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Does "seeing" have an effect on sct collimation?

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#26 Asbytec

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Posted 03 November 2013 - 08:52 PM

If the seeing is no good, and it never really is...

Well, never say never. :)

The tropics can put up some excellent seeing in modest apertures, anyway. I can regularly get down to 4 waves and sometimes less than one wave prior to final touch up on the first ring itself. (Had to sketch it one night, seeing was awful good...absolutely awful. I mean, inspiring awful.)

Really, though, it might be 7 of 10 is sufficiently good seeing in-focus with patients, but 8/10 and better allow for very close to focus inspection without the Poisson spot jumping all over the place and the first bright ring closing in on it.

Otherwise I totally agree with you, use a star in focus when seeing permits. If not, nothing wrong with a web cam and some free software (thanks Frank.) There is a certain beauty to a well collimated star image with an equally bright ring and a certain confidence you're getting the most your scope can offer especially when seeing cooperates, too.

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#27 Alph

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Posted 03 November 2013 - 09:13 PM

I have also seen collimation degrade awfully between horizontal/artificial star and pointed up at 60 degrees.


If that was the case then SCTs would not hold collimation on the other side of the meridian. There is definitely something wrong with your OTA. I would call Celestron.

#28 Simon Alderman

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Posted 03 November 2013 - 10:16 PM

"These forums can really get you down."

Well, I agree that it can be hard to read past some of the vitriol apparent in the remarks, but as the OP i feel like my question was answered. Plus I learned a few things I hadn't expected to. I'm looking forward to some good seeing so I can spend some time on dialing in the collimation on my little sct and I may look into metaguide at some point. As the Hotech system costs more than I have in my entire scope, I may hold off on that for a bit...;)

Thanks' to all for the info!

#29 HowardK

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Posted 04 November 2013 - 02:27 AM

I have also seen collimation degrade awfully between horizontal/artificial star and pointed up at 60 degrees.


If that was the case then SCTs would not hold collimation on the other side of the meridian. There is definitely something wrong with your OTA. I would call Celestron.


You are right....without mirror locks collimation is likely to change across the meridian......

That's why celestron now fit solid mirror locks....not 1 but 2 ....to stop the mirror flopping.

Any top planetary imager will test collimation on a star near the planet.
If there is a meridian flip then they will recheck.

For DSO's it's not relevant.
High mag planetary, double star stuff...it is.

I said my bit
If you do not think a large mirror titls on its bed of grease as it swings around the sky then you are entitled to that belief.

Just don't ask me to image Jupiter this season through your scope!
I'll use mine thanku!

#30 HowardK

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Posted 04 November 2013 - 02:29 AM

Guy in a furry hat...

My seeing is never better than 4/10.
I need Metaguide.

Great post.
Lovely sketch.

#31 Asbytec

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Posted 04 November 2013 - 03:26 AM

Howard, do you use larger aperture?

Yea, thank you, I had to sketch it never having really seen so close so clearly before (accustomed to average or lesser seeing most of my life.) It was stunning, one of those nights where you look skyward and offer a silent prayer of thanks. The colors were interesting, too. (Arcturus, IIRC)

But, the whole point being, seeing does affect our effort to collimate, IME. Good seeing allows one to really dial it in and, in conjunction with good seeing, good collimation really boosts the observing experience. If seeing is not as good, then the image is less steady and centering is less certain. Then you can use a tool that captures the disc and rings in real time. You're still optimizing a variable you can control.

Long before software tools were readily available (for SCT mostly), we used to use distant sun lit point sources before the stars came out and even check it after dark. Maybe fiddle with it. I am sure software makes it a snap and is more accurate.

#32 Alph

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Posted 04 November 2013 - 03:41 AM

If you do not think a large mirror titls on its bed of grease as it swings around the sky then you are entitled to that belief.


As I said call Celstron. Tell them to fix it.

#33 HowardK

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Posted 04 November 2013 - 05:08 AM

I called celestron UK today...or the importer should I say.

They told me to collimate on a real star near zenith to avoid mis collimation when the mirror tilts from horizontal.

Just like it says in the celestron manual.
Your scopes have very tight baffles so that your mirror never tilts..you are very lucky....you must have zero focus shift as well and never need to lock down the mirror locks if you have them.

