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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
PHD guiding defies logic.
      #4499106 - 04/05/11 08:53 PM

I have been trying to improve my guiding with PHD.
I attempted some ideas that I had - last night
to replace the tedious drift aligning process with
an illuminated reticule eyepiece.

I thought that if I was guiding on a star at the intersection of
the meridian & the equator - I could use PHD to help me
align the Azimuth & it did seem to yield some inconclusive results.
I was able to get it better by adjusting the azimuth screws.
However - I was unable to view a star on the Western or Eastern horizon
to check the elevation for polar alignment
but I looked at the display at the bottom of the screen & it was showing
North - 320, North 530, North 800 , North 350 etc.
All different values as it updated every second.

I thought - why do I need to look at a star on the horizon?
It is clearly showing a correction always North which is the polar axis.
I then played around with the elevation settings on my EQ6 mount.
It was set to 38 degrees - which is my latitude.
I tried setting it to 38.5 degrees, 39, 39.5, 40,
It was always still heading North on PHD.
I then tried to set it the other way:
37.5 degrees, 37, 36.5, 36, & guess what?
It still said on PHD that it was correcting for NORTH !!!
with similar values. 300 to 800.
(it was set to give a maximum excursion of 800)

Can anyone explain this as it seems to defy all logic?
What are the units of measurement for this 800 value that I get?


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Dan M
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Re: PHD guiding defies logic. new [Re: alpal]
      #4499545 - 04/06/11 01:19 AM

I used PHD for helping to drift align but only to capture images of the star I want to drift align against. I use StarTarg on top of PHD to watch the drift. It really is a joy to use.

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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Dan M]
      #4499598 - 04/06/11 02:24 AM

Quote:

I used PHD for helping to drift align but only to capture images of the star I want to drift align against. I use StarTarg on top of PHD to watch the drift. It really is a joy to use.




Thanks Dan - I'll check that out.
I'll also check out EQMOD which I hope allows you
to drift align & can give you more points
for pointing accuracy.

It is obvious that the numbers at the bottom of the PHD guiding screen are just garbage.
I also tried the graphs for Ra/Dec & dy/dx & they
were also misleading.
e.g - you can put the mount at different elevations &
the lines always end up at the top of the graph above the center line when they stabilise.
I am starting to think that there must be something fundamentally wrong with PHD guiding
unless someone can prove otherwise.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4500402 - 04/06/11 12:50 PM

I use PhD guiding for drift alignment all the time, and I've never seen what you describe.

The first thing you need to do is to physically rotate your camera in the focuser so that movement in RA and dec follow the horizontal and vertical axes of the display. If doesn't matter whether RA or dec end up on the horizontal sxis, just that moving in RA or dec doesn't result in diagonal star movement on the display.

Now, after doing an approximate polar alignment, begin the drift alignment procedure. Slew to a star near 0 dec and the meridian. Calibrate PhD and begin guiding. Now, open the graph, and switch the view from RA / dec to dx/dy. Also, select "off" from the dec guiding algorithm dropdown. You will see either dx or dy, whichever is the one corresponding with the dec axis, to begin drifting. Adjust the mount in azimuth until the drift goes away. You will most likely need to stop and restart guiding each time you move the mount.

Now slew to a star near 0 dec and the horizon. Calibrate PhD again (click on "brain", change dec guiding algorithm back to "resist switching" or "low pass filter", and check "force calibrate"). After PhD starts guiding, go to the graph and do the same thing you did before, this time making adjustments in alitude until drift stops.

You quickly see that, when you pass through ideal polar alignment, the reported direction of the drift will change. It sounds to me like you might have failed to recalibrate PhD when you slewed to a star near the horizon. You can't drift align in PhD with dec guiding active. Also, if you forget to switch from "RA/Dec" to "dx/dy" on the graph after turning off dec guiding, you will just see a flat line for dec corrections.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4501020 - 04/06/11 05:42 PM

Quote:

I use PhD guiding for drift alignment all the time, and I've never seen what you describe.

The first thing you need to do is to physically rotate your camera in the focuser so that movement in RA and dec follow the horizontal and vertical axes of the display. If doesn't matter whether RA or dec end up on the horizontal sxis, just that moving in RA or dec doesn't result in diagonal star movement on the display.

Now, after doing an approximate polar alignment, begin the drift alignment procedure. Slew to a star near 0 dec and the meridian. Calibrate PhD and begin guiding. Now, open the graph, and switch the view from RA / dec to dx/dy. Also, select "off" from the dec guiding algorithm dropdown. You will see either dx or dy, whichever is the one corresponding with the dec axis, to begin drifting. Adjust the mount in azimuth until the drift goes away. You will most likely need to stop and restart guiding each time you move the mount.

Now slew to a star near 0 dec and the horizon. Calibrate PhD again (click on "brain", change dec guiding algorithm back to "resist switching" or "low pass filter", and check "force calibrate"). After PhD starts guiding, go to the graph and do the same thing you did before, this time making adjustments in alitude until drift stops.

