Field rotation question

[RobertI]

Hope this is the right place to ask this.

I am looking to get an altaz mount for quick EAA/video astronomy sessions, and will be doing exposures up to 30 seconds. I have found a very useful table allowing you to calculate the max exposure for a given object's altitude/azimuth for an observer at a latitude of 50 degrees but I am hoping there is a simpler way of calculating max exposure times. Two questions:

1. Is there a chart available showing the maximum exposures without field rotation in each area of the sky (I seem to remember someone posting such a thing in one of the threads but I cannot find it)
2. Do any of the planetarium programs tell you the maxim exposure without field rotation when you select an object?

Thanks! ?

 

[2STAR]

I do EAA video on an Alt/Az mount, the camera I use easily does up to 2 mins without any star trails if that's  what you mean by field rotation , I am at 53 deg lat but I don't think it really matters when using the Atik Infinty what lat you are at. I may be wrong but that's based on images I have seen from EAA enthusiasts from many countries.

I suppose it will vary dependent on what Video imaging camera you will be using.

regards

eric 

[SteveNickolls]

Hi, 

I don't know the formula used to calculate a maximum exposure time but Joseph Ashley, in his, "Astro-photography on the Go Using Short Exposures with Light Mounts" over pages 95-97 (available from FLO-https://www.firstlightoptics.com/books/astrophotography-on-the-go-book.html) provides tables for a range of observer latitudes. I took the tables and worked roughly the values for my latitude (53 degrees) and use that to provide a ready reckoner of maximum exposures times for any given object as part of the imaging preparation phase each night.

I only use Stellarium and while it gives Alt and Az values for objects it doesn't give you maximum exposure times when Alt-Az imaging. 

For an observer at 50 degrees Latitude any object below 50 degrees Alt regardless of Az is ok to image for up to 30 seconds the time being greatest when an object is in the East or West (equal to or beyond  344 seconds!).

I recall that The Admiral posted a graph in the, 'No EQ Challenge Thread' showing the exposures times. It's a long thread so good luck hunting for it. The Mods would never let the thread be broken into more useful searchable form, shame really.

Cheers,
Steve

[vlaiv]

I'm not sure generic tables will be useful. Field rotation depends on couple of factors - your position, target position, frame size. And this is just for field rotation in seconds of arc. Next thing that you should look at is resolution at which you are working and what is acceptable field rotation with respect to this (2" rotation of corner is one pixel at 2"/pixel and 4 pixels at 0.5"/pixel).

This is how I would go about it: Find Alt/Az to Eq grid transforms. Fix one point on the frame (center) - in alt az coordinates find corner and transform into eq. Now move center in eq coordinates for duration of the frame (x seconds) - assuming that center of the frame will be "guide" point and stationary in both frames. Calculate AltAz motion of center (from eq coordinates on start and end). Translate corner in AltAz by same amount and convert to Eq. Look at the angle difference on the beginning vs on the end of x seconds period. This will be your rotation. Based on resolution - convert to pixels and see what is acceptable to you.

[RobertI]

Thanks for the responses guys. 

@2STAR - I'm not sure if we are talking the same thing or not, the field rotation I am referring to is the circular star trails around the centre of the image. At some points in the sky (such as very close to the zenith) you cannot expose for more than a few seconds with an altaz mount without significant field rotation, whereas at other points, such as due east or west, you can expose for several minutes. It doesn't sound like something you have had an issue with though? 

 

@SteveNickolls - thanks for the book ref, that is in fact where I got the tables that I referred to in my original post! I may end up using your approach in the end, but I was hoping there was something more visual or software driven. Thanks for the heads up on the non-eq challenge thread - @The Admiral - did you post something along the lines of a field rotation 'map'? 

