Not understanding EQMOD home position

[johneta]

Hi

Ive just started using EQMOD with AstroEQ, PHD2 and stellarium.
Everything seems to be talking OK but I dont understand the home position.
Im new to using DEC and RA coordinates also.

-I think home position is facing South  (Im in southern Hemisphere) along Meridian line, and with weights bar straight up and down. -Polar Aligning position

So thats DEC 90degs and RA is the Hrs that are currently at my location for the date and time of day.
I can get this off stellarium. But when I look at the Current RA in EQMod it differs from Stellarium.
(I have checked and both are set to the same Lat/Long and date/time)

For example, When I just checked, stellarium said it was around 1 Hr 50 mins

But EQmod (after just parking it) says mount is at around 19 Hr 50mins--this RA line goes through about East at this time on Stellarium.

Am I missing something or is there somewhere you can recommend I go to read up on this in order to learn me good

 

Thanks very much

John D

Edited September 1, 2019 by johneta

[Merlin66]

John,

similar system, but I use Cartes du Ceil.

when you park and unpark the coordinates should match.

 

 

 

 

[almcl]

4 hours ago, johneta said:

 

-I think home position is facing South  (Im in southern Hemisphere) along Meridian line, and with weights bar straight up and down. -Polar Aligning position

 Yes, that's right, 

4 hours ago, johneta said:

So thats DEC 90degs and RA is the Hrs that are currently at my location for the date and time of day.

Yes to the first bit (Dec 90) but at Dec 90 all the RA lines meet (see image below - northern hemisphere but the principle is the same) so any RA is correct if you are pointing directly at the pole:

[johneta]

Thanks

almcl  -"Yes to the first bit (Dec 90) but at Dec 90 all the RA lines meet (see image below - northern hemisphere but the principle is the same) so any RA is correct if you are pointing directly at the pole:"

 

So essentially the RA reading given by EQMOD at park (pointing at Pole) is random. I should ignore it and just give it a try -     GOTO an object?

[Merlin66]

I'm pretty sure that the polar home position assumes you have c'weights straight down, which means you're sitting along the meridian line where the RA is equal to the LST.

Think about it, the motors need to know where the starting point is (in RA and DEC directions) to issue subsequent commands to move delta Dec and delta RA during slew.

This should be confirmed when you look at the motor counter position (top left corner of the RA/Dec position display - just click to go through the options)

 

 

[johneta]

OK that makes sense. I just dont understand how the mount knows what hr line its pointing at because as almcl said- all the RA lines  combine at the SCP.

Heres an example of what I was playing with (without success) tonight:

Im at S 45 52' odd   and E 170 35'  time was 10 30pm local time. So when I look at Stellarium, the meridian running from SCP to the ground (South) is about 8hr
The line going up North towards North Pole is around 20hr.

Stellarium tells me the star Fomalhaut is at 22h 58m. -29degs 31'

So I start manually slewing to those coordinates in EQMOD watching the eqmod readout and watching the mount is going the right direction.
Home position says -Dec 90 degs and RA 2hrs odd ( but as above this RA figure is meaningless at the SCP?? )
I slew some way toward Fomalhaut in DEC then I slew a bit in RA, in the direction of the star.
As soon as I start slewing the RA in the direction toward Fomalhaut, the RA figure in EQMOD jumps to 14Hr odd. It makes sense that it could jump as it is now on a RA line away from SCP.

BUT I dont quite get how it knows what that should be. --from LST I guess.

Any way, I slew to DEC -29deg 31' - looks close. Then I slew in RA but I get to around RA 18hr and the star is lined up. Stellarium is telling me it should be 22hr 58' about 5hr difference.
BUT DEC seems to be bang on.

Also the EQMOD readout is telling me that the scope is east side pointing west I think?-(gear is packed away now)--but Im actually the reverse

In my mind there needs to be some sort of calibration to tell the mount where to start the RA hours and also what side of the mount the scope is on.

Sorry for all the detail -its hard to explain all this in words.

[johneta]

I had a thought.
I seem to be out about 5hrs.
I was only taking rough readings to speed up the process.
if its more like 6hrs out then 6hrs is 1/4 of 24hrs. And 1/4 of 360degs is 90 degs.
Is home position - counterweight bar horizontal - 90 degs difference.
I will check out.

[Merlin66]

I use EQMOD with Cartes du Ceil. 

The RA Dec of the target always match in EQMOD and CdC.

The home position, is definitely c’weigh straight down scope pointing to the SCP.

 

[symmetal]

In the standard Home position, weights down facing North (or South in the Southern Hemisphere) and just powered up with no sync alignment points set, the Eqmod RA reading will be 6 hours out from the RA of the Meridian and the Dec reads 90 degrees. This is because whenever it is unparked and the scope slews to point to an object on the Meridian the counterweight bar is always horizontal. The Eqmod RA then reads correct. If you unpark and slew to a point on the meridian very close to the celestial pole the scope just rotates 90 degrees in RA until the bar is horizontal.

