Alt/az mount for Spectro ?

[Gasman]

Hi peeps

Had a thought the other day about having a portable mount for spectro but with my imaging head on dismissed the idea of an alt/az mount because of the field rotation but the more I thought about it the more that seemed irrelevant at least from a spectroscopy point of view as it’s purely the light from the star (ie a point light source)  going through the spectroscope so maybe the angle at which it enters the slit is not important or is it?. So can I suggest an alt/az mount would be ok for spectro long exposures or anyone care to counter this please?

cheers

Steve

(Early happy new year to everyone !)

[Merlin66]

Steve,

Yes an Alt/Az can work very well. I had my 12" Lx200 on an alt/az for a while the issue I found for long exposures was guiding.

The Meade allowed PHD guiding, and ideally you need to guide on the target star in the centre of the field.

 

[Gasman]

Cheers for that Ken! Nice to hear from you again?!

Steve

[Merlin66]

The weather this year has been counterproductive in spectroscopy.....

What mount did you have in mind?

 

[Gasman]

Sorry for late reply!.

Not decided yet ,the plan is for camping with the grandkids maybe not a full Spectro outfit but perhaps a goto alt/az mount with my ed80 scope ,Nikon DSLR with star analyser so obviously sticking with the brighter stars and maybe with the Star analyser the guiding not that essential?.

Regards 

Steve 

[Merlin66]

Understood.

If you orientate the grating, the drift can be used to widen the spectrum......

 

[robin_astro]

Hi Steve,

Alt/Az is ok for slit/fibre fed spectrographs (as long as you are not looking for profiles along the slit eg through galaxies). In fact if you orientate the slit in the Alt direction it has the advantage that you are always orientated so the atmospheric dispersion does not affect the spectrum. As Ken said, you do need to guide on the target though rather than a field star if exposure times are long otherwise the target moves off the slit due to field rotation.

If you use slitless systems such as the Star Analyser with an Alt Az mount though, watch out for any background starts trailing through the spectrum (or the subtracted sky background zones) as the field rotates.

Cheers

Robin

[robin_astro]

4 hours ago, Merlin66 said:

Understood.

If you orientate the grating, the drift can be used to widen the spectrum......

 

There is no drift in the spectrum with a tracking Alt Az mount, only in the background stars.  A fixed mount can be used on bright stars with the Star Analyser, with the grating plus camera orientated with the dispersion in the Dec direction.

Robin

[Merlin66]

My bad..any tracking error will widen the spectrum. This can be used to advantage to widen the height of the recorded spectrum.

 

[robin_astro]

10 hours ago, Merlin66 said:

My bad..any tracking error will widen the spectrum.

I've not used a tracking Alt Az mount but as in general  both motors must run to track, isn't the direction of  any tracking errors  unpredictable and dependent on the pointing direction ?