You quickly see that, when you pass through ideal polar alignment, the reported direction of the drift will change. It sounds to me like you might have failed to recalibrate PhD when you slewed to a star near the horizon. You can't drift align in PhD with dec guiding active. Also, if you forget to switch from "RA/Dec" to "dx/dy" on the graph after turning off dec guiding, you will just see a flat line for dec corrections.




Thanks shams42.
I have sort of tried what you said above except for rotating the camera.
I was just wondering why it continues to read out a North error
& doesn't flip to South at some point when
adjusting the elevation?
I am not sure if I had resist switching or filter turned on.
I will have to revisit this.
I was trying to align the Dec without going near the horizon
as I couldn't see any stars close to the horizon from my location.
I wondered why that matters? - as the mount was still making North
adjustments which is the Dec axis.

This is my point:
PHD sees an error in elevation & adjusts for it -
no matter where your telescope is pointed therefore
why doesn't the error value FLIP at some stage
& go from North to South at the bottom of
the PHD guiding screen - when you change the elevation?
Have you ever looked at those numbers at the bottom of the screen?


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: alpal]
      #4501029 - 04/06/11 05:48 PM

By the way - PHD guiding knows where North & South are
without rotating the camera.
It sends out a command to the mount to go North & then South
when calibrating - the result tells PHD guiding the direction for adjusting the Dec axis.

Also - I was using the info from this link here:

http://www.njstargazer.org/PolarAlignment.asp

Edited by alpal (04/06/11 06:36 PM)


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4501297 - 04/06/11 07:54 PM

Quote:

By the way - PHD guiding knows where North & South are
without rotating the camera.




Yes. For guiding, it doesn't matter how the camera is oriented. But for drift aligning according to the procedure I outlined above, the camera needs to be aligned with RA / dec. This is because you will be looking at the "dx / dy" plot, and if the camera isn't properly rotated, declination drift will appear as errors in both x and y.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4501319 - 04/06/11 08:04 PM

Quote:


Thanks shams42.
I have sort of tried what you said above except for rotating the camera.
I was just wondering why it continues to read out a North error
& doesn't flip to South at some point when
adjusting the elevation?




I'm a bit puzzled by this as well; I've never seen that happen. At any rate, you should turn dec guiding off when you drift align, so you won't see ANY dec corrections.

Did you force a recalibration near the star that you are using near the horizon?

Quote:


I am not sure if I had resist switching or filter turned on.
I will have to revisit this.




It probably doesn't matter.

Quote:


I was trying to align the Dec without going near the horizon
as I couldn't see any stars close to the horizon from my location.
I wondered why that matters? - as the mount was still making North
adjustments which is the Dec axis.




It doesn't matter if you can see stars there or not, your guidescope can see stars where you cannot. At any rate, that's where the mount must be pointed for drift alignment to work properly. The logic of pointing near the horizon at zero declination is that azimuth errors will not induce declination drift there.

If you weren't pointed in the right spot and had an azimuth pointing error, that could cause the star to drift in declination -- perhaps consistently to the north.


Quote:


This is my point:
PHD sees an error in elevation & adjusts for it -
no matter where your telescope is pointed therefore
why doesn't the error value FLIP at some stage
& go from North to South at the bottom of
the PHD guiding screen - when you change the elevation?
Have you ever looked at those numbers at the bottom of the screen?




Yes, my numbers behave as they should. If I have dec guiding active, I will see PhD begin issuing commands in the other direction as I pass through the point of perfect polar alignment. That's why I'm wondering if you always calibrate near the star you are using for drift alignment (otherwise PhD may confuse its directions, I think) and if you are pointed at a star that is in the right place in the sky.

However, this is a secondary concern; you should have dec guiding turned off for drift alignment.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4501498 - 04/06/11 09:43 PM

Thanks Shams - I'll have another go at this & report back.

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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4501603 - 04/06/11 10:46 PM

Good luck!

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bluedandelion
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Re: PHD guiding defies logic. new [Re: shams42]
      #4501817 - 04/07/11 01:03 AM

Matthew,

Thank you for that quick tutorial for drift aligning using PHD.

This does assume that the guide scope axis is parallel to the main OTA doesn't it? Or in practice is that not a big factor since you are also autoguiding?

Ajay


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bardo
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4501871 - 04/07/11 01:59 AM

Ive had it happen several times. when its doing that I make way to obviously large corrections to make sure Im going the right way with adjustments or something isnt astray.
sometimes I need to do like 3/4-1 turn. the lat gauge is known to not be terribly accurate anyway, just a jist to go by. Im visually oriented so I like watching the graph.

also, make sure you give it some time between intervals. often when i make adjustments it will go one way for the first few minutes then start a slow trend in the opposite direction. I cant explain it but it does it. so I give it 3-5 minutes or so each time I make an adjustment just so Im clear. plus it gives a longer plot on the graph to go by making the guess at adjustments more accurate.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: bardo]
      #4501945 - 04/07/11 04:31 AM

Shams
Quote:


If you weren't pointed in the right spot and had an azimuth pointing error, that could cause the star to drift in declination -- perhaps consistently to the north.