 

@vlaiv - The tables do take into account latitude and object position, but I did also wonder about the effect of other factors such as frame size, resolution, etc. In none of the resources I've looked at does it mention these things, so I guess they must be assuming an average setup? For info I am using a small low res lodestar camera giving 2.5"/pixel for the big scope and 6"/pixel on the small scope. I am guessing that this kind of setup is far more tolerant to field rotation? Frankly I live with a bit of coma anyway (us EAA'ers are not that fussy!) so I guess a little field rotation will not be a massive issue. I am not sure how I go about doing the calcs you suggest (a bit advanced for me I think), but it also sounds like any tables/charts are only a very rough guide. Thanks for the info though, very useful. I'd still like to see a map just to understand the 'pattern' of field rotations on the different parts of the celestial sphere - it will help me to understand where to avoid.  

 

[vlaiv]

Maybe this will help:

http://www.stargazing.net/david/doublestars/fieldrotation.html

a lot of resources as well as map to help visualize (well sort of )

 

[vlaiv]

If you follow thru, one of the links on the first page is even better and more informative (but does not contain python scripts that you can use to create map for your observing location):

http://calgary.rasc.ca/field_rotation.htm

[The Admiral]

9 hours ago, RobertI said:

@The Admiral - did you post something along the lines of a field rotation 'map'? 

I did, but can't link to it. But I probably referred to this site (http://calgary.rasc.ca/field_rotation.htm) where you can find a lot of information. IIRC there is an equation given, and I used that in a spreadsheet to generate my own curves. I can't access that at the moment 'cos my PC is down.

Ian

Hah-ha! I'm too slow, I see Vlav has beaten me to it!

[ollypenrice]

14 hours ago, 2STAR said:

I am at 53 deg lat but I don't think it really matters when using the Atik Infinty what lat you are at. I may be wrong but that's based on images I have seen from EAA enthusiasts from many countries.

 

I think latitude does matter. After all, imagine imaging at one of the poles. Your alt-az mount is effectively equatorial in this circumstance because the objects in the sky never change in altitude, they simply rotate around the sky and your mount follows them. 

Olly

[The Admiral]

4 hours ago, ollypenrice said:

I think latitude does matter.

Indeed it does, as the Calgary link explains.

Ian

[RobertI]

Yes, it's been a voyage of discovery finding out about altaz imaging; at the poles there is no field rotation, at the equator there is so much rotation that imaging is very difficult, and midway between the two the pattern of maximum exposures across the sky is pretty complex and not that intuitive, that's what I am trying to get my head around. I think in reality, with the setups I have, i probably do not need to worry too much, I guess I shall have to try it and see.

[ollypenrice]

8 minutes ago, RobertI said:

 I guess I shall have to try it and see.

T'was ever thus!

Olly

[2STAR]

On 07/07/2018 at 12:00, ollypenrice said:

I think latitude does matter. After all, imagine imaging at one of the poles. Your alt-az mount is effectively equatorial in this circumstance because the objects in the sky never change in altitude, they simply rotate around the sky and your mount follows them. 

Olly

Who images at the Poles lol,  Colchester is the area in discussion I think, im no expert but  can only quote regarding Alt/Az and the Infinity from use in England

[ollypenrice]

8 hours ago, 2STAR said:

Who images at the Poles lol,  Colchester is the area in discussion I think, im no expert but  can only quote regarding Alt/Az and the Infinity from use in England

You miss my point, which was to illustrate why latitude affects alt-azimuthal tracking and field rotation. At the poles an Alt-Az mount is also an equatorial one. The closer it goes to the equator the greater the disparity between the tracking characteristics of Alt-Az and equatorial mounts.

Olly

Edit: To respond to a point in your first post, there is a distinction to be made between star trails and field rotation. A perfectly polar aligned EQ mount will never show field rotation but it may well produce star trails from tracking errors due to periodic error or other irregularities in the drives. On the other hand an Alt-Az mount, however perfectly it is locked onto a central star, will always suffer field rotation to an extent which is proportional to its distance from the poles. To what extent this shows depends on where in the sky you are imaging and on the resolution at which you are doing so. A coarse pixel scale will 'absorb' tracking and rotational errors while a fine one will not. This is the only way in which the choice of camera is relevant.

