Polar Alignment

[Ohgodwherediditgo]

I don't currently use any software for PA. I normally move Polaris in the polar scope to the 12 o'clock position, then use the clock on the setting circle to rotate the polar scope to the correct position and then make the alt / dec adjustments. That was until my last session, when i noticed big smudges instead of stars after leaving it far too long to check an image. I opened up the guiding assistant in PHD and let it run it's routine, after which it reported that my PA was 30 ' out, impressive, i thought ! I went through my usual PA routine and checked the assistant again to find i was within 2 ', so i settled for that. I have since invested £5 in one of China's finest laser pointers to avoid that mistake again.

Anyway, N.I.N.A has a new PA routine, currently on a nightly release that I'm thinking of updating to. I'm a bit hesitant to because I'm not sure if my imaging scope should be perfectly aligned with the polar scope ( which it isn't ) to get good PA using the imaging scope.

Is alignment between polar and main scope critical for using software ?

 

 

Edit: I suppose what I'm asking is if it is normal for software to compute an offset if they are not? 

 

Edited October 19, 2021 by Ohgodwherediditgo

[malc-c]

There are several technical terms for any misalignment of optical axis and the mount, and its the latter you are trying to PA not the telescope, which is why Polemaster was developed.  

One factor is cone error.  Ideally, with the scope pointing north in the home position, release the clutch and rotate the RA through 90 degrees so the weigh bar is horizontal, and the OTA is on the west side of the mount.  Rotate the DEC axis until Polaris is as central as possible in the field of view, using the adjustment screws in the dove tail to make the final adjustments.  Then rotate the RA through 180 degrees and repeat the process.  The idea is that you are forming a triangle with Polaris at the top, and the OTA at two points East and West of the mount.  With cone error removed from the main scope, you could repeat the process but this time with the guidescope, so basically removing cone error from that too, and making the two optical axis true to each other.    Depending on the setup you may experience flexure due to the mass of the equipment moving the C of G when actually on target, but  other than that, flexure is easier to deal with compared to a massive cone error.

As for whether the software works out the miss-alignment and uses that in its calculations, haven't got a clue... As far as know applications such as PHD2 simply uses a calibration routine to see how many pixels a star moves for a given pulse duration and then apples some maths to make the small pulses correct depending on the actual movement.  

[Ohgodwherediditgo]

Thanks for the detailed reply. I'll check for cone error as soon as the clouds clear enough. Unfortunately I will have to shim between the scope and dovetail if adjustment is required.

The polemaster had crossed my mind but I didn't want to throw money at the problem straight away, although it does seem to me to be the cream of the crop for pa. 

[malc-c]

To be honest, provided your PA is close, and your guiding gives you round stars then there is little need to go to the levels of trying to iron out all these things.

[Ohgodwherediditgo]

That's been the trouble. I can't go for longer than 2 mins without getting elongated stars. My subs at 2 mins are slightly elongated but stacking makes then pretty good. I'm trying to get to 5 mins to experiment with swamping the read noise of my dslr. 

I expect that checking for cone error then using software to assist will be near as damn it, well I'm hoping so. If not then I'll renew the mount bearings as Dec has more drag than I'd like. I did repack the tapered bearings recently    ( properly and with synthetic grease 🤔) and noticed one of the sealed bearings was a bit rough. 

[malc-c]

Is that 2min unguided or you having issues when guiding ?

[Ohgodwherediditgo]

That's guided. The scope is 1800mm  F9 though so not ideal for learning on. 

[StevieDvd]

18 hours ago, Ohgodwherediditgo said:

Is alignment between polar and main scope critical for using software ?

Edit: I suppose what I'm asking is if it is normal for software to compute an offset if they are not? 

 

No, the offset just has to be consistent during the PA process.

[malc-c]

33 minutes ago, Ohgodwherediditgo said:

That's guided. The scope is 1800mm  F9 though so not ideal for learning on. 

I'm not the best person to advise on this, but my first thought was the you are using a 328mm focal length guidescope to guide an 1800mm focal length scope.  The camera may not detect much movement through the 4.1 focal ration guidescope, but  naturally over 1800mm the drift would me more noticeable. Having said that the camera has a pixel size of 3.75um x 3.75um, giving 1280 x 960 image size, so I would have thought that the guide scope / camera would be able to maintain guiding without a noticeable drift over 2 minutes, especially with an NEQ6 as the mount.

Hopefully someone more knowledgeable who can do the maths and workout the degree of precision that the guide camera / scope combo gives.... and see if we are on the right track 

[Ohgodwherediditgo]

I used astronomy.tools before i bought the guide scope and camera :