I think you've answered the question here.
It seems that if you're pointed at any star,
other than one close to the East or West horizon
PHD is obtaining Azimuth movements
by moving North on the Declination!

That would explain what I'm seeing.
Therefore the fact that it reads Dec 500 or Dec 600
means nothing in regards to Dec.
It's actually compensating for Azimuth!


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bardo
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Re: PHD guiding defies logic. new [Re: alpal]
      #4501969 - 04/07/11 05:00 AM

thats the same principle if you're not level, but it shouldnt be so drastic. you should still be mostly correcting the right direction. to combat this go back and forth between az/alt adjustments incrementally.

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shams42
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4502536 - 04/07/11 11:17 AM

Quote:


This does assume that the guide scope axis is parallel to the main OTA doesn't it? Or in practice is that not a big factor since you are also autoguiding?
Ajay




I think it only assumes that that guide scope axis is roughly parallel to the RA axis, but I'm not sure about that. I don't think that the guide scope to main OTA axis is important at all at this stage.


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bluedandelion
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Re: PHD guiding defies logic. new [Re: shams42]
      #4502637 - 04/07/11 11:56 AM

That makes sense. Drift alignment is about getting the RA axis pointing at the celestial pole. So having the guidescope parallel to the RA axis and then doing the PHD align procedure should do it.

Thanks again.

Ajay


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shams42
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4503053 - 04/07/11 03:01 PM

Actually, what I said makes no sense. I think as long as the guidescope is pointed at the appropriate region of the sky, that's all that matters. After all, as soon as you move the mount in declination, the guidescope is no longer aligned to the RA axis of the mount!

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alpal
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Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4503476 - 04/07/11 06:30 PM Attachment (55 downloads)

Good news.
I finally got the mount to align with PHD guiding
by setting the Dec up on a star near the horizon.
The Azimuth was another story.
It ended up close but not perfect.
My mount Dec angle display was 0.5 degrees out.
Correct was 37.5 when it should have been 38 degrees.
I managed to get some subframes of Centaurus A _ NGC5128.
See pic - very quickly processed.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4504040 - 04/07/11 11:39 PM

Quote:

Good news.
I finally got the mount to align with PHD guiding
by setting the Dec up on a star near the horizon.
The Azimuth was another story.
It ended up close but not perfect.
My mount Dec angle display was 0.5 degrees out.
Correct was 37.5 when it should have been 38 degrees.
I managed to get some subframes of Centaurus A _ NGC5128.
See pic - very quickly processed.




Hey, that's great! Looks like the stars are nice and round, which is all you can really ask for.

I wouldn't worry about the latitude scale being a bit off; it's not applied very precisely at the factory.

Did you do a complete drift alignment (for altitude and azimuth) or just for altitude?


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alpal
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Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4504248 - 04/08/11 02:50 AM

Hi Shams,
The pic is not too bad considering that it's a tiny crop
of the total frame.
I did a complete drift algnment for altitude & azimuth.
I got the Dec exact.
I had no frame rotation after 150 minutes of subframes.
PHD was just hovering around +/- half a divison on the dy graph
when I guided.
The azimuth was mostly 1 division positive on dx & it actually
guided worse than before (when it wasn't properly aligned.)
I had to chuck out 19 of 30 x 5 minute subframes
leaving me only 11 resonable frames to stack.
That is another issue - I think.
I only have homemade tube rings which have too much
play in them - in my opinion.
I also need to tune the closed loop system by playing around
with the PHD settings.
I will try & get it right.

One thing I need to do is install a Canon program
to view more closely the frames as I take them.
(instead of just the preview on Canon EOS remote)
I didn't realise that the stars weren't round on all frames
until later when I magnified them up.
I will get this sorted one way or another.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4504799 - 04/08/11 11:15 AM

Wow, that's too bad. Can you post the details of your setup and your settings in PhD?

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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4505588 - 04/08/11 05:59 PM

Quote:

Wow, that's too bad. Can you post the details of your setup and your settings in PhD?



Sure - I just clicked on the "Brain" function.
These are the settings & although I tried 2 second
capture frames half way through, PHD lost track
altogether so I changed to 1 seconds frames.
The settings below were not altered:

RA Aggressiveness 36
RA Hysteresis 10
Dec guide mode Auto
Dec Algorithm Resist swit
Max Dec duration 520
Dec slope weight 5.00
Calibration (ms) 1540
Min. motion (pixels) 0.55
Search region (pixels) 15
Noise Reduction None
Time Lapse (ms) 0
Camera gain (%) 100
LE port Port 358
LE read delay 0
Force calibration tick
Use subframes no tick
Log info no tick
Disable guide output no tick


I'd be interested to know a good way of tuning my setup.
PHD has never given me 100% good frames.
Re-checking I actually got only 9 reasonable frames out of 30
when taking shots of that Centaurus A galaxy.
My previous time on the Southern Pinwheel galaxy
I got about 20 good frames out of 35.
None of the frames are absolutely perfect.

