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Modeling giving me the run-around.


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#1 DaveS

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Posted 11 September 2016 - 12:22 PM

I've been getting to grips with modelling for PA and collimation, but fear it's giving me the run-around. I haven't got as far a building a whole sky model yet. The following is copied from a conversation with Mike Paling and slightly edited.

 

What I've been doing is turning the motors on and letting Autofind tune the motors (Only getting 64% ATM, I did have 75% until I made some changes and rebalanced) before doing a Homefind, then slewing to the Park1 (Zenith) position and refining it with a spirit level before synching. In Cartes du Ciel I select a star (ATM either Vega or Deneb), and start capturing images, using the "continuous" setting in Maxim DL's camera control. When I have the star centred in Maxim's crosshairs I click "synch" in CduC. This will be my first alignment star. I then do a new Homefind Position.

 

This is now where things get "wobbly".

 

I click on Start Pointing File, whereupon a get a warning telling me to re-synch the mount else I'll get large offsets. I thought I *had* synched it. OK, select the same star from the Objects icon, slew (Still on crosshairs) and click the synch icon. Try again, same message. Oh well, start anyway.

Use the star I'm on as No1, click the target with the tick in it to say it's centred, then on to No2, go near meridian and fairly south, repeat, star No3 is to the east and on the equator (-ish), then No4 fairly high but not to near the pole (I tend to use alpha Cassiopeia ATM). Click Stop pointing file, and calculate the result.

 

Chances are I'll see crazy offsets measured in *hundreds* of arc minutes for all the parameters (Once I saw the PA offset measuring 1000 minutes!), even though each star was landing close to the centre of the frame.

 

Clear that model and start again, also looking at the bottom of the Autoslew window where it reports the alignment offset for each star.

 

This time I get sensible offsets (Even though I get the same warning about re-synching), so proceed to the PA tool, find star in the specified area and do the PA.

 

So far, so good, however....

 

I've not done a full model yet, as  when I've tried going across the meridian I get the crazy offsets again (Even though the target star is still hitting near the crosshairs), so something else to sort out. I tried to use Autogrid in Sequence but got a fatal error (See the thread about it).

 

So.... can anyone throw some light on what I'm missing out, or doing wrong? I've got the PA error down to no more than 1.5' on each axis, and the collimation down from 126 (126 what? Minutes?) to about 12.5


Edited by DaveS, 11 September 2016 - 12:25 PM.


#2 GeorgeCarey

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Posted 11 September 2016 - 02:20 PM

I have never bothered with the zenith park position. It is a long time since I did a polar alignment or pointing model but if I remember correctly I follow roughly this method:

Switch motors on and let magnetic find do its stuff. ( I call it that to avoid confusion with Homefind and Homeposition).

Clear old configuration.

Move scope to near zenith and do a find Homeposition (Autoslew looks for the zero points on the encoders).

Find a bright star with the aid of Cartes du Ciel. Get it centred.

Now do a Sync in Cartes du Ciel (I have never used the Autoslew Sync button).

..........You do a new Homefind here..............no need for this.........

Then start the pointing model and move in succession to the stars in one hemisphere you have chosen, all with similar Declination.

Ignore the stuff about Synching on a star - you did this on the first star.

Finish and calculate model.

Then do the bit about moving to a Meridian star to do the Polar shift thing.

Clear configuration and repeat the polar find over and over until it is good.

Now clear configuration. Sync on a star in the hemisphere you chose for Polar alignment.

Try a 6 or 7 star model in each hemisphere. You should find the stars in your 'good' hemisphere quite easily, and provided the collimation is reasonable, you should find stars in the other hemisphere.


I assume you are not using Sequence and Pinpoint to do the pointing models? Maxim, Pinpoint and Sequence make it very easy - almost fully automatic.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#3 RamaSpaceShip

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Posted 11 September 2016 - 05:42 PM

Hi Dave,

 

Never do a sync while making a pointing model as it can confuse the subsequent measures.

 

When I do a pinting file, I always check the values in the bottom line of Autoslew. It the offsets are too big, i remove the star, as I have seen that it creates wrong models. I never do a meridian flip as I use a bent pier.

 

Best regards.

 

Bernard


Edited by RamaSpaceShip, 11 September 2016 - 05:44 PM.


