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Forum Index : Microcontroller and PC projects : Question about ASTRO

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Frank N. Furter
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Joined: 28/05/2012
Location: Germany
Posts: 1095
Posted: 11:40am 22 Feb 2026
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Hi,
to calculate the hour angle for my telescope, I need the sidereal time in addition to the right ascension of the object.

The sidereal time is calculated in the background for the ASTRO command.
Is there a way to read out the sidereal time (possibly with PEEK)?

Frank
Edited 2026-02-23 17:37 by Frank N. Furter
 
Frank N. Furter
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Location: Germany
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Posted: 07:09pm 25 Feb 2026
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Is there really no way??? That's a shame! Then I'll have to calculate the sidereal time separately after all... Very annoying...  

Frank
 
PilotPirx

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Joined: 03/11/2020
Location: Germany
Posts: 125
Posted: 12:03pm 26 Feb 2026
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Hi Frank,
unfortunately, I can't help you,
but I would like to know which GoTo mount you have for your telescope?
Is it an open interface?

Peter
 
matherp
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Joined: 11/12/2012
Location: United Kingdom
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Posted: 12:07pm 26 Feb 2026
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The sidereal time is returned by GPS(SIDEREAL) but there isn't currently an option to  get it from a defined location and time without a GPS
 
Frank N. Furter
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Joined: 28/05/2012
Location: Germany
Posts: 1095
Posted: 07:00pm 26 Feb 2026
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@PilotPirx:

The telescope mount is an old Vixen Super Polaris:
https://www.photoinfos.com/astronomie/VixenSuperPolaris.htm

The mount and telescope are mounted on a portable column.
The main tube itself is a Newtonian with a 5 1/4" inch mirror.
My mount is still equipped with two stepper motors, and I have a simple electronic tracking system for it.

Maybe I can upload a picture of it here sometime (if anyone is interested)...

I also have a kadadioptrically shortened 5 1/4" inch Newtonian telescope - it would be interesting to build a tracked, azimuth mount for it. It would be great to control it with the new commands.


@matherp:

Thank you for your reply! Is there no way to trick the system into thinking it has GPS? (e.g., by sending an artificially generated GPS telegram from one hardware pin to another?)

I have already considered using a routine to change the right ascension until the azimuth outputs 180° as the result. Then the entered right ascension should correspond to the sidereal time.


Frank
 
matherp
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Posted: 07:41pm 26 Feb 2026
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In the next beta I'll add a parameter to the location command that returns the sidereal time for the time specified
 
Frank N. Furter
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Posts: 1095
Posted: 07:50pm 26 Feb 2026
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That's great! Thank you very much!!!      

Frank
 
matherp
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Posted: 02:48pm 06 May 2026
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Frank

I'm in the process of implementing a SLEW command into the astro code. This will take the current DEC and RA ( assuming polar aligned equatorial mount) and output the moves requred to move to a target new RA/DEC. The code is intended to handle meridian flip.
Please could you review this concept:

  Quote  New signature

SLEW dRA1, dDec1, flipRA, flipDec, dRA2, dDec2, mountRA, mountDec, RAs, DECs, LST

Takes 5 input parameters:
Current position - mountRA/mountDEC
Desired new position - RAs/DECs
Sidereal Time - LST

6 outputs
Initial slew : deltaRA1, deltaDEC1
If flip is needed :
flip direction - flipRA, flipDec
second slew deltaRA2,deltaDEC2

Then the Basic code looks like this

LOCATE date$, time$, lat, lon, lst
ASTRO Saturn alt, az, ra, dec
SLEW dra1,ddec1, flipra,flipdec, dra2,ddec2, mount_ra,mount_dec, ra,dec, lst

drive_RA(dra1) : drive_Dec(ddec1)

IF dra2 <> 0 THEN
 drive_Dec(flipDec)            ' physical flip
 drive_RA(flipRA)              ' RA compensation
 drive_RA(dra2)            ' fine correction to target
END IF

mount_ra = ra : mount_dec = dec


Note ddec2/deltaDEC2 is always zero but is there for consistency

Edited 2026-05-07 00:50 by matherp
 
Frank N. Furter
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Posted: 06:22pm 07 May 2026
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Peter,

somehow, I don't quite get the point yet.
So I have a mounted telescope that is pointed at an object. The sidereal time corresponds to the star that is currently at its highest point in the sky.
To calculate the coordinates of a new celestial object with known right ascension and declination, all I have to do is find the difference between them, right?

So what does your SLEW command do, or what is the benefit of using it?

Frank
 
matherp
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Posted: 06:43pm 07 May 2026
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Here is the explanation. Obviously you are correct if no flip is needed but the move is complex if it is. The new function covers both. My scope will be completely computer controlled so if I ask it to point to a new object it needs to know the full process for that. Likewise, even tracking a flip is needed if the meridian is crossed.

  Quote  A telescope flip (usually called a meridian flip) is a manoeuvre that a German Equatorial Mount (GEM) must perform when a target crosses the local meridian. It’s a geometric necessity of the mount design, not a software quirk.

⭐ The core idea
A GEM cannot track an object indefinitely on one side of the sky because the telescope tube will eventually collide with the mount or tripod.
So when the target crosses the meridian (the north–zenith–south line), the mount must:

Move the telescope to the opposite side of the RA axis while keeping it pointed at the same coordinates.
That is the flip.

🧭 Why the flip is required
A GEM tracks by rotating the Right Ascension (RA) axis.
But:

The RA axis has limited rotation before something hits something else

The counterweight bar must always point down for stability

Cables, camera, and focuser all have physical limits

If the mount kept tracking past the meridian without flipping, you’d get:

OTA hitting the tripod

Counterweights rising above the telescope

Cable strain or motor housing collision

So the flip is a mechanical safety requirement.

🔧 What physically happens during a flip
A meridian flip is a two‑axis manoeuvre:

1. RA rotates by exactly 180° (12 hours)
This moves the telescope from the east side of the mount to the west side (or vice‑versa).

2. DEC rotates to re‑acquire the same declination
Because flipping RA alone points the telescope to the mirror position, the DEC axis must rotate to bring the telescope back onto the original target.

After these two motions, the telescope is pointing at the same RA/DEC, but the tube is on the opposite side of the mount.

🕒 When the flip happens
A flip is triggered when:

The target’s Hour Angle becomes slightly positive (i.e., it has crossed the meridian)

Or the mount hits a user‑defined limit (e.g., “track 5° past meridian”)

Edited 2026-05-08 04:54 by matherp
 
Frank N. Furter
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Joined: 28/05/2012
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Posts: 1095
Posted: 08:11pm 07 May 2026
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Okay, that makes sense! I think this command can be very useful.

Frank
 
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