Meridian flip, Dec axis before and after.

[SteveBz]

Hi Guys,

I have an old C8-N with drives and a handset with an ST4 port.   I'm using an Arduino to do small jobs around my setup like exposure control, electronic focusing, guiding and small GOTOs through the handset (large ones take too long).

The first three work nicely, by my GOTO is all over the place and I've just realised that whether I'm looking toward the Eastern horizon or the Western one must mean that I need to invert my Dec  movements.  Is this the case?  Is this the only effect of the meridian flip?  The RA axis keeps on chugging (and I don't mean collecting for charity) in the same old direction, but Dec needs be inverted.

Is there a proper name for these two modes, like "Western mode" and "Eastern mode", or before and after midnight?

What else should I look out for?

Thanks

Steve.

[andrew s]

Yes a flip reverses the Dec axis. Also when you pass the pole the Dec changes sign wrt the Dec motion. It can get quite complex especially if the scope can point at the same point from either side of the meridian!

Regards Andrew

[SteveBz]

37 minutes ago, SteveBz said:

Is there a proper name for these two modes, like "Western mode" and "Eastern mode", or before and after midnight?

Ok, I've just realised, while mowing the lawn, it's probably "Ante-Meridian" and "Post-Meridian" or am and pm for short.

It's amazing what the smell of freshly mown lawn mixed with petrol fumes can do for your grey matter.

[SteveBz]

4 minutes ago, andrew s said:

when you pass the pole the Dec changes sign wrt the Dec motion

Hi Andrew,

So for am vs pm I need a minus sign on the dec axis.  Is that the only change?

At the moment GOTO takes me way out of the way, but doing GOTO plus a GOTO with signs reversed does take me back to where I started (eg Vega).

Thanks for your help.

Steve.

[SteveBz]

19 minutes ago, SteveBz said:

Ok, I've just realised, while mowing the lawn, it's probably "Ante-Meridian" and "Post-Meridian" or am and pm for short.

It's amazing what the smell of freshly mown lawn mixed with petrol fumes can do for your grey matter.

So I just Googled it and of course am and pm are ante meridiem and post meridiem, so it doesn't really work.

1) What are the correct terms, please?

2) I think I'll probably use am and pm for the time being because they are quite short, plus they sound right if you're not a Latin scholar.

[SteveBz]

More Googling gives:

https://www.grammarphobia.com/blog/2014/03/ante-meridiem-vs-antemeridian.html

and

http://svatantranatha.blogspot.co.uk/2014/08/solar-noon.html

both interesting articles.

[andrew s]

I remember getting into this many years ago via "Telescope Control"  by Mark Trueblood and Russell Merle Genet. Now way out of date on the computer side but the basics are still correct. ( I have a copy I could lend you but would like it back.)

Don't forget the hour change and time zone corrections. I found a bug in the Astro-Physics firmware years ago for those on the meridian (time zone zero) which they acknowledged but was still not fixed some years later.

I assume you are working in hour angle HA and local sidereal time LST etc.

Good luck.

Regards Andrew

[SteveBz]

14 minutes ago, andrew s said:

Don't forget the hour change and time zone corrections. I found a bug in the Astro-Physics firmware years ago for those on the meridian (time zone zero) which they acknowledged but was still not fixed some years later.

Hi Andrew,

I may take you up on the offer later, but for now I just want to be able to star-hop remotely.  As it was, I hope to hop from Vega to Beta Lyrae to M57 just by entering the relative coordinates of the objects concerned, but actually I ended up half way across the universe in the other direction.  Mainly, I believe, because I hadn't understood the reversal of Dec under a meridian flip.

Thanks for the offer,

Steve.

PS I'm beginning to think am and pm is confusing and that East and West are better.  What do you think?

[andrew s]

19 minutes ago, SteveBz said:

I'm beginning to think am and pm is confusing and that East and West are better.  What do you think?

I don't really know which is better. I do however, think you should be using the Hour Angle of the target. This then unambiguously tells you where it is relative to your meridian at any given time. But you still then need to know if it is above or below the pole.  

It is a very long time since I considered this seriously and could mislead you if I tried to get into details. Patrick Wallace (http://www.tpointsw.uk/ ) who has worked on the pointing of most world class telescopes always says if there are two options there is a 50:50 chance you will get it wrong however hard you think about it!

Regards Andrew

[SteveBz]

5 hours ago, andrew s said:

I don't really know which is better. I do however, think you should be using the Hour Angle of the target. This then unambiguously tells you where it is relative to your meridian at any given time. But you still then need to know if it is above or below the pole.  

It is a very long time since I considered this seriously and could mislead you if I tried to get into details. Patrick Wallace (http://www.tpointsw.uk/ ) who has worked on the pointing of most world class telescopes always says if there are two options there is a 50:50 chance you will get it wrong however hard you think about it!

Regards Andrew

Is HA different from RA (which is also measured in hours0/

Steve.

[andrew s]

Yes it is Hour Angle is measured from your meridian so changes with time. While RA is fixed on the sky.. Google for the formula that relates RA HA and local sidereal time.

Regards Andrew

[SteveBz]

16 hours ago, andrew s said:

Google for the formula that relates RA HA and local sidereal time.

Hour Angle to Right Ascension

1. Convert Local Sidereal Time and Hour Angle into decimal hours.
2. Subract Hour Angle from Local Sidereal Time.
3. If result is negative add 24.
4. This is the Right Ascension in decimal hours.

http://129.79.46.40/~foxd/cdrom/musings/formulas/formulas.htm

Update: Here is a better one.

https://docs.kde.org/trunk5/en/kdeedu/kstars/ai-hourangle.html