#4 GeorgeCarey

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Posted 11 September 2016 - 05:53 PM

Hi Bernard.
Do you not sync on the first star?
I have always thought that you need to do that so that slewing to the sequence of stars in your model is likely to get close enough to the stars to locate them on the CCD.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#5 lukepower

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Posted 11 September 2016 - 06:25 PM

Guys,

 

always sync before doing a pointing file, and then never  do it while building a pointing file.

 

What I could think is that maybe sync'ing close to the meridian could confuse Autoslew about which pierside it is currently pointing, but this is just a guess. Maybe the polar alignment is quite off, that would explain the subsequent measurements being off by alot...


Lukas Demetz
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www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#6 DaveS

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Posted 11 September 2016 - 06:34 PM

Thanks guys

 

I'll have another go the next clear night, though I'm still "under the weather". I'll leave out the zenith sync and go straight to CduC for my first sync and keep away from the meridian. due to my view, using stars east of the meridian makes sense anyway. Autoslew is reporting PA errors as no more than single digit minutes of arc, the latest being under 2' for both axes.

 

Finally I've uninstalled 5.19 and installed 5.20, and it connects OK, which it didn't before. I think Stellariumscope was interfering with things. I did a trial Autogrid with Sequence and didn't get the fatal error. Hooray!



#7 MikePaling

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Posted 11 September 2016 - 07:08 PM

Hi you guys ...

 

This is all a very timely and useful topic for me.

 

I am almost in the same position as Dave ..... just waiting for a clear sky to start my very first PA on my DDM60.

 

After reading the new Autoslew manual, many many times, the process of making a PA and subsequent Pointing Files was starting to make some sort of sense for me .... and now all of the above comments are reinforcing my confidence of getting things right :D !

 

Fingers crossed !

 

Regards Mike



#8 DaveS

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Posted 11 September 2016 - 09:04 PM

OK.

Done another 4 star PA pointing file, getting better. Went from Homefind to first star without zenith sync, but.....

 

My first star (Deneb) was so far off I had to use a RDF and gamepad to centre it. After that the subsequent 3 stars (Avoiding anything too close to the meridian or south or near the pole) hit very close to the centre of the crosshairs, now the calculation gave me errors of about 2.4' before correction, and 0.34 and 1.5' after.

 

I think I'll call it a night as I need to turn in early and anything more will likely take more time.

 

Happy for now!



#9 RamaSpaceShip

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Posted 12 September 2016 - 12:05 AM

Hi Bernard.
Do you not sync on the first star?
I have always thought that you need to do that so that slewing to the sequence of stars in your model is likely to get close enough to the stars to locate them on the CCD.

As Lukas wrote, I sync before starting the pointing file.



#10 GeorgeCarey

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Posted 12 September 2016 - 12:34 AM

Hi Dave,
It is normal for the first star to be way off. You have no pointing model yet so Autoslew has no idea what is going on.
If Deneb is your first choice, get it centred with your joystick or whatever method works.
When I am finding a first star I get behind the scope and use the tube ring knobs like a rifle sight. It works well!
When Deneb is perfectly centred, sync on it with Cartes du Ciel and then off you go.

When you have your first pointing model working you can sync on a star and then go to the Mount menu.
Then do 'set new Homeposition' .
Remember to save parameters in the File menu or it will be forgotten.
When you next switch on you should find that Autoslew can go to stars very accurately.
My last pointing model was made in June last year and still works well.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#11 MikePaling

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Posted 12 September 2016 - 11:06 AM

Hi George ....

 

I was interested to read your comments above saying " It is normal for the first star to be way off".

 

I made my first trial sync on a star using CdC and Autoslew last night ... and it was way out ... it must have been approx. 5 degrees out. I gave up at this point thinking something must be wrong with my general alignment of my DDM60.

 

From your experience is my first sync normal ... or .... very much abnormal????

