ADC- essential for planetary?

[Rusted]

8 hours ago, IB20 said:

I have just been observing Jupiter and it was quite clearly displaying blue and red fringes at opposite sides of the disc. There were moments where details (EBs) on the disc were showing but there were also plenty of blurred and wobbly moments. It is quite windy outside though and checking the jet stream location, it is just clipping my skies.

So my questions are:

Is AD more likely to show in poorer seeing conditions? Anecdotally that seems to my experience so far.

Would an ADC help with removing these coloured fringes in conditions similar to these? This was in a 3” achromatic f10 refractor.

 

A 3" f/10 should show very little false colour. It is almost equivalent to a 4" at f/15.
Poor seeing conditions often lead to increased visibility of chromatic aberration.
Low altitude objects similarly. A combination of both do not lead to eternal joy.

 

 

[globular]

On 15/07/2020 at 12:15, andrew s said:

The atmospheric dispersion is along the parallactic angle and is only vertical at the meridian.  The bubble level may be misleading if your observations deviate too far from there.

I'm using an alt-az mount and have my shiny new ADC ready to try on my next clear night.

Hopefully most of the time I'll see red and blue colour fringing.... and then I align the ADC 90° to a line through the centre of the colours.....  
... but if there is no obvious colour fringing, but still sharpness issues that the ADC can help with, I'd like to know how to correctly align the ADC.

If I'm observing something at the meridian then it will have a 0° parallactic angle and the ADC should be at 0° from horizontal.... yes?  (hence the bubble level on some ADCs)
But if my object has a 30° parallactic angle, say, then the ADC should be at 30° from horizontal???
And how do I calculate the parallactic angle?  Does skysafari give it out for me?
And if I have my diagonal rotated, say 10°, should the ADC angle relative to the horizontal still be 30° or now be 40°?

My head hurts 🙃

[markse68]

Hi Glob- you took the plunge! Congrats- hope you find it useful and not too frustrating. I had to look up those angles and still don’t understand them but i don’t think you need to worry about them as the ad is worse at lower altitudes and will be vertically oriented. But yea you’ll need to compensate rotation of diagonal just as i have to compensate rotation of dob ota. I think you can do it by removing ep and holding a horizontal straight edge in front of scope and noting orientation in view through scope but i just fiddle with it till it gels.

Good luck with it

Mark

[andrew s]

12 hours ago, globular said:

I'm using an alt-az mount and have my shiny new ADC ready to try on my next clear night.

Hopefully most of the time I'll see red and blue colour fringing.... and then I align the ADC 90° to a line through the centre of the colours.....  
... but if there is no obvious colour fringing, but still sharpness issues that the ADC can help with, I'd like to know how to correctly align the ADC.

If I'm observing something at the meridian then it will have a 0° parallactic angle and the ADC should be at 0° from horizontal.... yes?  (hence the bubble level on some ADCs)
But if my object has a 30° parallactic angle, say, then the ADC should be at 30° from horizontal???
And how do I calculate the parallactic angle?  Does skysafari give it out for me?
And if I have my diagonal rotated, say 10°, should the ADC angle relative to the horizontal still be 30° or now be 40°?

My head hurts 🙃

Try this https://astronomy.stackexchange.com/questions/34086/how-to-calculate-parallactic-angle-from-a-fixed-alt-az-position

Regards Andrew 

[globular]

Thanks Mark.
I suspect, once I'm outside and trying it, all will be well.  I went with a single knob control version to adjust prism-prism angle and rotation to make it as easy to operate as possible. But, as you might have noticed, I like to understand the theory as well as trying it in practice. 

I get that the rotation has to be perpendicular to the parallactic - because the dispersion happens along the parallactic - but I thought this angle changes as distance from meridian increases and not as altitude changes.  (e.g. at all altitudes long the meridian the parallactic angle is always vertical - so the ADC is always horizontal. But away from the meridian it's not vertical so the ADC is not horizontal).

Thanks Andrew. 
I found that same formula for the parallactic angle and have written an app for my phone that allows me to read out loud the inputs (that I get from SkySafari) and it will speak back the angle.....   but I can't seem to get it to produce sensible results. Either the formula is wrong or my inputs are not in the correct format. 🤔

It's probably a degrees vs radians thing in the sin, cos, tan functions....?

Could you perhaps give an example location, time and object and provide the inputs in the correct formats to slot into the formula?

[markse68]

Hi Glob, found this which i think confirms my thoughts:

”When doing long-slit spectroscopy of point sources, determining the the proper rotator angle which aligns the slit along the atmospheric dispersion direction is trivial with an Alt-Az telescope. Simply put, the slit should always be oriented perpendicularly to the horizon, which also happens to be perpendicular to the altitude axis of the telescope.”

http://www.not.iac.es/instruments/alfosc/paralang.html

Mark

[globular]

I'll just run with keeping it horizontal.  Pretty easy to do in an alt-az.  Trial and error will see me through.