Or maybe you only look at objects below 20 degrees?

#34 Alph

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Posted 04 November 2013 - 11:34 AM

They told me to collimate on a real star near zenith to avoid mis collimation when the mirror tilts from horizontal.


That's right! That means you can also collimate SCT in the horizontal position. Obviously you can't use a real star when the OTA is in the horizontal position. What you can use is an artificial star or the HotTech Collimator. Let's put that urban legend and myth to rest.

#35 HowardK

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Posted 04 November 2013 - 12:04 PM

Ok alfie

I give in
You are right
You CAN collimate an SCT horizontally with an artificial star
I have been misinformed
Thanx for your input on this

#36 WesC

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Posted 04 November 2013 - 12:41 PM

...in his opinion.

#37 DesertRat

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Posted 04 November 2013 - 02:39 PM

There is no urban legend concerning horizontal collimation that needs to be put to rest. One can perform a collimation procedure horizontally using an artificial star or using a laser system.

But the acceptable amount of coma and/or astigmatism the intended application can tolerate varies mostly on the image scaling, be it visual or electronic. That (and other points not covered here) will determine the best collimation procedure.

For viewing DSO's at 100-150X one can accept a fair amount of coma without excellent collimation, especially in average or worse seeing.

For high resolution imaging of planets the requirement is pretty severe, meaning 1/14 wv rms of coma or something approaching the classic 'diffraction limited' definition falls well short of the needed accuracy of collimation.

Attached is a 3X IR image of Aldebaran at approx 0.033"/pix taken in mediocre seeing (64 frames of > 2000). It represents the minimum acceptable collimation error I tolerate. The star here needs to be moved about 45 arc sec to the right. Experience has shown that a collimation error greater than this can be damaging in green and blue light, which are much more sensitive, and show coma more dramatically when seeing permits. To overcome dispersion it is essential to collimate with a star as far overhead as practical.

Stars are plentiful and free. Video cameras are inexpensive, and the software tools are free as well.

Glenn

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#38 RossSackett

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Posted 04 November 2013 - 02:42 PM

After viewing the HOTECH laser collimation videos I can see why some people are so invested in horizontal collimation--the gear is awfully expensive and the adjustments very involved compared to star collimation. Sure would be a waste if it turns out that collimation shifts with scope elevation, as many of us have experienced. I could be mistaken but I found no mention of turning the focuser knob counterclockwise to seat the mirror or locking the primary before beginning the procedure--you could waste a lot of time making twitchy adjustments if you ignore those steps. Maybe it's a great product and repays the investment; Rod Mollise seems to like them and that says a lot to me. But I've seen apparently well-aligned scopes that were still out of optimal collimation in a star test. Newts are SO much easier...

#39 Eric63

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Posted 04 November 2013 - 04:34 PM

You guys just need a Mak!! :grin: It will hold collimation from one good seeing period to another while a SCT might lose collimation before another good night of seeing permits a star test. Hotech Lasers and webcams, what are those? Just show me Polaris! :roflmao:

(Sorry I couldn’t resist)

#40 drollere

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Posted 04 November 2013 - 07:47 PM

i am also not much of a fan of hotech equipment, and leave it at that. but even with good equipment, the fact that you can see a problem in a star image means that you can also see directly how to fix the problem -- no hardware required.

yes, you can only do approximate collimation in bad seeing, but the bad seeing will make your collimation error inconsequential anyway. you typically will get the accuracy of collimation you can use.

if your scope has a central obstruction, use the extrafocal side of focus that shows you the tiny poisson spot, not the intrafocal side that shows you the black hole in a bagel. the poisson spot, like the airy disk, is surprisingly robust against seeing. in poor seeing the problem is not the center of the defocused image, it's the rapidly moving circumference that makes collimation difficult.

collimation is maintenance, not configuration. you should routinely (like, why not, every night before you start observing) examine a slightly defocused star image. this lets you check the "mirror seeing" or thermal mirror plumes (usually as an apparent diffraction "wedge" arising from the center of the mirror), bad collimation, as well as atmospheric currents.