Is there a way of tuning this double axis closed loop
by perhaps putting in a command from the hand controller on say a x1 forced movement &
looking at how PHD handles the disturbance on the graphs?
(and adjusting accordingly)


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4505851 - 04/08/11 09:07 PM

Your settings look kind of off. With RA aggressivenes that low, you are hardly making any corrections in RA. Try these settings and see what happens:


RA Aggressiveness 85
RA Hysteresis 15
Dec guide mode Auto
Dec Algorithm Resist switching
Max Dec duration 1500
Dec slope weight 5.00
Calibration (ms) 5000
Min. motion (pixels) .2
Search region (pixels) 15


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alpal
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Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: shams42]
      #4506222 - 04/09/11 02:03 AM

Thanks Shams,
I'll try those figures you've given me.
I have played aroud with RA Aggressiveness.
100% was too much & caused worse guiding.
I know I'm close & I hope it just needs a bit of tweaking.
cheers
Alpal


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alpal
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Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: alpal]
      #4506424 - 04/09/11 07:19 AM Attachment (39 downloads)

There might be a clue to the problem.
Look at this tiny crop of one of the worst frames.
You can actually see 2 stars instead of one.
Surely that must be caused by the external guiding scope
shifting it's position?


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: alpal]
      #4506432 - 04/09/11 07:30 AM Attachment (33 downloads)

In contrast this was one of the better frames but still not
perfect pinpoint stars.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4506589 - 04/09/11 09:48 AM

Do you know if the direction of elongation corresponds with RA or dec?

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jmiele
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Re: PHD guiding defies logic. new [Re: shams42]
      #4506649 - 04/09/11 10:09 AM

If it's DEC and you were way off in polar aling, could be that guider based on settings can't keep up witth the amount of error being produced. I don't know PHD well so I can't recommend settings changes but I agree with shams, identify DEC vs RA first. Joe

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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: jmiele]
      #4506656 - 04/09/11 10:12 AM

Hi Guys - the Dec was spot on.
No frame rotation after 2.5 hours.


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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4506697 - 04/09/11 10:36 AM

Can you stack your frames without aligning stars and post result? I want to see if the errors are in a constant direction or if they curve.

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shams42
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Re: PHD guiding defies logic. new [Re: alpal]
      #4506698 - 04/09/11 10:36 AM

Can you stack your frames without aligning stars and post result? I want to see if the errors are in a constant direction or if they curve.

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shams42
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Re: PHD guiding defies logic. *DELETED* new [Re: alpal]
      #4506701 - 04/09/11 10:38 AM

Post deleted by shams42

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Peter in Reno
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Re: PHD guiding defies logic. new [Re: shams42]
      #4506988 - 04/09/11 12:54 PM

Your scope is pretty big and may be too big (or too long) and heavy for your NEQ6 to handle. That might be why you are not getting consecutive good subs. You must have a hard time adjusting the latitude with heavy load while drift aligning. What is the total load including main scope, guide scope, cameras, etc?

Do you have a small refractor to test? If you can image with a small scope and get nice round stars, then extra heavy load may be the culprit.

Peter


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bardo
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4507386 - 04/09/11 04:14 PM

differential flexure

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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4507461 - 04/09/11 04:57 PM

Quote:

I only have homemade tube rings which have too much play in them - in my opinion.




If that's the case, it's differential flexure. You have to get rid of any flexure you find before fine tuning PHD settings.

I notice you have 80mm guide scope. Is it Orion Short Tube 80? If so, they are famous for focuser flop. Check both focusers (image and guide scopes) for any flop. How is your guide scope mounted: 3-point rings or clam shell rings? I would use clam shell rings since they grip the scope much better than 3 point rings.

Also check to make sure the mounting of primary mirror is tight. If it shifts a tiny bit while tracking, then that's another kind of differential flexure similar to infamous mirror flops in SCTs scopes with moving primary mirrors.

Once you get everything tightened, then you can start tweaking PHD guide settings. I would revert back to default PHD settings and go from there.

Bottom line, get rid of the flexure first. Also check to make sure your scope plus equipment are not too heavy for your mount to handle. The carrying capacity of Atlas EQ-G or NEQ6 is about 40lbs for visual observing. The imaging capacity is about half which is probably no more than 20-25lbs.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: shams42]
      #4507880 - 04/09/11 08:54 PM

Quote:

Can you stack your frames without aligning stars and post result? I want to see if the errors are in a constant direction or if they curve.




OK - I'll do that - I never thought of that -
it's a good idea.


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4507888 - 04/09/11 08:58 PM

Quote:

Your scope is pretty big and may be too big (or too long) and heavy for your NEQ6 to handle. That might be why you are not getting consecutive good subs. You must have a hard time adjusting the latitude with heavy load while drift aligning. What is the total load including main scope, guide scope, cameras, etc?