 

Regards Mike



#12 GeorgeCarey

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Posted 12 September 2016 - 08:26 PM

I am assuming your mount does not have absolute encoders.
When you switch on Autoslew it does not know where it is pointing.
Trying to slew to a star is not possible.
When you go to the near zenith position and do a Homefind Autoslew finds the zero point in the encoders but still does not know where in the sky it is pointing. It is a bit like using a protractor to measure angles. All we have done so far is figure out where the zero on the protractor is.
You then locate a star by trial and error. When you Sync on the star in CdC you at last have got a valid measurement.
Now comes the first slew... At this stage you haven't got a good polar alignment yet and have no idea if the optical axis of your scope is aligned with the mount's dec axis (the collimation angle). That means that the first slew is likely to miss the CCD centre by a lot and sometimes does not land on the CCD at all.
When you have done a good polar alignment things should be much better. Most stars in one hemisphere should be close to centre.
A big collimation error means that the other hemisphere will be not so good. The first pointing model will iron out collimation error and all the other errors and away you go.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#13 RamaSpaceShip

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Posted 12 September 2016 - 08:55 PM

Hi Mike,

 

Did you do a home find at the start of your session? It is mandatory in order to adjust the origin of the (inrfemental) cencoders.

If you don't do that, you can have a huge error on the first star, and your pointng may not be acciurate at all.

 

Best regards.

 

Bernard



#14 MikePaling

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Posted 12 September 2016 - 10:16 PM

Hi Bernard ....

Yes .... I always start with a home find before I start doing anything else.

 

So .... is an error of 5 degrees on my first slew normal .... or abnormal?

 

Regards Mike



#15 MarkS

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Posted 12 September 2016 - 11:15 PM

Mike,

 

Since my set-up is mobile, and subject to variations in alignment between one session and the next, I never know how far off that first slew will be.

 

So here's what I do - and it never fails:

 

After homefind, I slew to the zenith, and wait for the first star to appear in the evening sky. I slew to it via TheSkyX, and centre it with the Gamepad on my visual finder (which is a super-light Quikfinder so as not to mess with balance issues). This is necessary to get the star onto the camera sensor.

 

How far off that star is to start with doesn't matter to me, as long as it is in the finder view. It is dependent on how close I got the initial physical alignment. But usually the star is within about 2 degrees. So my answer to your direct question as to whether 5 degrees is normal or not is that it looks high but it would be perfectly manageable with the approach described here. However, I would want to set the system up physically more accurately in the first place.

 

Then I centre the star on the camera and synch TheSkyX. This sets the initial alignment well enough for the PA routine (once it is dark enough!)

 

The config in place for this is usually whatever I used for the previous session. This is not critical.

 

Hope this helps.

 

Mark



#16 GeorgeCarey

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Posted 12 September 2016 - 11:16 PM

5 degrees does seem excessive. I think when I was new to the DDM mount my first errors were about 1 degree max. My CCD field of view is 38 x51 minutes and stars would sometimes be oOf the CCD.

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#17 MikePaling

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Posted 14 September 2016 - 07:30 PM

Hi ...

I am hoping to carry out my first PA tonight as the sky looks as if it might be clear enough :-)

 

However ... I am starting to get confused about Synching within Autoslew ... and which button to click on after slewing. Several people have said that no synching is needed during making appointing file as it leads to errors.

 

Am I correct that after slewing the scope until the chosen star is central in the eyepiece I click on the target icon with a green tick in it (see attached image) ??? ... I think tha it's correct name is "Centre confirmation button" .... and NOT the plain red target button???

 

This particular icon only appears when pointing files are being done.

 

Mike

Attached Files



#18 GeorgeCarey

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Posted 14 September 2016 - 09:23 PM

You MUST sync on the first star. Not on any of the other stars in the pointing model. You confirm those with the icon with the tick.
It is years since I did a manual pointing file so I can't remember what that icon looks like. You will soon know if you clicked on the right icon because it then says "slew to the next star".

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#19 DaveS

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Posted 14 September 2016 - 09:46 PM

I found tonight that syncing on the zenith position after doing the homefind greatly helps the first slew hit the target. This time Deneb was very near the centre of the crosshairs.
Unfortunately when I tried an Autopoint run I only had 4 solves, so something's out.
I'll take my software stick in to work tomorrow and print out the manuals, much easier to study than on the screen.

#20 RamaSpaceShip

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Posted 15 September 2016 - 10:18 AM

I'll take my software stick in to work tomorrow and print out the manuals, much easier to study than on the screen.

Hi Dave,

 

I prefer the screen. The paper is missing some mandatory features like the search box, cut and paste, etc...

 

Best regards.

 

Bernard






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