In reality, in your home position the scope will not be pointing exactly at the CP. Eqmod assumes it is when it's powered up first time and uses this to work out where to go on your first Goto. When you centre the target and issue a 'Sync' command the pointing model will note the error from your first Goto. Other Sync commands from different Gotos will add more points to this pointing model to help counteract the fact that your mount may not be perfectly polar aligned. If it is perfectly aligned then only one Sync command after your first Goto is necessary.

When you then 'Park' the scope it returns to its home position. Because of your 'Sync' modified, and therefore more accurate pointing model, the physical home position is not pointing exactly at the CP, and Eqmod will report where it now believes it's pointing so the Dec probably reads around 89 degrees or so. The RA now reported will be 6 Hrs different from the RA Eqmod believes it's pointing to, which can be any RA value between 0 and 24 hours depending in which direction your scope home position in reality deviates from actually pointing to the CP. The closer the Home position is to actually pointing to the CP, the more 'random' this Home RA reading will appear to be. This RA reading however does tell Eqmod (along with the Dec reading) in which direction the Home position is off from actually pointing to the CP and uses it for future Gotos to make them more accurate.

The physical pointing 'accuracy' of the Home position isn't important, as you can tell Eqmod that pointing to anywhere in the sky is the 'Home' position and it will use that for the first Goto. The more inaccurate your 'standard' Home position is from actually pointing to the CP just means the first Goto will be further away from the target. Centreing the target and 'Syncing' will then correct this Home position error.

There is nothing 'special' astronomically speaking about the standard Home position. It's just a safe position to leave the scope. You can easily remove the scope without worrying that the counterweights might inadvertantly swing around under gravity and cause damage or injury.  

If you delete the alignment points then the 'standard' Home position is again assumed to be pointing at the CP and the Dec again reads 90 and the RA is 6 hrs from the Meridian.

Alan  

Edited September 2, 2019 by symmetal

[johneta]

Thanks 
Merlin 66 -- when you say RA of EQMOD and cdc always match - what does the RA match to? The meridian hr line from the CP to the ground?

symmetal-- I partially understand what your saying. I will read again later. - So if I choose my first star to sync with and its 6hrs wrong and I correct it in eyepiece, and then sync it, will this big 6hr error be a problem?

I assume the second star I choose will be substantially more accurate, and the third more so etc?       Note my PA is very good as I have a permanent setup.

[symmetal]

2 hours ago, johneta said:

symmetal-- I partially understand what your saying. I will read again later. - So if I choose my first star to sync with and its 6hrs wrong and I correct it in eyepiece, and then sync it, will this big 6hr error be a problem?

The first star you sync on shouldn't be 6 hours wrong. It's only the home position which reports this 6 hour difference as the home position has the counterweight bar in the wrong position when viewing on the meridian. As soon as you do your first goto from park Eqmod should automatically correct this 6hr RA 'difference'. As I mentioned above if you goto an object on the meridian from park the scope rotates RA by 90 degrees (6 hours) automatically to make the RA correct. If you goto an object which isn't on the meridian it will automatically rotate RA the correct amount from park (between 0 and 90 degrees as required) to report the correct RA on the target.

As you have a permenant setup with good PA your first goto should put the object fairly well centred in the eyepiece if you use a modest focal length eyepiece to start with. Centering the object and syncing should be all you need to do.

On your previous post you were slewing manually but that shouldn't make any difference to the Eqmod readout. For a first test just slew to a target that's close to the meridian but away from the Celestial Pole and you should find your counterweight bar is now near horizontal and the RA reads correct.

 I shouldn't worry too much about the Side of Pier settings.  It's terminiology is confusing. The first line of the Ascom Standards Side of Pier states

Quote

For historical reasons, this property's name does not reflect its true meaning. The name will not be changed (so as to preserve compatibility), but the meaning has since become clear.

I believe with the latest EQAscom, the  Side of Pier is best left set to 'Pointing'.

Alan

Edited September 3, 2019 by symmetal

[johneta]

Im at work so cant give it to much thought at the moment, but I was doing that GOTO last night and was always way off.

Will try again 

[Merlin66]

John,

I gave you some misinformation.....

1. The counters on the motors give a Dec position and an RA position based on their "zero" position - usually set at the home position.

2. When in the home position (pointing at SCP c'weights down) the Dec counter shows "-90 deg" and the RA counter "180 deg ( - the azimuth reading for the meridian.)

3. The RA shown is six hours above the LST.

4. When you slew to a target, the RA and Dec increment to the target settings (and agree with CdC)

5. When you re-park to home position - the counters return to the readings as per 2 above and park RA is incremented to LST plus 6 hours.

See the attached images.

Read through the EQMod manual (in the EQMod folder) for details on how it handles GOTO accuracy - based on triangles of known points.

 

[johneta]

Thanks again everyone for the great advice.

I've got it sorted now.   Yay!

As I mentioned I am just newly running AstroEQ with EQMOD and Stellarium. And I have used it successfully with PHD.

The problem was my motor direction was the wrong way 🙄.