Do you have a small refractor to test? If you can image with a small scope and get nice round stars, then extra heavy load may be the culprit.

Peter




The scope is pretty big.
I do have a hard time adjusting the latitude.
I had to replace the lower latitude bolt as it had bent slightly.
I have since lowered the weight from 18Kgs to 12 Kgs
( which is now 26 lbs total load )
I do have a smalller 4.5" Newt which I could do tests with.


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4507904 - 04/09/11 09:06 PM

Quote:

Quote:

I only have homemade tube rings which have too much play in them - in my opinion.




If that's the case, it's differential flexure. You have to get rid of any flexure you find before fine tuning PHD settings.

I notice you have 80mm guide scope. Is it Orion Short Tube 80? If so, they are famous for focuser flop. Check both focusers (image and guide scopes) for any flop. How is your guide scope mounted: 3-point rings or clam shell rings? I would use clam shell rings since they grip the scope much better than 3 point rings.

Also check to make sure the mounting of primary mirror is tight. If it shifts a tiny bit while tracking, then that's another kind of differential flexure similar to infamous mirror flops in SCTs scopes with moving primary mirrors.

Once you get everything tightened, then you can start tweaking PHD guide settings. I would revert back to default PHD settings and go from there.

Bottom line, get rid of the flexure first. Also check to make sure your scope plus equipment are not too heavy for your mount to handle. The carrying capacity of Atlas EQ-G or NEQ6 is about 40lbs for visual observing. The imaging capacity is about half which is probably no more than 20-25lbs.

Peter




I am using the Orion short tube.
The focuser extension tube looks really low quality.
Maybe that is moving slightly?
The Orion focuser is only held on by 4 philips head screws
which 1 week ago were loose - I did however tighten them.
Just checked it now - it feels tight.
The primary mirror has 3 large adjusting screws with strong
springs behind them.
I don't think they would move much between 2 x 5 minute frames but you have got me thinking.
I have the 2 x 3-point rings not clam shell rings.

Thanks for the tips - I have plenty to consider.
I think that sub-frame with the 2 stars per star image
is a dead give-away - something is moving between the
guide scope & the main scope.
cheers
Alpal


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alpal
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508098 - 04/09/11 11:27 PM Attachment (31 downloads)

Quote:

Quote:

Can you stack your frames without aligning stars and post result? I want to see if the errors are in a constant direction or if they curve.




OK - I'll do that - I never thought of that -
it's a good idea.




OK here is a stack of 9 in a row - with a frame missing -
without alignment checked - in Deep Sky Stacker.
Clearly the mount is not guiding in Right Ascension.
Remember - I was using Dither Master.
This is a tiny crop of the full frame.


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alpal
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508109 - 04/09/11 11:36 PM Attachment (35 downloads)

And here is a full frame as above.
I think Dither Master causes it to drift
in between frames while it takes 30 seconds to
organise the dither -
if RA is not spot on - it will drift between frames.


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508149 - 04/10/11 12:09 AM

I think it's flexure. The star trails are not curved. If they were curved then it would be bad polar alignment. I can count nine star steps in the cropped image.

I used to own an Atlas EQ-G and Celestron 11" SCT guided by Orion Short Tube 80. I was getting exactly what you are getting. I was getting image shifts in the same direction at every sub. I concluded mirror flop flexure due to moving primary mirror in C-11. I solved the problem by replacing guide scope with Off Axis Guider. OAG practically eliminated flexure.

OAG might be difficult for your scope due to very limited back focus. You would probably need the thinest OAG.

You said there may be some play in tube rings, start there. I have read Newtonian mirror mounts can cause the mirror to shift if either improperly mounted or low quality mount is used. Double check to make sure the mirror is well supported. What about secondary mirror? Check that as well.

Can you post a nice picture of your setup including guide scope, cameras, etc?

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4508179 - 04/10/11 12:29 AM Attachment (24 downloads)

Thanks Peter.
I just took a pic now.
here is a link: http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2355_A_large.jpg

or a smaller version:


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alpal
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508244 - 04/10/11 01:21 AM

As for an OAG - Off Axis Guider.

I have tried to avoid that.
I would have to shift the mirror up a bit to allow
for it & also it's not always possible to find a guide star
using an OAG.
I tried to take the Tarantula Nebula the other night
which is inside the LMC & I couldn't find a guide
star even with the SSAG guider - on 1 second capture.
I would need to go to 5 second capture but my RA alignment
wasn't precise enough.
It's not an easy hobby!


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508274 - 04/10/11 01:45 AM

SSAG is probably the least sensitive autoguider in the market. I use to own one and never liked it so I replaced it with a high sensitivity Lodestar autoguider and I have never had to hunt for guide stars with OAG. I have guided to as low as 0.2 second.

Not only it's not an easy hobby but an expensive one!!!

