Astroart Forum

Hello,
If I am correct, when saving Plate solve parameters the Region size(RA and DEC) values and the Reference stars value are not saved.
So, having plate solved and saved the parameters, I later do a Telescope control GOTO. I insert J2000 RA and DEC values, tick the Jnow box and press Goto.
I then open the Centre tab (I will Center using Telescope coordinates then Exposure and Centre which includes Plate solving):
Reference stars is showing (a Default value? of) 5 and the Search zone is limited to one of the four available: 1 x 1, 2 x 2, 3 x 3 and 4 x 4.
Question 1: Under what conditions would one choose a different number of stars to that used for a successful plate solve?
Question 2: Why only those specific Search zone values?
Question 3: If I also ticked the Jnow box would the coordinates be precessed a second time?
Question 4: Under what circumstances would someone select this Jnow?

There is has obviously been provision made for more functionality or situations than meets the eye?
Regards
Patrick.

Hello,
1) With only 4 stars there's the risk of a false positive. However if the field of view is small, or for some other reason it's not possible to get consistently more stars, there's no choice. The number of stars is different from the Plate Solve window because the Centre window usually works with short exposures. 5 stars is a good compromise. 6 stars would be ideal if it's always possible to get them (even near the galactic pole).

2) To help avoiding false positives. If a GOTO is off by several degrees there's a problem anyway. In this case, after having solved the problem, use the Plate Solve window with a higher zone (example 50x40 degrees) then Sync the telescope with the option "Last image".

3) No, JNow is just a kind of coordinate system.

4) If the telescope is working in JNow.
Ideally a telescope should work in J2000 (i.e. it receives and sends J2000 coordinates, converts them and moves the mount correctly). Several older telescopes cannot do that.

Fabio,
Thank you for your rapid and fulsome response - very clear now.
If one does a GOTO using Jnow coordinates it makes sense to me to plate solve to Jnow coordinates during the Centre process. In fact after a GOTO using Jnow coordinates, if Jnow is not selected at the Centre page a warning is given.

A final question if I may? I use the GAIA catalogue which has J2000 coordinates. Does this mean that plate solving is actually done in J2000 coordinates and then corrected to Jnow when required?
Patrick.

Patrick wrote: ↑

31 Oct 2022, 23:39

Fabio,
Thank you for your rapid and fulsome response - very clear now.
If one does a GOTO using Jnow coordinates it makes sense to me to plate solve to Jnow coordinates during the Centre process. In fact after a GOTO using Jnow coordinates, if Jnow is not selected at the Centre page a warning is given.

A final question if I may? I use the GAIA catalogue which has J2000 coordinates. Does this mean that plate solving is actually done in J2000 coordinates and then corrected to Jnow when required?
Patrick.

Astroart's platesolving engine uses a highly compressed file: Astroar8.dat, which is probably derived from the GSC. I'd guess that it uses J2000 coordinates.

Hi, everything about plate solving and astrometry is J2000.

If your telescope needs to work in JNow, then enable the options "JNow" both in the Goto and in the Center page of the telescope window. Probably in the future it will remain only one option for JNow, currently that is just a duplication.

Dunnmunro,
Thanks for replying.
As I said I use the GAIA Catalogue.
I am not into guessing.
Regards,
Patrick.

Fabio,
Thank you.

Dunmunro wrote: ↑

01 Nov 2022, 19:17

Patrick wrote: ↑

31 Oct 2022, 23:39

Fabio,
Thank you for your rapid and fulsome response - very clear now.
If one does a GOTO using Jnow coordinates it makes sense to me to plate solve to Jnow coordinates during the Centre process. In fact after a GOTO using Jnow coordinates, if Jnow is not selected at the Centre page a warning is given.

A final question if I may? I use the GAIA catalogue which has J2000 coordinates. Does this mean that plate solving is actually done in J2000 coordinates and then corrected to Jnow when required?
Patrick.

Astroart's platesolving engine uses a highly compressed file: Astroar8.dat, which is probably derived from the GSC. I'd guess that it uses J2000 coordinates.

Hi, it would be really useful to know which catalogue the plate solving engine is based on as for astrometry measures of comets and minor planets to the MPC, it is required to mention the source.

Regards
Andrew

Hi, the plate solving engine cannot be used to send positions to the MPC, it's too risky since the precision will be unknown. It's designed only for speed and reliability, so only for telescope centering.

Once the image is plate solved, the procedure is:

1) Click the Atlas icon to center the Star Atlas to the image coordinates.
2) If needed, zoom the Atlas (mouse wheel) to display more or less the same field.
3) In the Atlas, click the icon "Reference stars automatic".
4) Select 50-200 stars and click OK.
5) Click the icon Astrometry, verify the O-C, enable Optical distortions if needed, press OK.

So, the command Astrometry deletes the previous plate solve and replaces it with a good quality one.

Now the image is ready for the MPC: the OCs are verified, and the latest catalog is used (the one selected in the Atlas).

This is a manual procedure which takes about 10 seconds. If really needed it could be made fully automatic with a script command, which could be added into a future Service Pack.

Excellent, thanks Fabio. That’s exactly the process I follow. I was just unclear what catalogue the astrometry was based around but you’ve clarified that it’s the one used by the Star Atlas.

Andrew