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Guiding Advice


Scooot

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I had a go at testing my guiding last night with my new setup, which is an Asiair, Asi 120 mini guidescope on the original star adventurer.

I went through the plate solving polar alignment routine, which was quite straightforward and I managed to get it quite close according to the app. However when I looked through the star adventurer’s polar scope it was a long way off, Polaris was above 12 o’clock instead of just below 9 o’oclock. It seemed a big error for my polar scope to be out that much, but anyway I went on to test the guiding.

I haven’t used guiding before but I got it working ok. My right ascension was quite good but the graph of my dec showed an error and that it was 13.59”. My dec is turned off because the star adventurer only tracks the right ascension but even so 13.59 seems a lot for something that should be polar aligned. According to an Asi tutorial I should have still been able to see a plot of the declination even if it was off.

The graph is below. I’m going to have another go as soon as possible to see where I’ve gone wrong but if anyone has any ideas about this I’d love to know thanks.

3B60E3AC-C914-45C1-A159-FFF3ABDB785A.thumb.png.74ffb55305b9bfb85ccc5458749173b8.png

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There’s a similar issue on the Zwo asiair Facebook page which basically blames it on poor polar alignment, not sure if you’re a fb user, I’m not, I just borrow someone’s tablet to have a look now and again. The thing I found with the star adventurer is it is prone to losing pa after you’ve positioned the scope on target just from the movement. I upgraded the wedge and it helped keep pa.

that said I’ve had the Asiair now for 6 months and I’ve used it twice so I’m not an expert. it was bought for a portable setup to take to the local dark sites but lockdown put and end to that, and well I can’t seem to tear my self away from APT and the laptop at the mo  

 

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12 minutes ago, Andy R said:

There’s a similar issue on the Zwo asiair Facebook page which basically blames it on poor polar alignment, not sure if you’re a fb user, I’m not, I just borrow someone’s tablet to have a look now and again. The thing I found with the star adventurer is it is prone to losing pa after you’ve positioned the scope on target just from the movement. I upgraded the wedge and it helped keep pa.

that said I’ve had the Asiair now for 6 months and I’ve used it twice so I’m not an expert. it was bought for a portable setup to take to the local dark sites but lockdown put and end to that, and well I can’t seem to tear my self away from APT and the laptop at the mo  

 

Many thanks, I’m not an FB user either. I’ll try and be a bit more careful and lock everything down properly and see if it’s any better. 

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14 minutes ago, Andy R said:

Can I ask what tripod your using? I was using the cheapish plastic skywatcher mount and found nipping up Allen bolts at the top where the legs join the base helped too

I’m  using a berlebach report 833 at home which is quite sturdy. I also have a lighter gitzo which I use if travelling.

I’ll take a pic, but I’m out at the moment.

 

Edited by Scooot
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Do уou know what time is used to calculate total / RA / Dec error from?

I counted about 54 line segments above and your guide cycle is 3 seconds from the image. That gives about 160 seconds of graph and I'm going to assume that error figures are calculated from the graph - so again ~160s.

You have about 14" of error in DEC from those 160s - this equates to 0.0875"/s of drift.

This equates to 20' of polar alignment error.

To put it into perspective - you are aiming your RA axis about 2/3 of a full Moon away from celestial pole. On the other hand - it's not bad if you consider that placing RA axis right on Polaris will create about 44' of polar alignment error - or about twice of what you have.

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25 minutes ago, vlaiv said:

Do уou know what time is used to calculate total / RA / Dec error from?

I counted about 54 line segments above and your guide cycle is 3 seconds from the image. That gives about 160 seconds of graph and I'm going to assume that error figures are calculated from the graph - so again ~160s.

You have about 14" of error in DEC from those 160s - this equates to 0.0875"/s of drift.

This equates to 20' of polar alignment error.

To put it into perspective - you are aiming your RA axis about 2/3 of a full Moon away from celestial pole. On the other hand - it's not bad if you consider that placing RA axis right on Polaris will create about 44' of polar alignment error - or about twice of what you have.

No I don’t know what time is used Vlaiv. I know my time and location were correct in the app though.
 

After I’d polar aligned my supposed error with the polar align routine,which uses a plate solve method, was less than 1 minute on the Ra and about 2 minutes for the dec. All I’ve then done is rotate to Altair to start guiding to test it.  2/3 moon is a long way off. I wonder if something was loose.

You say the RA is out, the RA plot is ok, it’s the dec that’s out or is my understanding of this lacking :) 

Edited by Scooot
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50 minutes ago, Andy R said:

Can I ask what tripod your using? I was using the cheapish plastic skywatcher mount and found nipping up Allen bolts at the top where the legs join the base helped too

Here’s the pic, I’ve mounted the Asiair on the opposite side of the scope & I’m powering it with 8 aa batteries.

4E35F374-69B0-4F70-B913-06E585DBB400.thumb.jpeg.48955ec068604361df87bfc39992f5bb.jpeg

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22 minutes ago, Scooot said:

No I don’t know what time is used Vlaiv. I know my time and location were correct in the app though.

Sorry, I was not precise about what I meant.