I do not know enough about Newtonian scopes so I am not sure how much I can help you. I do know that in order to handle a large scope requires a strong mount with very good tracking and it's not going to be cheap. Try downsizing to smaller scope and go from there.

Good luck.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4508288 - 04/10/11 02:08 AM

Quote:

SSAG is probably the least sensitive autoguider in the market. I use to own one and never liked it so I replaced it with a high sensitivity Lodestar autoguider and I have never had to hunt for guide stars with OAG. I have guided to as low as 0.2 second.

Not only it's not an easy hobby but an expensive one!!!

I do not know enough about Newtonian scopes so I am not sure how much I can help you. I do know that in order to handle a large scope requires a strong mount with very good tracking and it's not going to be cheap. Try downsizing to smaller scope and go from there.

Good luck.

Peter




Thanks for the advice Peter.
It's great to have the opinion of others.

I now wish I'd bought a Lodestar camera but still -
I am pushing the limits with 1220mm focal length &
magnifying the stars up so much as I have here.
I was going to try a carbon fiber tube to cut down the weight
but none are available for the size I need for an 8" mirror
and a 1 inch gap each side.
Making my own carbon fiber tube is too big a job.

I am thinking of making some better tube rings & that might help.

An OAG wouldn't work without a Lodestar camera/guiding system
& it all means a lot more money.

If I was using a smaller/lighter 500mm focal length telescope
with my present system I know it would work.
It would divide my error by more than a 1/2 & the
ligher weight would make it easier for my mount to guide.
(also less length means less leverage on the mount)
That's a pity because my 8" telescope is a fine instrument
when properly collimated.
It has a 1/20th wave primary P to V
& a 1/12th wave secondary P to V.
It also has a great focuser & Carl Zeiss flocking paper inside the whole telescope tube.


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jmiele
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508796 - 04/10/11 10:50 AM

My SBIG 402 is super sensitive if you are looking for something else. It is however, expensive as guiders go. Joe

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bluedandelion
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Re: PHD guiding defies logic. new [Re: jmiele]
      #4509140 - 04/10/11 01:35 PM

Alpal,

Is your imaging scope the 8" F/6 reflector? If so I think the problem might stem from the long moment arm. From the image I can see where the tube rings are usually located. You are loading up the front of the scope with the ST80 and guide camera (3 lbs at least from my experience) plus your DSLR. I would suspect that the main OTA would flex quite a bit with such loading.

One solution might be to move your guidescope back to locate it directly over the saddle plate of your mount.

Ajay


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4509210 - 04/10/11 02:11 PM

To do process of elimination, test your mount with your smaller scope to make sure your mount is behaving well and also make sure there is no flexure anywhere. Take 10 minute subs and analyze the images.

Like I said before, I had problems with Atlas EQ-G, C-11 and guide scope. So I tested with 80mm F/6 (or F/6.5) EON guided by ST80 using the same Atlas EQ-G and the guiding was perfect. I didn't drift align. I used polar scope. The guide scope was supported by Orion clam shell rings and they were rock solid.

Are your guide scope rings supported by plastic screws? Looking at Orion web site showing ST80 guide scope package, the screws look like nylon or plastic. If that's the case, the screws are too soft and can cause flexure.

Peter


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509758 - 04/10/11 07:44 PM

I found this link from another CN thread. He uses 8" Newt on the same mount as yours. Take a look at his pictures from his web site:

http://webpages.charter.net/paastroman/

I think his scope is F/4 which is quite a bit shorter than yours.

You might want to contact him and you might get better advice than me.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509941 - 04/10/11 09:29 PM

Quote:

I found this link from another CN thread. He uses 8" Newt on the same mount as yours. Take a look at his pictures from his web site:

http://webpages.charter.net/paastroman/

I think his scope is F/4 which is quite a bit shorter than yours.

You might want to contact him and you might get better advice than me.

Peter




Hi Peter,
I am at work so can't type much now.
His stars are nice & round & he does have the
guide scope on the tube rings.
I'll look into this later.
cheers
Alpal


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Nils_Lars
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509963 - 04/10/11 09:42 PM

I would guess the ST-80 is causing the flexure , that focuser tube with an extension is notorious.

I would put a block and a strap between the SSAG and the Newt so it cant move , once the focus is set it wont need to move anyway.


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alpal
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Re: PHD guiding defies logic. new [Re: Nils_Lars]
      #4510082 - 04/10/11 10:54 PM

Quote:

I would guess the ST-80 is causing the flexure , that focuser tube with an extension is notorious.

I would put a block and a strap between the SSAG and the Newt so it cant move , once the focus is set it wont need to move anyway.




Thanks Nils,
The extension tube is rubbish.
It wobbles inside the focuser until you do it up tight.


