Synscan, Polar alignment and Star alignment

[matthasboldlygone]

Using Synscan on a HEQ5 Pro mount. Would a very good 3 star alignment compensate for a not so perfect Polar Alignment. Will the software work out it's not perfectly polar aligned.

I've seen youtubers saying just point to N on the tripod roughly towards Polaris, set your rough Latitude and then spend the time on the star align, while others are saying that is wrong and you MUST start with a very good polar alignment.

Cheers

[Freddie]

PA and star alignment are not related. PA aligns your mount and allows it to accurately track by just rotating in RA. Star alignment just tells the mount where the stars are. Having good star alignment WILL NOT compensate for poor PA in any way.

[Ronclarke]

If you have a Polarscope on the mount, check using Polaris's positon on app like 'polar scope' and get as near as you can on the circle!

I used this for 5 years and did AP with it!

Good Luck

Ron

[Jiggy 67]

1 hour ago, Freddie said:

PA and star alignment are not related. PA aligns your mount and allows it to accurately track by just rotating in RA. Star alignment just tells the mount where the stars are. Having good star alignment WILL NOT compensate for poor PA in any way.

Not strictly true.

If you read the Synscan manual it clarifies the position on star alignment.

For a one star alignment accurate polar alignment is required. I would suggest that with accurate PA Synscan knows where it is starting from better.

For a two star alignment less accurate polar alignment is required, I assume because it can correct any errors in PA with the second star

A three star alignment is not required at all unless you suffer from cone error, the only purpose of the third star is to correct for cone error and has no impact on pointing accuracy with no or little cone error

Although it is correct that good star alignment does not compensate for poor PA

Edited July 15, 2020 by Jiggy 67

[Freddie]

The mount will only drive in RA when it is tracking. If the mount is not accurately Polar aligned no amount of star aligning will be able to compensate for poor PA. Poor PA will cause a drift in DEC over time and as the mount will not drive DEC when tracking, star alignment obviously can’t overcome that!

[RayD]

2 minutes ago, Jiggy 67 said:

Not strictly true.

If you read the Synscan manual it clarifies the position on star alignment.

For a one star alignment accurate polar alignment is required. I would suggest that with accurate PA Synscan knows where it is starting from better.

For a two star alignment less accurate polar alignment is required, I assume because it can correct any errors in PA with the second star

A three star alignment is not required at all unless you suffer from cone error, the only purpose of the third star is to correct for cone error and has no impact on pointing accuracy with no or little cone error

But that is only accounting for the PA error when identifying star positions, it isn't correcting PA error when tracking, as Freddie notes.  Doing a 2 or 3 star alignment just means the stars will be close to the centre of the reticle when slewing between them but, if the PA is out, the stars will soon move out of centre when tracking them.

[Jiggy 67]

1 minute ago, RayD said:

But that is only accounting for the PA error when identifying star positions, it isn't correcting PA error when tracking, as Freddie notes.  Doing a 2 or 3 star alignment just means the stars will be close to the centre of the reticle when slewing between them but, if the PA is out, the stars will soon move out of centre when tracking them.

I'm not disputing that or what Freddie says, I'm just pointing out that PA and star alignment ARE related, whereas Freddie said they aren't, the better your PA, the more accurate your star alignment and the fewer stars you have to use

[RayD]

2 minutes ago, Jiggy 67 said:

I'm not disputing that or what Freddie says, I'm just pointing out that PA and star alignment ARE related, whereas Freddie said they aren't, the better your PA, the more accurate your star alignment and the fewer stars you have to use

Ok I see what you're saying.  However, the OP specifically asked "Would a very good 3 star alignment compensate for a not so perfect Polar Alignment".  Well the answer is no, in this respect they are unrelated as star alignment has no effect whatsoever on polar alignment and the stars will soon move out of the FOV if it's a long way out.

[Jiggy 67]

4 minutes ago, RayD said:

Ok I see what you're saying.  However, the OP specifically asked "Would a very good 3 star alignment compensate for a not so perfect Polar Alignment".  Well the answer is no, in this respect they are unrelated as star alignment has no effect whatsoever on polar alignment and the stars will soon move out of the FOV if it's a long way out.

I apologise for confusing things.....though in my defence, in my original post I agreed with Freddy that good star alignment does not compensate for poor PA

[RayD]

Just now, Jiggy 67 said:

I apologise for confusing things.....though in my defence, in my original post I agreed with Freddy that good star alignment does not compensate for poor PA

Not an issue and no defence needed, we are all just trying to offer help.  PA and star alignment is something that often come up and, as basic as it seems, can be pretty daunting and confusing when starting out as they do often appear to go hand in hand.

[AstroMuni]

1 hour ago, Freddie said:

The mount will only drive in RA when it is tracking.