#41 azure1961p

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Posted 04 November 2013 - 08:40 PM

Exactly Bruce. If the seeing us so poor the star can't be as collimated as a better night - what's the net loss then??? Conversely on the best night with the stilleat air will allow the cleanest diffraction patterns and in the kind of conditions probably needed to see the difference. It kind if takes care of it itself it would seem. Now if the best collimation were oy possible in the worst nights - well then that'd be a problem.

Pete

#42 DesertRat

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Posted 04 November 2013 - 11:07 PM

Bruce & Pete,
I agree with you to a point. However in lucky imaging which may occur hours after a collimation check , the seeing may have improved enough that the check or adjustment was useful. Even on a sub par night the seeing may improve for enough moments to be useful in imaging. I almost always check collimation, but rarely make changes unless the configuration has changed.

Back to the OP's question, if I were only doing visual collimation in poor seeing I would not make any corrections, unless the error was huge, like a decentered donut. If it was known to be ok the last time seeing was better, I would not make any corrections.

The benefit of video collimation is that lucky imaging of an airy pattern can facilitate small corrections even in average seeing.

Eric,
I have a IM180 and an Edge14 SCT. Both hold collimation quite well.

Ross ,
SCT collimation is one of the easiest to perform. There are only 3 screws to adjust, and most good SCT's hold collimation well for months. Sometimes the endless threads on collimation in this forum may give people the impression it is a big problem. It is not.

Glenn

#43 PGW Steve

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Posted 04 November 2013 - 11:59 PM

There seems to be some firm beliefs in regards to OTA position and mirror flop when it comes to collimation.

I think it is a known fact that there is some degree of primary mirror movement in standard C and M brand SCT's.

My first example will be collimating straight up. The primary mirror should be neutral and sitting as close to its mechanical centre as possible. This will yield collimation at '0'. As I slew around the sky, and my mirror shifts and amount of '1', my optical train will be out by '1' from the value I collimated at.

Now if you collimate in a horizontal position, and slew up to the zenith, you will be out by '1'. Carry on slewing to a direction opposite to where you collimated and you can approach a worst case scenario of '2'....albeit at the horizon where you won't be observing anyway unless you are trying for Omega Centauri from Ottawa Ontario.

Collimating at zenith has less air mass, and less atmospheric affect on seeing, no disputing this. Perhaps even the tube currents at zenith being straight up vs boiling the star image in the tube will be more stable. I've never gone from say 60 degrees to zenith in a period of a minute to verify this, but the science makes sense.

There is a lot of merit to collimating at zenith on an actual star.

I've used CCD Inspector for CCD assisted collimation and I am happy with the results.

Metaguide sounds like a nifty product, does it work with a DMK41??

#44 freestar8n

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Posted 05 November 2013 - 02:50 AM

If the seeing us so poor the star can't be as collimated as a better night - what's the net loss then???



Seeing changes over time and the situation for collimation is similar to the one for focus. Software can help find true focus for a real star in-situ even when the seeing makes it hard to tell visually, and if you image for some time and the seeing improves, you will reap the benefit of that optimal focus. Similarly, video and software lets you collimate optimally even if the seeing isn't ideal, in-situ using a real star, so that when seeing improves, you "see" it.

If the collimation isn't optimal and you just leave it that way, there will still be nights of different performance where things are better one time than another - but on the really good nights you may be limited by whatever collimation state the 'scope is in - and you won't know it.

Where this really matters is planetary video - and although those guys tend to keep their methods to themselves - I am pretty sure they collimate all the time. With skill, which they appear to have, they may be able to do well just by looking at the fuzz around jupiter's moons, for example, while studying the video and making small adjustments. This is completely in-situ at the precise altitude of the object because it is collimating on the actual object with the actual imaging train.

Frank

#45 freestar8n

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Posted 05 November 2013 - 04:04 AM

examine a slightly defocused star image.



A good defocused star image is neither necessary nor sufficient for a good in-focus Airy pattern. Decentering of a spherical secondary is a form of apodization - not aberration.

Frank

#46 Namlak

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Posted 07 November 2013 - 10:36 PM

I see this idea that an SCT pointed straight up has it's mirror at the mechanical center when it seems to me that the mirror is being held up by the off-center focusing screw and leaning against the baffle tube, off-center.






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