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BlueGrass
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Re: PHD guiding defies logic. new [Re: alpal]
      #4510163 - 04/10/11 11:46 PM

Alpal,
Moving to a 9x50 finder / guider setup I think would help alot. Once the SSAG and finder is put in focus, there is no flex. This is the 'home brew' version of the Kwiq guider package. Many of us have made them and they perform very well. I mount mine atop all my imaging setups using a V clamp with a V plate. This allows a very rigid assembly that can be moved front to back to help in balancing. You've probably seen the threads that cover making one. Phd has no problems guiding at the 9x50 FL.

Found the thread from last year, that shows some of these 9x50 units and also some posts with in-depth discussion of guiding performance and results...
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3929567/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1


Edited by BlueGrass (04/11/11 12:06 AM)


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alpal
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Re: PHD guiding defies logic. new [Re: BlueGrass]
      #4510389 - 04/11/11 05:06 AM

Thanks guys for all the tips.

I can't do them all right now but what I can do quickly & easily
is mount the Guide Scope directly onto the tube rings.
OK - the telescope itself will flex slightly but it should
have a resonance that won't last longer than 1/2 a second
& that would be minimal compared to a 5 minute sub-frame.
That way the quidescope is at the closest possible point
to the mount itself & I'll see if that improves things
& get back to you all here.

I think that will make a considerable improvement.
After that I can change the 6 plastic screws on the guide scope rings to metal.

Also - I have a friend who is working on an idea to make some better
main tube rings out of aluminium.
There are no standard tube rings available for the Sonotube size
which has an outside diameter of about 10.5"

After that I'll try the finder scope option.


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alpal
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Re: PHD guiding defies logic. new [Re: alpal]
      #4510495 - 04/11/11 06:57 AM Attachment (22 downloads)

OK - I changed it over.
There is one pic below & some more here on photobucket:

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2360_A.jpg

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2359_A.jpg

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2362_A.jpg

I just need a clear night to test it.


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4510756 - 04/11/11 10:10 AM

For the screws in tube rings, I wouldn't recommend metal screws touching the guide scope. I suggest screws with special tip material like Delrin so they won't scratch the guide scope. Here is a sample:

http://www.admaccessories.com/Miscellaneous_DTTS.htm

Peter


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bluedandelion
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4511246 - 04/11/11 01:38 PM

Alpal,

That looks much better. The dovetail plate for the ST80 will give the tube rings an extra bit of stability. With the setscrews on the ST80 extender tightened down, see what you get.

Await your results curiously.

Ajay


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4511303 - 04/11/11 02:01 PM

What kind of dovetail are you using? Standard NEQ6 (Vixen style) or Losmandy "D" style? If you are using Vixen style, I highly recommend getting "D" style since it's quite a bit wider and will provide much greater stability for a heavy tube.

http://www.admaccessories.com/D_Series_Universal.htm

You will also need "D" saddle.

http://www.admaccessories.com/D_Series_Saddles.htm

I am a little concerned of your tube rings. They look a little thin. They may flex differently depending on where the scope is pointing to.

Parallax makes great tube rings. They have 10.3" and 11".

http://www.parallaxinstruments.com/ring.htm

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4511662 - 04/11/11 05:11 PM

Quote:

For the screws in tube rings, I wouldn't recommend metal screws touching the guide scope. I suggest screws with special tip material like Delrin so they won't scratch the guide scope. Here is a sample:

http://www.admaccessories.com/Miscellaneous_DTTS.htm

Peter




Thanks Peter -I'll organise some of those.


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alpal
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Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4511666 - 04/11/11 05:12 PM

Quote:

Alpal,

That looks much better. The dovetail plate for the ST80 will give the tube rings an extra bit of stability. With the setscrews on the ST80 extender tightened down, see what you get.

Await your results curiously.

Ajay




So do I - I am looking for an improvement.


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4511677 - 04/11/11 05:19 PM

Quote:

What kind of dovetail are you using? Standard NEQ6 (Vixen style) or Losmandy "D" style? If you are using Vixen style, I highly recommend getting "D" style since it's quite a bit wider and will provide much greater stability for a heavy tube.

http://www.admaccessories.com/D_Series_Universal.htm

You will also need "D" saddle.

http://www.admaccessories.com/D_Series_Saddles.htm

I am a little concerned of your tube rings. They look a little thin. They may flex differently depending on where the scope is pointing to.

Parallax makes great tube rings. They have 10.3" and 11".

http://www.parallaxinstruments.com/ring.htm

Peter



I am using the standard dovetail that arrived with the NEQ6.
I would prefer a wider type.
I will try anything to make this setup work.
I'll check those dovetails out.

The Parallax tube rings which would fit are perhaps
the 10.3" OD Tubes (Celestron C-9.25) for $250.
I'll order some if they'll fit.
That's a good price.

Also - wouldn't it be nice to put the guide scope
on proper tube rings too?


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alpal
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Re: PHD guiding defies logic. new [Re: alpal]
      #4512738 - 04/12/11 06:00 AM

Update:
I just measured the exact size of the telescope's diameter
with a circometer.

Therefore: 10" Sonotube once painted = 10.36" outside diameter.

