AP900 Rotating R.A. Setting Circle


Wenhan Chang
 

Hi All,

I just acquired a used AP900 and am learning how to use it. One thing that puzzles me is about the rotating R.A. setting circle (i.e. the black ring with 0-24 markings;  here is a video description: https://youtu.be/amu2G4Np0kk).

The DEC setting circle doesn't move, which puzzled me a little bit. George kindly taught me that "RA changes, as the earth not only rotates, but also revolves around the sun, thus sidereal time is changing every day and the RA circle has to be able to change to compensate".

If I understand correctly, this means that one need to manually adjust the R.A. setting circle every day to compensate for sidereal time changes? How to do that? I couldn't find a fixed reference point on the RA body to align the R.A. setting circle to.

Please correct me if I'm wrong and any suggestions are greatly welcome!

Thank you very much.
Best regards,
Wenhan


Roland Christen
 

Probably the easiest to Google how to operate a setting circle.
Here's what I found:

https://www.cloudynights.com/topic/645389-how-to-understand-this-ra-setting-circle/

here's the jist:
 
Do your normal polar alignment.
Find some know object in the sky (star or DSO).
rotate the RA and DEC setting circles to match those known coordinates.
lock the RA and DEC setting circles down.
 
Now use a book to look up the RA and DEC of anything you want to see and move the mount to the matching coordinates and you should be on the object.

By the way, the 900 setting circles are driven which means that as long as you have the motors running, the circles are accurate. On the 900 the Dec circle doesn't move, it is locked into place and will be accurate as long as you are reasonably well polar aligned. No need to ever unlock it. The RA circle moves easily and you simply rotate it manually to match the known object. A bright star will do.

Rolando

-----Original Message-----
From: Wenhan Chang <sjtu.wenhan@...>
To: main@ap-gto.groups.io
Sent: Fri, Feb 11, 2022 6:11 pm
Subject: [ap-gto] AP900 Rotating R.A. Setting Circle

Hi All,

I just acquired a used AP900 and am learning how to use it. One thing that puzzles me is about the rotating R.A. setting circle (i.e. the black ring with 0-24 markings;  here is a video description: https://youtu.be/amu2G4Np0kk).

The DEC setting circle doesn't move, which puzzled me a little bit. George kindly taught me that "RA changes, as the earth not only rotates, but also revolves around the sun, thus sidereal time is changing every day and the RA circle has to be able to change to compensate".

If I understand correctly, this means that one need to manually adjust the R.A. setting circle every day to compensate for sidereal time changes? How to do that? I couldn't find a fixed reference point on the RA body to align the R.A. setting circle to.

Please correct me if I'm wrong and any suggestions are greatly welcome!

Thank you very much.
Best regards,
Wenhan

--
Roland Christen
Astro-Physics


Wenhan Chang
 

Hi Rolando,

Thank you very much for the detailed explanation! This is my first mount with a RA setting circle; and I naively thought it is only for finding Park Position 3 and should be locked into place. Now I totally understand the logic behind it and how to use it.

Best regards,
Wenhan


M Hambrick
 

Hi Wenhan

Different mount, but I used to have an Astro-Physics 800 mount, and The RA circle was held in place by friction. There were no lock screws. The setting circles were graduated in 10 minute increments. I cannot remember if there was also a vernier scale as well, but if it had one it would have allowed resolution to within 1 or 2 minutes. I think that might have made a vernier scale out of masking tape. It sounds like your 900 mount uses a similar concept for holding the RA setting circle in place.

I keep hoping that one day Roland will announce that they are going to start putting setting circles on their mounts again, and that everyone who has a mount can order a kit to retrofit their mounts with setting circles. :>)

Mike


Roland Christen
 

The 900 had a vernier scale. Only problem is that knowing how a vernier works is even more baffling than knowing how to use a setting circle. It's like having a stick shift on a car - does anyone know how to make that work?

Rolando

P.S. I drove a 67 Camero stick shift once and you needed a very strong leg muscle to push the clutch down. Not for the faint of heart.



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Feb 11, 2022 6:56 pm
Subject: Re: [ap-gto] AP900 Rotating R.A. Setting Circle

Hi Wenhan

Different mount, but I used to have an Astro-Physics 800 mount, and The RA circle was held in place by friction. There were no lock screws. The setting circles were graduated in 10 minute increments. I cannot remember if there was also a vernier scale as well, but if it had one it would have allowed resolution to within 1 or 2 minutes. I think that might have made a vernier scale out of masking tape. It sounds like your 900 mount uses a similar concept for holding the RA setting circle in place.

I keep hoping that one day Roland will announce that they are going to start putting setting circles on their mounts again, and that everyone who has a mount can order a kit to retrofit their mounts with setting circles. :>)

Mike

--
Roland Christen
Astro-Physics


Wenhan Chang
 

Hi Mike,

Thank you for sharing your experience. The RA setting circle design of AP900 is quite impressive, as Roland has mentioned: it is driven by the motor, but stays in place if RA clutch is loose. It's also how I feel about the Rotating Pier Adapter: clever design coupled with precision machining make amazing products.

Hi Roland,

About a decade ago, I borrowed a stick shift car to pass the driving exam, as required by the government. A year later, I forgot how to shift already...


Best,
Wenhan


M Hambrick
 

Nowadays, if you have a car with a manual transmission, it is as good an anti-theft device as you can find. I learned to drive in a '68 ford pickup truck with a "three on the tree" manual transmission. It was ugly. Very ugly.

If you travel to Europe (especially Germany) and you can drive a manual transmission, you wall always get a better car when you go to a car rental agency if you specify a manual transmission. Asking for an automatic transmission will almost guarantee that you get a "granny" car. If you specify a manual transmission, your chances of getting a Mercedes or BMW go up exponentially.

Mike


Peter Nagy
 

On Fri, Feb 11, 2022 at 05:51 PM, Roland Christen wrote:
P.S. I drove a 67 Camero stick shift once and you needed a very strong leg muscle to push the clutch down. Not for the faint of heart.
 
I had a 1964 Corvette with 4 speed Muncie transmission and the clutch was absolutely perfect. Not too stiff or soft. That was an awesome car to drive.

Peter


Wenhan Chang
 

Hi Mike, Hi Peter,

These are great reasons for me to start "learning" how to drive a manual car again! The last time I use the term "clutch & accelerator" was when teaching someone how to ski (making a ski turn is like driving a manual car: you release the ski pressure first, i.e. clutch, and re-engage while changing ski direction, i.e. accelerator)...

Best,
Wenhan