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Thread: Oops... Embarrassed by Astrotortilla!

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    Phil Leigh's Avatar
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    Default Oops... Embarrassed by Astrotortilla!



    So, I went out last night to replace the tripod with the new pillar mount. It was-3c...
    Anyway, managed to do it fine... Very impressed by the stability and ease of levelling... And the height is perfect for me and my site.

    So, I THOUGHT I had the pillar in the right place and a "close enough" polar align... I started guiding through intermittent clouds and noticed that the PHD graph was basically a small height, rapid frequency triangle waveform... Osc was .11 and RMS 0.3

    Not sure what was going on so I tried a 5 minute exposure and got round stars... But something wasn't right.

    Then I remembered the discussion earlier about AT and polar alignment... So I thought I'd try the PA tool in AT( sorry about the acronyms!).
    This tool is quite clever but it only measures the PA error, it doesn't really help you fix it(like Alignmaster does).


    So to cut a long story short, after 5 minutes, AT gleefully informed me that I had an error of >7 degrees in ALT and 5 degrees in AZ...

    Wow! - and I'm still getting round stars at 5 mins with guiding+PEC

    Further investigation is warranted... I'll see how far I can push the exposure time and then set about correcting the PA.

    regards
    Mr Embarrassed.
    Telescope: SWEquinox ED80 Pro (Schott/Ohara Fluorite)+Baader Steeltrack, C8 XLT Mount: Skywatcher HEQ5 Pro, Rowan belt drive on Pillar mount+EQDIR+ADM saddle/dovetails Camera: JTW1100D mono , CentralDS 600, ASI120MM. ASI120MC, IDAS & Baader NB 2-inch filters, TS Filter Drawer System Accessories: TV Nagler T4 12 mm, Baader 8-24 MkIII, Ortho 5mm, Aspheric 31mm, Meade 12mm reticle, TV Powermate 2x & 2.5x, JMI+BAST Motorfocus+FCUSB, TV 0.8x FR/FF, Celestron 0.63FR, Orion RACI & Mag Mini, Canon IS 15x50 Bins
    Software: EQMOD/Stellarium/Autostakkert!/Registax/BackyardEOS/PixInsight,PHD2,AlignMaster, AstroTortilla, Straton

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    Default Re: Oops... Embarrassed by Astrotortilla!

    7 degrees in ALT and 5 degrees in AZ
    Glad your setup is working for you Phil. That really is amazing that you got 5 min exposures with the polar align that far off..?? The PHD graph tells the story, with a .11/.3 you should be able to image for a half an hour. I think there's something amiss with the AT numbers...?

    Curious to see what your follow up post says. Thanks for the nice report.

    Best,

    Paul
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    Default Re: Oops... Embarrassed by Astrotortilla!

    Quote Originally Posted by Phil Leigh View Post
    So, I went out last night to replace the tripod with the new pillar mount. It was-3c...
    Anyway, managed to do it fine... Very impressed by the stability and ease of levelling... And the height is perfect for me and my site.

    So, I THOUGHT I had the pillar in the right place and a "close enough" polar align... I started guiding through intermittent clouds and noticed that the PHD graph was basically a small height, rapid frequency triangle waveform... Osc was .11 and RMS 0.3

    Not sure what was going on so I tried a 5 minute exposure and got round stars... But something wasn't right.

    Then I remembered the discussion earlier about AT and polar alignment... So I thought I'd try the PA tool in AT( sorry about the acronyms!).
    This tool is quite clever but it only measures the PA error, it doesn't really help you fix it(like Alignmaster does).


    So to cut a long story short, after 5 minutes, AT gleefully informed me that I had an error of >7 degrees in ALT and 5 degrees in AZ...

    Wow! - and I'm still getting round stars at 5 mins with guiding+PEC

    Further investigation is warranted... I'll see how far I can push the exposure time and then set about correcting the PA.

    regards
    Mr Embarrassed.
    Holy Cow, Phil,

    I agree with Paul, errors that large in PA would seem like the mount is leaning like a drunken Sailor...

    Are you sure its not 0.7 and 0.5 degrees? Even those values should be visible by eye if you are setting up in a location you use regularly... I'm assuming you did not change the elevation/Latitude setting of the head when changing to the pillar? As long as all is plumb and level, I would expect the change to be in tenths od a degree at worst...
    Thanks, Mitch - K1FH
    And sidekick Shadow, the precocious one
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    Default Re: Oops... Embarrassed by Astrotortilla!

    It's toooooo cold for me tonight, but tomorrow is supposed to be clear and a little warmer so I will follow up tomorrow... will try and get a better "visual" PA using a true north compass and clinometer in the daytime.
    Maybe AT was freaked by the large error... but I hear what you guys are saying and it makes sense.
    Thanks
    Phil
    Telescope: SWEquinox ED80 Pro (Schott/Ohara Fluorite)+Baader Steeltrack, C8 XLT Mount: Skywatcher HEQ5 Pro, Rowan belt drive on Pillar mount+EQDIR+ADM saddle/dovetails Camera: JTW1100D mono , CentralDS 600, ASI120MM. ASI120MC, IDAS & Baader NB 2-inch filters, TS Filter Drawer System Accessories: TV Nagler T4 12 mm, Baader 8-24 MkIII, Ortho 5mm, Aspheric 31mm, Meade 12mm reticle, TV Powermate 2x & 2.5x, JMI+BAST Motorfocus+FCUSB, TV 0.8x FR/FF, Celestron 0.63FR, Orion RACI & Mag Mini, Canon IS 15x50 Bins
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    Default Re: Oops... Embarrassed by Astrotortilla!

    dpending on the guide star, if in the camera's FOV, it will guide fine on that star but over a couple hours you might notice field rotation when stacking.
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    Default Re: Oops... Embarrassed by Astrotortilla!