Not local time, I'm sure your local time and location are correct. I meant - for how long have you been guided before DEC RMS error became 13.7". It did not start out that high - it grew over to that value. Either we need that time, or how many samples your guiding app uses to calculate RMS error. With PHD2 you can set number of samples to display / use in calculation.

image.png.cd2e9112f379cadbdd7e4a122f0f8541.png

Here I marked number of samples for guide graph and RMS error - in this case 100 samples were used. If your guide cycle is for example 3 seconds - that means that RMS error displayed is for last 300s of guiding. We can use this time and RMS error to calculate drift rate and hence polar alignment error - which I tried to do by counting "points" on the graph and using 3s exposure length.

27 minutes ago, Scooot said:

After I’d polar aligned my supposed error with the polar align routine,which uses a plate solve method, was less than 1 minute on the Ra and about 2 minutes for the dec. All I’ve then done is rotate to Altair to start guiding to test it.  2/3 moon is a long way off. 

If you have 1' minute in altitude and 2' minutes in azimuth (these are really errors in altitude and azimuth rather than RA and DEC) - combined error is ~2.24'.

This is very low polar alignment error - drift rate would be 0.6" per minute. Depending on your imaging resolution - it would take many minutes before frame would move single pixel. It would take something like 20-30 minutes before DEC RMS error would reach 13.7"

If above screen shot was taken after half an hour of guiding then everything is fine - your drift rate is low, but it did accumulate to almost 14" over half an hour.

30 minutes ago, Scooot said:

You say the RA is out, the RA plot is ok, it’s the dec that’s out or is my understanding of this lacking :) 

Yes - I'm not talking about RA tracking error here - I'm talking about RA axis pointing direction. RA counters Earth's rotation and hence needs to be parallel to axis of rotation of Earth - and point to celestial pole.

When I say that RA is out - I meant not properly aligned with axis of rotation of the Earth. Nothing to do with RA tracking error (which is difference between the speed at which Earth is rotating and the speed at which the mount is tracking).

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33 minutes ago, vlaiv said:

Sorry, I was not precise about what I meant.

Not local time, I'm sure your local time and location are correct. I meant - for how long have you been guided before DEC RMS error became 13.7". It did not start out that high - it grew over to that value. Either we need that time, or how many samples your guiding app uses to calculate RMS error. With PHD2 you can set number of samples to display / use in calculation.

image.png.cd2e9112f379cadbdd7e4a122f0f8541.png

Here I marked number of samples for guide graph and RMS error - in this case 100 samples were used. If your guide cycle is for example 3 seconds - that means that RMS error displayed is for last 300s of guiding. We can use this time and RMS error to calculate drift rate and hence polar alignment error - which I tried to do by counting "points" on the graph and using 3s exposure length.

If you have 1' minute in altitude and 2' minutes in azimuth (these are really errors in altitude and azimuth rather than RA and DEC) - combined error is ~2.24'.

This is very low polar alignment error - drift rate would be 0.6" per minute. Depending on your imaging resolution - it would take many minutes before frame would move single pixel. It would take something like 20-30 minutes before DEC RMS error would reach 13.7"

If above screen shot was taken after half an hour of guiding then everything is fine - your drift rate is low, but it did accumulate to almost 14" over half an hour.

Yes - I'm not talking about RA tracking error here - I'm talking about RA axis pointing direction. RA counters Earth's rotation and hence needs to be parallel to axis of rotation of Earth - and point to celestial pole.

When I say that RA is out - I meant not properly aligned with axis of rotation of the Earth. Nothing to do with RA tracking error (which is difference between the speed at which Earth is rotating and the speed at which the mount is tracking).

Oh I see, I knew it would be my understanding. :) 
I wasn’t guiding for very long at all before I took the screenshot, maybe 4/5 minutes, not longer than 10 minutes anyway. I can’t answer about the cycle times etc, it’s all very new to me. When I next set up to try it again I’ll get some more screenshots and information .

Thanks very much for the explanations, I understand a bit more about it now.

 

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Well that was successful, just had another go and everything worked perfectly. This time I made sure everything was locked properly and secure and I was particularly careful when touching anything on the mount etc.

My first manual polar alignment was actually very good, just the altitude out a bit. See below.

AE6F3D3C-BD77-42F4-A2AF-DCF0F1B70D0B.thumb.png.8a058581a34d99b5553efe7c3a845522.png
 

So I made some small adjustments to the altitude and settled for this:

F89D2E90-FFAA-4776-A8CF-A685CEE11027.thumb.png.06ecaecd942c07ccc728a030eae92e98.png
 

I then pointed the scope at Altair again and started the guiding, the graph is on this screenshot.

157EC290-BD06-402E-980D-D38D1B738919.thumb.png.60f1f021180caa29895738ebbe5f4c71.png
 

and finally here is a screenshot from my IPad showing a 5 minute exposure. The tracking looks great to me 😀

1FCC0FAB-0FC8-4656-85EC-7F22E8CE6CD3.thumb.png.cfa94df1c5f770bfcdbe8382647fc14f.png
 

So I’m chuffed to bits with this setup now. I’m sure they’ll be many more advantages as well once I learn to use it a bit more.

Thanks everyone for your help. It seems it was just me as expected. :) 

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