The rings at Parallax instruments are 10.3" -
slightly too small ( 1.5mm ) but I sent the guy an email
& we'll see if he can supply me with some tube rings that will fit.

Also I read the whole 4 pages about using a finder scope
as a guide scope & that was very interesting.
It shows another method I could use which lightens the load
& still works due to sub pixel accuracy of PHD guiding.

Further note:
Parallax can machine the 10.3" rings to fit so I ordered some.

Edited by alpal (04/12/11 07:34 AM)


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4513103 - 04/12/11 10:41 AM

Congrats on getting Parallax rings.

If you want "D" style dovetail, do not order it until you receive the rings. The holes separartion for mounting the rings on the dovetail may be English (2") or metric (60mm). If you already know the hole separation or spacing in new tube rings, ADM Accessories sells both English and metric versions dovetail. I highly recommend to get it. I also suggest to get the longest length and get two of them, the second dovetail for mounting on top of the rings for guide scope. Don't forget the "D" style saddle as well. You won't regret getting wider and stiffer dovetails.

See http://www.admaccessories.com/D_Series_Universal.htm and you will see what I mean by holes separation or spacing.

Ask ADM for the recommended length dovetail for your scope. ADM are very friendly and will customize at very little or no extra cost.

You can ask Parallax whether the mounting holes spacing is English or Metric and how wide they are.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4513779 - 04/12/11 05:14 PM

Thanks Peter - you're giving me great advice.
It's difficult to work out what dovetails to order.
Maybe I'll have to wait until I receive the tube rings first
but it must be possible to work out the dovetail arrangement.

I will also check out those special 6 screws.

I would like to get it all quickly as good imaging time may be soon.

These forums are so powerful as I can get advice which
is just not available anywhere else.


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4513798 - 04/12/11 05:25 PM

After receiving new equipment, let us know the outcome. It may take a while since you live down under and shipping from USA may take a while. From ADM web site, it listed one Australian dealer, hopefully they will have what you need in stock.

Good luck.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4513815 - 04/12/11 05:35 PM

Quote:

After receiving new equipment, let us know the outcome. It may take a while since you live down under and shipping from USA may take a while. From ADM web site, it listed one Australian dealer, hopefully they will have what you need in stock.

Good luck.

Peter




It's ok - just got a reply.
Parallax has all that stuff & will do everything for me.
It should arrive ready to go!


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4513834 - 04/12/11 05:43 PM

That's great news. Hopefully this will solve your flexure issues and your NEQ6 mount is beefy enough for your big scope and astrophotography.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4513949 - 04/12/11 06:48 PM

Quote:

That's great news. Hopefully this will solve your flexure issues and your NEQ6 mount is beefy enough for your big scope and astrophotography.

Peter




Hi Peter - it should be.
I had the Dec & RA adjusted to take out the backlash recently.
I am feeling confident now.
I was getting some quite good frames even with those lousy homemade tube rings & the guide camera in the wrong spot
so we'll see & I'll post my results here & PM
you so you'll know when I post.
cheers
Alpal


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4513995 - 04/12/11 07:12 PM

I am curious of the real source of the flexure in your setup. You are pretty much replacing more than one component which makes it difficult to really track down the source of the flexure. I kind of would like to know but that would waste your time and not worth the effort. I would be mad after replacing all the components and you still see image shifting in between frames. That would be very annoying and I hope that won't happen.

It happened to me when I was using a guide scope guiding an SCT with moving primary mirror. I spend extra money on trying to get this system to work by getting stiffer components and none of them worked because the source of the flexure was the moving primary mirror and the guide scope cannot be in sync with moving primary mirror. So I threw out the guidance system and replaced it with OAG and that solved everything.

If you do still see image shifts in between subs with new parts, probably the next place to look at is the primary mirror mount which I hope that won't happen.

That's the price of this hobby.

Peter


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alpal
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Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4514082 - 04/12/11 07:49 PM

Quote:

I am curious of the real source of the flexure in your setup. You are pretty much replacing more than one component which makes it difficult to really track down the source of the flexure. I kind of would like to know but that would waste your time and not worth the effort. I would be mad after replacing all the components and you still see image shifting in between frames. That would be very annoying and I hope that won't happen.

It happened to me when I was using a guide scope guiding an SCT with moving primary mirror. I spend extra money on trying to get this system to work by getting stiffer components and none of them worked because the source of the flexure was the moving primary mirror and the guide scope cannot be in sync with moving primary mirror. So I threw out the guidance system and replaced it with OAG and that solved everything.

If you do still see image shifts in between subs with new parts, probably the next place to look at is the primary mirror mount which I hope that won't happen.

That's the price of this hobby.

Peter




That's ok - I'll keep going until I get it right.
I will have some different DSLR pics to offer everyone
on this forum since few people are offering pics
from an 8" f6 Newt.
It means galaxies are larger on the prime focus frame
compared with the normal f5 & f4 pics etc.


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