    Well, here's a bit of a follow-up. My PA was way out for reasons I can't imagine - not sure what happened. So I redid it with AlignMaster and now it is (almost) perfect. Tracking and goto's are excellent. I can get 10 minute guided subs no problem. As part of the testing I took a 5 minute and 10 minute sub and stacked them in PixInsight (no darks/bias/flats!) - a bit of gradient treatment and colour calibration plus a centre crop and this is what came out...

    On the next clear night I'm going to try to measure my PA with AT again...
    Attached Images Attached Images
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    Default Re: Oops... Embarrassed by Astrotortilla!

    Excellent results Phil, especially for only 15 minutes of data to work with. Looks like the mount is ready for a new camera, so you can 'push the limits' and 'go where no man has gone before'...? Looking forward to your progress.

    Regards,

    Paul
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    Default Re: Oops... Embarrassed by Astrotortilla!

    Phil,
    Perhaps you've accidentially demonstrated the power of PHD. My scope is on a scope buggey, so it's "generally" polar aligned to the extent that I can place it in the same place night after night. Frankly, I dismissed the need for fine tuning polar alignment, figuring that PHD would make up for my laziness. I live in an orange area, so long exposures without extreme light pollution fogging are out of the question, which might also mask some of the flaws in my PA.

    I agree with Jeff, PHD doesn't know how well or how poorly your PA is, it just keeps pushing and pulling to keep a point of light in the same spot. What happens to all of the other stars that PHD isn't watching is where the importance of PA comes in. So I would expect you PA alignment errors to show up in field rotation.

    I'm sure there's a way to calculate the correlation between PA errors and field rotation as the distance from your guide star increases, but it's way beyond my ability to do. Also, I would expect that the effect of field rotation would become smaller as your field of view shrinks, and thus the distance from your guide star (assuming it's in your FOV) shrinks.
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    Default Re: Oops... Embarrassed by Astrotortilla!

    From my experiments so far I have discovered that it doesn't matter which star I choose to guide on from the many choices (usually) available - the PHD graph seems similar regardless. I would expect field rotation to lead to star trailing - but I'm not getting any (up to 10 minute subs anyway - I can't go beyond that at my home location because of skyglow/LP) - Maybe tonight I'll try and find where the breaking point is. I'm getting 75-80% good subs - the rest are damaged by some kind of random non-periodic spike (so not removed by PEC) in DEC that I haven't tracked down yet but it may be balance/cable/stiction related. Because it is non-periodic it is hard to track down. It may be that this is the sort of problem that can be dealt with by hypertuning but that will have to wait until next year - I'm out of funds.

    Maybe relevant is that the targets I've been shooting are all near the zenith?
    regards
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    Default Re: Oops... Embarrassed by Astrotortilla!

    Quote Originally Posted by Phil Leigh View Post
    From my experiments so far I have discovered that it doesn't matter which star I choose to guide on from the many choices (usually) available - the PHD graph seems similar regardless. I would expect field rotation to lead to star trailing - but I'm not getting any (up to 10 minute subs anyway - I can't go beyond that at my home location because of skyglow/LP) - Maybe tonight I'll try and find where the breaking point is. I'm getting 75-80% good subs - the rest are damaged by some kind of random non-periodic spike (so not removed by PEC) in DEC that I haven't tracked down yet but it may be balance/cable/stiction related. Because it is non-periodic it is hard to track down. It may be that this is the sort of problem that can be dealt with by hypertuning but that will have to wait until next year - I'm out of funds.

    Maybe relevant is that the targets I've been shooting are all near the zenith?
    regards
    Phil
    Phil,

    It has been my experience that near the "Zenith" is when you will get the poorest tracking. PHD will do a lot to help resolve this however the graphs will be more spikey when the scope is pointed straight up or near straight up.

    One experiment I have been trying is to have an aluminum bar about 34 centimeters long, 1.3 cm thick, 2.5 cm wide with holes drilled & tapped along its length, held to the front end of the dovetail bar under the scope and extended out the east side. On this bar I have an additional small weight, a steel ring about 8 cm dia. 2 cm thick, that I can put at different positions along the bar extending at right angles on the east side of the telescope. With this bar and weight installed, I then balance the OTA/mount as normal...

    The concept is that the added weight off to the side near the center of the mount will shift the neutral balance point such that when imaging near the zenith, the balance is still biased to the east side and the OTA does not reach the neutral balance until the telescope is pointed about 15-20 degrees west of center.

    This added weight seems to help as long as it and the bar are added before balancing however I need more experimenting time with it.

    At this point in time I am operating with the bias weight at the very end of the bar and only conclusion I have reached is it definitely does help when imaging near the zenith however I suspect the weight I am using needs to be twice as large. The current steel ring is too small in mass as compared to the combined weight of the OTA/guidescope and two cameras and presently I am cheating by adding a cast iron 7.5 cm C-clamp to the bar along side the weight... Part of the issue is the primary OTA having a finderscope mounted on the west side which offsets the effectiveness of the present east side bias weight I am using.

    However winter weather has me shut down and I probably will not get back to experimenting with the east side bias weight until spring again...
    Thanks, Mitch - K1FH
    And sidekick Shadow, the precocious one
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