So the AstroEQ is brand new, (originally inspired by almcl in another post)  I have built and installed the electronics and the stepper motors  etc.
I had the guiding working well with PHD. Next step was to get GOTO working.
In the AstroEQ Config settings you can choose the directions of the 2 motors (forward or backward) as of course initially the software doesn't know which direction they are rotating the mount.
 I took a stab at the direction that seemed right.   When I came to testing out the GOTO - because of my lack of understanding about RA/DEC coordinates
and the initial slew movements away from the polar position, -that initial movement away from the SCP seemed like odd movement to me and really threw me.

It looked like it was going the opposite direction toward the star that it should. So I updated the Config a few times, reversing the direction of the motors, both together and individually. But I couldn't see a movement that seemed sensible to me.
So I chased my tail for a bit until your explanations of the initial 6hr difference and the mount slewing 90degs in RA if aiming at an object on the Meridian etc.

I found I needed the RA motor Forward and the DEC motor Backwards and all came right. I synced 4 or 5 stars easily and GOTO was very accurate.

So thanks again, your help is much appreciated. When you're going round and round in circles it can seem impossible at times.

I hope some of this can be of use to others too.

Regards John D

[symmetal]

Phew! Glad you got it sorted out John D. I admit I'd glossed over the fact you were using AStroEQ and that the basic mount parameters are therefore unknown to EqAscom/Eqmod and need to be initially set.

At least you've learned a bit about Home positions now that you wouldn't have been aware of, if everything had worked first time. 

Alan

[johneta]

Agreed I thought the same. Good to be on the learning curve.
Makes it more fun, if a little taxing.

[almcl]

17 hours ago, johneta said:

So the AstroEQ is brand new, (originally inspired by almcl in another post)  I have built and installed the electronics and the stepper motors  etc.
I had the guiding working well with PHD. Next step was to get GOTO working.
In the AstroEQ Config settings you can choose the directions of the 2 motors (forward or backward) as of course initially the software doesn't know which direction they are rotating the mount.
 I took a stab at the direction that seemed right.   When I came to testing out the GOTO - because of my lack of understanding about RA/DEC coordinates
and the initial slew movements away from the polar position, -that initial movement away from the SCP seemed like odd movement to me and really threw me.

I found I needed the RA motor Forward and the DEC motor Backwards and all came right. I synced 4 or 5 stars easily and GOTO was very accurate.

Well done, once you get it configured, AstroEQ is a great bit of kit isn't it? 

[symmetal]

Another way to confirm that the RA in the Home position is actually pointing East/West, is if you unpark and slew to an object in the East or West. The counterweight bar doesn't move. It just slews in Dec. In the Northern Hemisphere my home position RA reports it as facing West. From your first post it appears it faces East in the Southern Hemisphere. 

Alan

[johneta]

Yeah AstroEQ seems great. I converted my SW EQ5 mainly to try and tame backlash and get better RA accuracy. Backlash improved alot (still bad because of worm gear though) and RA figures in PHD are OK now, and more importantly more stable overall, so I did my first 2 nights of imaging with the new kit and only had 1 bad shot. (5min subs)

I certainly recommend it, although there is a heck of alot of little hurdles to overcome to get it running. Thats Astro I guess though.

Edited September 5, 2019 by johneta

[johneta]

I'll try PEC with EQMOD next 😬

[johneta]

Alan - wouldnt it only slew in DEC,  if I GOTO an object in east or west that happened to be perfectly lined up with that arc of the DEC?

if it was off just a bit , it would have to also turn the RA axis as well to get to it ??       uh oh.. not understanding again 🙂

[Merlin66]

John,

The mount will slew in both RA and Dec to find the target star.

[symmetal]

2 hours ago, johneta said:

Alan - wouldnt it only slew in DEC,  if I GOTO an object in east or west that happened to be perfectly lined up with that arc of the DEC?

if it was off just a bit , it would have to also turn the RA axis as well to get to it ??       uh oh.. not understanding again 🙂

Sorry John, rather than just East or West I meant to say the line that passes from East to West through the CP. I was mixing my Equitorial/Azimuth grids. 

Alan

[symmetal]

2 hours ago, johneta said:

I'll try PEC with EQMOD next 😬

If you're guiding there isn't much to gain using PEC. I tried it with mine when I first started, assuming the guiding corrections needed would be less and so lead to better guiding. But in reality there was no noticeable difference in the guiding. It's also easy to get your PEC corrections out of sync with the mount worm position unless you're very careful in setting the mount position when it's turned off and on.

Alan

[Stub Mandrel]

On 01/09/2019 at 05:12, johneta said:

So thats DEC 90degs and RA is the Hrs that are currently at my location for the date and time of day.

Yes, but hours, minutes and seconds in SIDEREAL TIME, not 'clock time'.

Think of it as rotating the scope to point north and 0 degrees dec. Then increasing the DEC to 90 degrees.

It's just a convenient repeatable place to set the scope when you switch off and on. EQMOD will remember where ever you left the scope when switched off but any other position is much harder to find again, even approximately.

 

For PEC try the 'predictive PEC' routine in PHD2, I find it is very good and improves my guiding after two worm rotations.

 

Edited September 5, 2019 by Stub Mandrel