Camera alignment (or not) for widefield

[stevepsheehan]

Hi all. 

Probably a silly question, but one I just can't seem to get my head around, so bear with me please 🙂

If I'm piggybacking a DSLR, typically with either a 50mm or 135mm lens, on my ED80, on a nicely polar aligned equatorial mount, does the camera NEED to be pointing at the same target as the main scope, or can it be swung round, to say 60 degrees to the main scope, and still avoid trailing? 

It seem to me, that assuming the rig is all solid and is tracking nicely, the relative positions that the scope and camera are pointing at, should stay the same... won't they? Or am I missing something obvious?

I'm just thinking that IF that works, then I could set up so I could track the widefield from, say, 30 degrees east of meridan, to 30 degrees west of meridan, without have to consider flippng the mount, at stupid o'clock in the morning.

Thanks in advance,

Steve  

[PeterCPC]

If you think about it, if you are guiding, the guidescope will probably not be pointing in exactly the same place as your scope and that works ok. The point is that you are aligning the mount and not the scope so the mount will always track once aligned.

Peter

[ollypenrice]

There is a problem, potentially. If you have a polar alignment error and are autoguiding, you will get field rotation centered on the guide star. If your guide star is in the middle of your imaging field the middle of the frame will be perfect but the further away you look from the centre of the chip the more you'll see trailing. What you're proposing would be an extreme case of this off-guidestar field rotation, meaning that it would risk being obtrusive even with a low level misalignment.

Warning: I'm not very good at these 'spacial awareness' things!

Olly

Edited April 24, 2020 by ollypenrice
Typo

[stevepsheehan]

Thanks, both of you. 

Olly, I can't get my head around 3D spacial awareness probs either, hence the question 😉  I guess I'll just have to try it and see what happens. Next time it's clear, I'll give it a go, and report back.

Steve

[andrew s]

@ollypenrice point about field rotation is correct if you are guiding. It's not an issue if  just tracking. There are other types of error due to the apparent sidereal rate being different in different parts of the sky and you get the apparant celestial pole  moving with altitude which are both elliminated by guiding when the guide scope and camera are aligned. 

I think the best approach is to just try it. Experiment is king in these situations

Regards Andrew 

 

 

Edited April 24, 2020 by andrew s

[rickwayne]

The main scope, if I'm interpreting you correctly, is just serving as a place to bolt your camera on. So you're equivalent to a ballhead mounted on a tracker, which works fine no matter which way you point the camera.  And if you're autoguiding through the main scope, it will still work OK. Think of it this way, the autoguiding, if absolutely perfect, will cause the guide star to stay exactly centered in the scope's FOV all night. And a perfectly-tracking mount without autoguiding will do...exactly the same thing.

Now, any field rotation due to polar misalignment will certainly be less well-compensated by guiding on a star far away from the camera FOV. But guiding will not in itself induce field rotation.