How to work out field rotation with asiair

[powerlord]

Hi chaps - is there a way from the plate solve to work out what the field rotation is ?

With the asi1600 connected to the newt it's not obvious at all what 'up' is. so when I check framing in sky safari I don't know what offset to set the field rotation too.

I suppose taking a long exposure of a dso then working it out from there manually, but figured there might be a more obvious way I'm missing?

stu

[GoldTop57]

You mean the camera angle? I found this useful: https://www.youtube.com/watch?v=ZmqyfZTabgg

 

 

[powerlord]

Thanks though having watched it I'm none the wiser. Seems that the angle asiair is showing is relative to something other than a rotation from 'level' on polaris... Instead, I looked at one I took 4 nights ago (m51),  saw that the tail in up. And worked out that was a rotation of 0. So then I could move 30 from that to get to what I wanted. Pity its not simpler.

[symmetal]

The plate solving angle is the angle of the camera sensor edge orientation compared to a line through the sky joining all points at the same RA. In other words the long axis of the camera is parallel to, (at the celestial equator), or elsewhere tangential to a circle in the sky joining all points at the same declination. 'Up' can be considered parallel to this imaginary line joining all points at the same RA.

It's easiest to use Stellarium for example and overlay the sensor outline using the oculars plugin and see how the sensor orientation changes as you move around the sky. At the celestial pole the sensor overlay spins around rapidly for tiny changes in pointing position. 

It looks like you're plate solving at Polaris. You should never do this. Lines of RA which are say 1 hour apart, are well spaced near the celestial equator, but get closer and closer together the higher your target declination, until they all converge to the same point at the celestial pole. Likewise the circle joining all points at the same declination gets smaller too, and is a single point at the pole. If you are not absolutely perfectly polar aligned, and no-one actually is, a small pointing error near the pole will likely show a large error in RA and/or DEC, and likewise a large error in the angle shown by platesolving. These values shown are really meaningless.

To accurately get your camera angle correct always plate solve and adjust the required camera angle at a low declination. Then slew and platesolve to your intended target and your camera angle will stay the same wherever you're pointing. It's usual to set the platesolving camera angle to 0 degrees, or sometimes 90 degrees to best frame your target. 

Alan

[ollypenrice]

You'll simplify your imaging life if you set your camera to have its edges aligned with RA and Dec. This makes adding data from subsequent nights easy and simplifies mosaics. The orientation can be landscape (long side along RA) or portrait (long side along Dec) but I use one or the other.  Quite honestly I'd rather shoot two panels than rotate the camera to fit a target into a single frame, though this hardly ever arises. It also makes guiding troubleshooting much easier because you know which axis is which.

To set the camera this way just eyeball it relative to dovetail or counterweight arm then shoot a 10 second sub while slewing slowly on one axis. This will produce star trails but are they parallel with the chip? Rotate the camera and repeat until they are. You'll be very glad you did this, believe me.

Olly

[Richard_]

I don't bother with the readout on ASI Air. My telescope (refractor) has a field rotator with angular measurements. What I do is take an image of my target and try to rotate my camera such that some features are horizontal or vertical (like two bright stars) field rotator angle. 

I then open whatever software I use for planning and framing (eg stellarium or telescopius) and rotate the field of view in software until it matches the image I took with stars in a line as best I can. I take note of the rotation and work out the difference (eg 20° in software and 90° for my field rotator means there is a difference of 70°). 

When I plan a shoot where rotation is critical (like a mosaic) I use the above process to ensure that my camera will image with the same rotation as I planned in software which. So if I frame up in telescopius and it happens that the rotation is 45°, per my example we established a delta of 70° so I should rotate my camera to read 45+70=115°. I take a test image of each panel and compare back to telescopius before I execute the plan. So far, its worked well with my ASI Air Pro in Plan mode. 

[Richard_]

In response to Olly's comment, that sounds pretty logical and makes sense. I'd like to give that a go (up until now, I've always rotated my camera such that the image orientation matches how I see the sky, unless I mosaic and need to rotate of course). 

[scotty38]

How about and this is just an idea.....

In NINA there's a framing assistant and you can also connect a Manual Rotator, so you could set up the virtual kit to mimic your own and then:

1. Choose a target in the Sky Atlas

2. Send that to the framing assistant

3. Rotate/Frame as required

4. Slew to/Centre/Rotate the target.

NINA will then tell you how many degrees and whether to rotate the camera clockwise or counter clockwise to frame the target as needed.

I have never tried it like this as I use it with real kit connected but I can't see why it would not work for you. Once you have the starting point, both physically and in NINA, the same you should be good to go.

Edited January 18, 2022 by scotty38

[powerlord]

tbh I've never had issues before as I don't tend to 'stuff' the frame. But last night the targets and FL meant I had to worry about orientation.

I just found the easiest way was to image a single sub of the target, look at it.. work out how much I wanted to turn it, and then turn it. it worked fine tbh.

stu