Is this the case only while using Syncscan or with EQMOD direct connection (not via the handcontroller) as well?

Edited July 15, 2020 by AstroMuni

[Freddie]

The mount will only drive DEC if it is being guided. 

[AstroMuni]

9 minutes ago, Freddie said:

The mount will only drive DEC if it is being guided. 

I learn something new each day 🙂

[matthasboldlygone]

Good exchange of ideas here. Consensus is the Polar Alignment is really important and should be done as accurately as possible. I guess I was looking for shortcuts, next clear night I'll be out crawling around under the scope trying to peek through the PA scope and twiddling knobs and screws 👍

[Freddie]

Accurate PA is very important for imaging. Close will however do if you are just visual.

[jambouk]

Close PA is also “OK” for imaging if taking short subs and/or using a short focal length. There is accurate and there is accurate. If your polar scope is well aligned to the R.A. axis of the mount, then a polar scope alignment is generally good enough for 30 second subs with a scope or lens under 1000mm in my experience, unless you are going for astrophotographer of the year. 
James

[Jiggy 67]

 

3 hours ago, matthasboldlygone said:

next clear night I'll be out crawling around under the scope trying to peek through the PA scope and twiddling knobs and screws

You don’t have to crawl around looking through the polar scope, just get one of these, they’re a God send 
 

https://www.firstlightoptics.com/adapters/astro-essentials-right-angled-eyepiece-for-polar-scopes.html

[cletrac1922]

16 hours ago, matthasboldlygone said:

Using Synscan on a HEQ5 Pro mount. Would a very good 3 star alignment compensate for a not so perfect Polar Alignment. Will the software work out it's not perfectly polar aligned.

I've seen youtubers saying just point to N on the tripod roughly towards Polaris, set your rough Latitude and then spend the time on the star align, while others are saying that is wrong and you MUST start with a very good polar alignment.

Cheers

Matt

Where I am we do not have a Polaris, like the northern hemisphere to polar align

With my EQ5pro mount, what I do is have the north leg facing south, and using a compass align the mount allowing for 12 deg east, magnetic variation

Then do a 2 stay alignment, and I am within a couple of degrees of object want to observe

John

  

[AstroMuni]

6 hours ago, cletrac1922 said:

With my EQ5pro mount, what I do is have the north leg facing south, and using a compass align the mount allowing for 12 deg east, magnetic variation

Thats an interesting one. 👍 The manual states that you can do alignment using Octans and I thought thats what would be done down south 🙂

[AstroMuni]

On 15/07/2020 at 11:57, matthasboldlygone said:

Using Synscan on a HEQ5 Pro mount. Would a very good 3 star alignment compensate for a not so perfect Polar Alignment. Will the software work out it's not perfectly polar aligned.

Over and above what has been discussed so far.....

I just re-read the manual. Looks like the software does figure out its not properly aligned and it allows you to fix it as well. This is section taken from Synscan manual

• 11.3 Polar Alignment without Polar Scope
• The polar alignment function can help users to polar align an equatorial mount accurately.
• Here are the operating instructions:
• Complete a 2-star alignment or a 3-star alignment. At the end of the alignment, the Syn- Scan hand control will display the polar alignment error (refer to Section 3.3). Users can use the data to determine whether it is necessary to adjust the polar alignment.
• Press the “MENU” shortcut key, and then access to sub-menu “Alignment\Polar Alignment ”, press the ENTER key to proceed to the next step. .......

etc.......

 

[matthasboldlygone]

55 minutes ago, AstroMuni said:

Over and above what has been discussed so far.....

I just re-read the manual. Looks like the software does figure out its not properly aligned and it allows you to fix it as well. This is section taken from Synscan manual

0. 11.3 Polar Alignment without Polar Scope

   The polar alignment function can help users to polar align an equatorial mount accurately.

   Here are the operating instructions:

   1. Complete a 2-star alignment or a 3-star alignment. At the end of the alignment, the Syn- Scan hand control will display the polar alignment error (refer to Section 3.3). Users can use the data to determine whether it is necessary to adjust the polar alignment.

   2. Press the “MENU” shortcut key, and then access to sub-menu “Alignment\Polar Alignment ”, press the ENTER key to proceed to the next step. .......

etc.......

 

That's an interesting one if it tells me how far out it is, maybe I'll spend some time today RTFM

 

[kfong]

On 15/07/2020 at 04:15, Freddie said:

PA and star alignment are not related. PA aligns your mount and allows it to accurately track by just rotating in RA. Star alignment just tells the mount where the stars are. Having good star alignment WILL NOT compensate for poor PA in any way.

What you have said in just 4 sentences clarifies what I could not find in many manuals...from a beginner, THANK YOU for this valuable clarification.

[Freddie]

A blast from the past but glad the comment helped.