with my WO ZS61ii and more with the dinky Askar FMA180. Maybe the aggro of dealing with the greenery around  me is simply not worth it.

I'm lucky with my mount, it's the cheapest EQ goto mount available, I bought it used from someone who seemed to know what they were doing with adjusting a somewhat wayward dec axis, I've no further aspirations at my age for bigger mounts and longer scopes!

I'll probably submit my Asiair PHD2 stats from last night for help with analysing them.

[Vroobel]

I don't know the WO ZS61II, but I'm pretty sure that the Askar FMA180 gives you undersampled images, so you may find e.g. blocky stars. it depends on your camera(s) of course. You can run dithering every few frames which moves the images randomly a little bit. If you tack the data with drizzling, e.g. x2, the stars become round, your nebulas and galaxies get details and you can process them with better results. After that, you can resize (resample) them to reach the optimal size of the image. Unfortunately, the guiding is needed for that.

 

[TiffsAndAstro]

I think blind dithering is possible without guiding in Nina.

Never tried it

[OpusElite]

Under the Tools menu in the full version of PHD2 there is a Calibration Assistant which has a 'Slew' button at the bottom to take the mount to the Celestial equator. I'm not sure if it's on the Asiair version of PHD though.

[LaurenceT]

7 minutes ago, OpusElite said:

Under the Tools menu in the full version of PHD2 there is a Calibration Assistant which has a 'Slew' button at the bottom to take the mount to the Celestial equator. I'm not sure if it's on the Asiair version of PHD though.

Thanks for that, however it doesn't appear as an option in the Asiair which is probably yet another reason to try full PHD2/NINA. Also if I slewed to the celestial equator in my garden I'd just be seeing trees although I could use that as a starting point to elevate the scope.

[LaurenceT]

On 21/09/2024 at 13:11, Vroobel said:

I don't know the WO ZS61II, but I'm pretty sure that the Askar FMA180 gives you undersampled images, so you may find e.g. blocky stars. it depends on your camera(s) of course. You can run dithering every few frames which moves the images randomly a little bit. If you tack the data with drizzling, e.g. x2, the stars become round, your nebulas and galaxies get details and you can process them with better results. After that, you can resize (resample) them to reach the optimal size of the image. Unfortunately, the guiding is needed for that.

 

I've just processed this in Siril, it's the Akar FMA180 taken using the asi533mc Pro. From what I have seen using Astronomy Tools it is indeed undersampled, do these stars seem "blocky" as you mentioned as a possibility? I can see some (one?) but not many.

 

Edited Monday at 11:53 by LaurenceT

[Vroobel]

I didn't want to say that the stars are literally square. 🙂 Try to stack the data using the 2x drizzling if you can and check the stars then - even the tiniest ones should be round and nice. Be aware that the result file is 4x bigger. 

[LaurenceT]

1 minute ago, Vroobel said:

I didn't want to say that the stars are literally square. 🙂 Try to stack the data using the 2x drizzling if you can and check the stars then - even the tiniest ones should be round and nice. Be aware that the result file is 4x bigger. 

I'll do that later, thanks. I do think that the stars look a bit "weird" though, I used Starnet, I'm not sure I like the result!