[The Admiral]

And more to the point, even Colchester is not immune from the effects of field rotation 

Ian

[RobertI]

6 minutes ago, The Admiral said:

And more to the point, even Colchester is not immune from the effects of field rotation 

Ian

Something which I will doubtless find out in due course!  

BTW, I still haven't found the map I was after. Must be around somewhere.  

[The Admiral]

I might add Eric (2STAR) that video astronomy is perhaps not quite representative of ordinary astro imaging because I'm guessing that individual frame durations used are quite short. So short in fact that the field rotation isn't normally visible, and still wouldn't be once the frames are stacked.

I've imaged a target for a couple of hours total duration when Alt-Az imaging (not video), but by using a sub length of only 30 seconds the rotation is not visible. If I had been able to us a sub length of a couple of minutes, it certainly would have been. Mind you, at such long durations the tracking movements of the mount would be much in evidence anyway.

Ian

[The Admiral]

4 minutes ago, RobertI said:

BTW, I still haven't found the map I was after. Must be around somewhere.  

Robert, what does the map show? The variation of angular rotation rate/maximum exposure time vs alt and az ftom your neck of the woods, or how it varies across the globe? The Calgary link does show something like the latter.

Ian

[The Admiral]

Robert,

I might have posted something like this before. Not a map exactly, but a visualization of the variation with position of sky from the south of England. Any help? I used a limiting field rotation of 0.1° rather than 1/8 deg as others have used.

Ian

[RobertI]

Hi Ian,

The map shows a projection of the celestial sphere, a bit like the Philips planisphere rendition of the sky, and numbers dotted around the sky showing the max exposure in that part of the sky. 

Your graph is much more accurate though and much more useful! Thanks for taking the time to post.  ?

I guess if I still feel the need I can create my own map based on you data.  

[happy-kat]

Then there is the capability of the mount to track for longer without doing any sticky jerks, so longer exposure may not be possible due to mount limitations irrespective of field rotation.

[The Admiral]

57 minutes ago, happy-kat said:

Then there is the capability of the mount to track for longer without doing any sticky jerks, so longer exposure may not be possible due to mount limitations irrespective of field rotation.

Too true. That's my main limitation, and with my mount I reject a significant proportion due to that.

Ian

59 minutes ago, RobertI said:

Your graph is much more accurate though and much more useful!

Thanks Robert, but precision isn't really the name of this game! I don't recall seeing a map such as you describe, though others may do.

Ian

[happy-kat]

I do it was in one of the links given way back on the No EQ thread but I also found the map much harder to interpret.

[The Admiral]

51 minutes ago, happy-kat said:

I do it was in one of the links given way back on the No EQ thread but I also found the map much harder to interpret.

This site's search is a bit limited, but using Google to home in on "field rotation" and "the no eq challenge" thread I've found some links around 5th April 2016 posted by both Steve Nickolls and Silver Astro, but these links no longer work. I guess they may have been the source for the sought for map, as the context fits.

Ian

[RobertI]

Thanks for comments Ian and Happy Kat and thanks for searching in the No Eq thread Ian.

I've had a go at translating the graph into a very rough map (based on a screenshot from SKySafari), to show the 'no fly' zones for someone who is imaging at 30 seconds and 20 seconds. I don't know if the zones are circular, oval or other shape, but I just wanted to understand what the rough pattern was. I think the zones may be slightly bigger than on the diagram, as SkySafari seems to compress the scale closer to the zenith and expand it closer to the horizon. It's not scientific, but I think I am happy now....unless someone can tell me I have got it all wrong!!

Some interesting spehrical geometry coming into play, if that's the right phrase. It seems counter intuitive that the zones either side of the zenith should be the same, given that one side contains the celestial pole, but that what the graph says......! My guess is that as you increase latitude and get closer to the geographic North pole, the cirlces reduce to zero, conversely as you get closer to the equator, they get bigger, still touching. May be wrong, interesting if true though.