Is one star alignment enough

[AstroRookie]

Hello,

is the following procedure any good; my last imaging session (dates back from mid september, cloudy nights ever since ...), I had a go on the Andromeda galaxy. This is how I proceeded:

• searched beta Andromeda, and synced to it
• searched Andromeda galaxy and synced to it
• back to beta Andromeda to use it as the star to focus on
• back to Andromeda and start imaging

I was wondering now, will the guiding software (phd2 in my case) have to correct more then if I would have aligned on 3 stars?

AstroRookie

[ollypenrice]

13 hours ago, AstroRookie said:

Hello,

is the following procedure any good; my last imaging session (dates back from mid september, cloudy nights ever since ...), I had a go on the Andromeda galaxy. This is how I proceeded:

• searched beta Andromeda, and synced to it
• searched Andromeda galaxy and synced to it
• back to beta Andromeda to use it as the star to focus on
• back to Andromeda and start imaging

I was wondering now, will the guiding software (phd2 in my case) have to correct more then if I would have aligned on 3 stars?

AstroRookie

Absolutely not. The 3 star alignment is entirely internal to the mount and is concerned with helping the mount find the target accurately. On a short hop from Beta Andromeda to M31 a single star alignment will be plenty good enough to locate the target. Once found, the mount's internal planetarium has no further role in the proceedings.

PHD will lock onto you chosen star and follow it. The only thing PHD needs is for your polar alignment to be accurate. It cannot correct for that. If your mount isn't polar aligned accurately enough the guidestar will be held in place by PHD but the image will slowly rotate around it during the shoot. The longer the subs you take the better the PA has to be. Slight rotation between subs will be corrected by the stacking software.

Olly

[michael.h.f.wilkinson]

I have frequently not used any star alignment on my Great Polaris mount with Synscan GoTo upgrade, because finding the target was easy by star hopping. As long as the PA is good, the scope tracks solidly, and guiding is easy.

[carastro]

I always do a one star align, just pick a bright star close to your target.

Carole 

[gilesco]

So from what Olly has said, it sounds as if I don't actually have to do a star alignment at all? Once Polar Aligned I can just slew somewhere and Plate Solve my position?

[Budgie1]

35 minutes ago, gilesco said:

So from what Olly has said, it sounds as if I don't actually have to do a star alignment at all? Once Polar Aligned I can just slew somewhere and Plate Solve my position?

Yep, my normal routine is as follows:

• Polar align with SharpCap
• Use Polaris to focus the camera
• Use Carte de Ciel to slew to where it thinks my target is
• In APT I take an image and Plate Solve the image, then sync the mount/CdC and use the GoTo++ facility in APT to centre the target
• Start PHD2 guiding and start the imaging plan in APT

I think you can skip my third item and just use APT to slew to target after an initial plate solve and sync, but I've not tried that as I like having CdC running.

[gilesco]

6 minutes ago, Budgie1 said:

Yep, my normal routine is as follows:

• Polar align with SharpCap
• Use Polaris to focus the camera
• Use Carte de Ciel to slew to where it thinks my target is
• In APT I take an image and Plate Solve the image, then sync the mount/CdC and use the GoTo++ facility in APT to centre the target
• Start PHD2 guiding and start the imaging plan in APT

I think you can skip my third item and just use APT to slew to target after an initial plate solve and sync, but I've not tried that as I like having CdC running.

So on my CGX, I have, and certainly if I'm doing visual observing without a PC, usually done a 2-star alignment, and then it asks me for up to 4 calibration stars. Are those calibration stars just fine tuning the alignment, or do they serve as some sort of error correction for the motors (like a rough PEC type thing)?

[Budgie1]

4 minutes ago, gilesco said:

So on my CGX, I have, and certainly if I'm doing visual observing without a PC, usually done a 2-star alignment, and then it asks me for up to 4 calibration stars. Are those calibration stars just fine tuning the alignment, or do they serve as some sort of error correction for the motors (like a rough PEC type thing)?

If I was doing visual observing without a PC then I would do a 2 or 3 star alignment so the mount is as accurate as possible with the on-board GoTo function for star hopping. I use a Skywatcher mount and I don't remember the Synscan handset asking for calibration stars after doing a 2 or 3 star alignment, so I'm not sure.

Since starting back into astro imaging I've always had the mount connected to the laptop. When I first started I used CdC to do the alignment, only using the hand controller for fine adjustment. Now I use the EQDIR cable, so I don't have the hand controller connected at all.

[bottletopburly]

If your platesolving syncing on stars goes out the window, I use APT and  use astap as the platesolving program , using go to++ is bang on target 🎯 

[gilesco]

4 minutes ago, Budgie1 said:

If I was doing visual observing without a PC then I would do a 2 or 3 star alignment so the mount is as accurate as possible with the on-board GoTo function for star hopping. I use a Skywatcher mount and I don't remember the Synscan handset asking for calibration stars after doing a 2 or 3 star alignment, so I'm not sure.

Since starting back into astro imaging I've always had the mount connected to the laptop. When I first started I used CdC to do the alignment, only using the hand controller for fine adjustment. Now I use the EQDIR cable, so I don't have the hand controller connected at all.

Yes, when I was visually I was doing the 2 star + 4 calib all the way, by the 3rd or 4th Calibration it was bang on target and needed no adjustment. I just wondered as the default calibration star selections seem to be all over the sky, so just wondered if it was doing anything more than just fine tuning the perceived position.

[ollypenrice]

1 hour ago, gilesco said:

So from what Olly has said, it sounds as if I don't actually have to do a star alignment at all? Once Polar Aligned I can just slew somewhere and Plate Solve my position?

That's right. The plate solution will tell the mount where it's pointing and that's all it needs to know in order to find the target and return to it if necessary. The key thing is that Tracking and guiding have nothing to do with target finding. They are independent processes.

BTW, my Mesu mounts only have a one star alignment option anyway.

Olly

[Budgie1]

13 minutes ago, gilesco said:

Yes, when I was visually I was doing the 2 star + 4 calib all the way, by the 3rd or 4th Calibration it was bang on target and needed no adjustment. I just wondered as the default calibration star selections seem to be all over the sky, so just wondered if it was doing anything more than just fine tuning the perceived position.

Thinking back about 8 years, when I used my Celestron 127SLT, I seem to remember the instructions for the Nexstar controller suggested a 3 star alignment using stars as far apart as possible and in different areas of the sky. This may have had the calibration stars on it as well, I don't remember, but I know by the third star the alignment was about spot on. So I can only think that the 4 extra calibration stars on you CGX are just, like you say, to fine tune the calibration.