Guiding for wide-field imaging?

[Hallingskies]

Am looking to put a 135mm f3.5 lens on an old SXV-H9 for a wide-field set up.  At around 10 arcsec per pixel, I wouldn’t have thought autoguiding was necessary provided my polar alignment was good.  The mount is a Vixen GPDX and is pretty good in terms of PE.

Do you regular wide-field imagers bother with guiding?

[vlaiv]

At 10"/pixel I don't think you really need guiding.

If mount is good enough to have P2P periodic error such that it is less than 5" and polar alignment is good enough, again less than 5" drift over course of single exposure - that is almost the same as guiding at half of resolution - guide precision recommended most of the time. P2P of periodic does not need to be less than 5" for whole worm cycle - just for duration of exposure. As a bonus you will get "natural" dither if there is slight drift due to PA or PE.

[Hallingskies]

Thanks, Vlaiv.  The Vixen mount I have has a peak to peak PE of around 5" so I should be fine - it's much better than my NEQ6 in that respect. Just wondered if all the folk imaging with these Samsung lenses I keep reading about used autoguiding at all. Shouldn't be a need with a half-decent mount, as you say. 

[Vixen4eva]

Do you have Skysensor 2000 on your GPDX?

Alan

[Scooot]

I don’t have guiding, but I keep exposures below I minute, usually 30 seconds. I use my Samyang 135mm with a star adventurer. If it helps I could post the opening sub and the last of a run if you’d like to see the extent of drift.

[Vixen4eva]

I use a canon 200 mm f2.8 lens on a canon 450D with my Vixen photoguider. With the crop factor i`m imaging at around 300 mm. I can easily take subs of 2 mins or more with reasonable polar alignment. The Photoguider is basically a Vixen GP mount with just the RA drive. I`m going to try mounting my Canon lens and camera on my Vixen GPDX to see how i get on...

Alan

[Hallingskies]

13 hours ago, Vixen4eva said:

Do you have Skysensor 2000 on your GPDX?

Alan

Used to, but I got fed up with the flimsy plug/socket from the handset, the slightest flex sending the mount shooting off in RA.  Replaced it all with the Skysensor EQ5 upgrade kit and it runs under EQMOD/PHD now with no problems.

The skysensor was utterly brilliant when it worked though, pointing accuracy was superb. 

[Hallingskies]

12 hours ago, Scooot said:

I don’t have guiding, but I keep exposures below I minute, usually 30 seconds. I use my Samyang 135mm with a star adventurer. If it helps I could post the opening sub and the last of a run if you’d like to see the extent of drift.

That would be useful, thanks for the offer.

[Scooot]

51 minutes ago, Hallingskies said:

That would be useful, thanks for the offer.

30 seconds taken at 21:43:03

30 seconds taken at 22:52:28

Triangulum- both at F2.8 ISO400 with the 135mm Samyang

[Stub Mandrel]

2 minutes at 135mm is fine with decent PA.

 

[Hallingskies]

2 hours ago, Stub Mandrel said:

2 minutes at 135mm is fine with decent PA.

 

I shall probably be looking at 20min subs and will see what I get.  I had some success with 5min subs but that was 2x2 binned and resolution was not good, as can be imagined.

[Hallingskies]

3 hours ago, Scooot said:

Triangulum- both at F2.8 ISO400 with the 135mm Samyang

Blinked them in Astroart and there seems to be only a slight amount of drift (maybe a star diameter) over the 69 minutes between your two images.  That's very encouraging.  What method do you use for polar alignment?

[Scooot]

6 hours ago, Hallingskies said:

Blinked them in Astroart and there seems to be only a slight amount of drift (maybe a star diameter) over the 69 minutes between your two images.  That's very encouraging.  What method do you use for polar alignment?

Just manually by eye but I check it again after I’ve put all the gear on and focused. When I posted it I thought it looked quite good, I had to check I hadn’t posted the same image twice  That must have been one of my better efforts. 

I use the counterweight so I can see though the polar align sight wth the camera in place. I use the polar align app and  I have a right angle viewfinder on the star adventurer to save my back.

[Scooot]

Maybe that one was a fluke, here are two more. I don't think this is quite as good.

taken at 00:01:39

Taken at 01:27:55

[vlaiv]

I think polar alignment error is given too much significance when in reality it is probably the least contributing factor.

Here is my reasoning: with any sort of attention one should be able to get polar alignment of 6' or less - just for comparison, finger held at arms length is about one degree - 6' is one tenth of that so it is angle that can easily be observed with naked eye (one fifth of full moon). With any sort of polar scope one should get at least that much precision in polar alignment - if proper procedure is followed.

With 6' of PA, imaging at equator (dec 0 - worst case) will lead to drift rate of about 1.5" per minute. Any sort of wide field imaging where pixel scale is more than 5-6"/pixel should be able to do up to couple of minutes with minimal impact of PA drift (up to two minutes it is less than recommended guide error - half of imaging resolution).

Just for comparison, most "economy" or light weight mounts have periodic error in range of 15-30" P2P or even more (without pec). Look here for general reference:

https://lambermont.dyndns.org/astro/pe.html

If we take this number and divide with general figure for worm period of about 10 minutes (some mounts will have less than this, some more, but let's say this is average), even if we assume uniform motion (non uniform sine will be even worse) - this means PE rate of at least 3" per minute. In case of 15" P2P error, for one cycle, mount needs to complete full period so peak to peak will happen in half a period meaning 5 minutes (from "bottom" to "top" peak and then back down to "bottom" again in full period) - 15"/5 = 3" per minute. With higher P2P and non uniform PE there will be times when drift rate is even worse than this. So you see in most cases drift due to PE will be at least double that of drift caused by PA error.

 

[Hallingskies]

Well, I’ll take your word on the maths, it all looks sensible...???

But I want to try 20 min subs in H-alpha.  Looks like I don’t need to bother setting up guiding as the mount’s a good ‘un, but I think I will be very careful on polar alignment.   That has a right sort of feel to it.

If the clouds ever clear I’ll have a go and then post the result.

Thanks for the replies everyone....??