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Equipment Discussions >> Cats & Casses

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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Skunky]
      #6173679 - 11/03/13 12:37 PM

Quote:

Quote:



This is simply not true when you use modern methods based on hardware and software. MetaGuide has allowed collimation based on the Airy pattern in mediocre seeing for many years. Times change and techniques change.

Frank




Another plug, free advertising, from Frank..

You can also use a Hotech Advanced Laser Collimator. And you don't even need a star..





Excuse me skunk spliff......

Frank's Metaguide is FREE ...no plug from him needed as there's nothing in it for him.

Metaguide collimates on an in focus real star with the diffraction ring clearly seen in awful seeing....Pickering 3-4...... Ask me to post an image of the airy disc and diffraction ring from my SCT...


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6173695 - 11/03/13 12:53 PM

These forums can really get you down.

Hotech cannot be used for perfect, final collimation.
You have to collimate on an IN FOCUS STAR in the sky with the scope pointing up (primary will tilt) and adjust the secondary on the airy pattern with that DIFFRACTION RING.... there is no other way....if the ring is not whole and evenly illuminated then it is not perfectly collimated.

This is why Hotech tell you to finish the job on a real star.
This is why Celestron say in all their manuals to collimate on a real star in the sky or a sunlight glint on a high pole...as long as the scope is pointing UP say 45 degrees plus.

An SCT new from Celestron or Meade, etc. will have to be unboxed, set up, mounted and finely collimated on a real star high in the sky...if high power planetary or double star observing is your thing.

If the seeing is no good, and it never really is, then the only way is with a cheap webcam, a laptop, Barlow and Frank's most wonderful FREE to all users Metaguide software which will clearly show that illusive diffraction ring surrounding the bright Airy disc... A few tiny tweaks on a collimation screw later and that rough but useable factory new or Hotech adjusted optical instrument will be in spot on collimation on an IN FOCUS STAR.

'Nuff said'


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Skunky
sage


Reged: 09/16/13

Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6173858 - 11/03/13 02:28 PM

 

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RossSackett
Carpal Tunnel
*****

Reged: 08/17/07

Loc: Memphis, TN
Re: Does "seeing" have an effect on sct collimation? *DELETED* new [Re: Skunky]
      #6173941 - 11/03/13 03:20 PM

Post deleted by RossSackett

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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: RossSackett]
      #6173955 - 11/03/13 03:28 PM

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

There's no doubt...fine collimation has to be done pointed up at an in focus star or glint of sunlight.

Some people don't want to accept stuff that they don't know.
I hope skunkman enjoys his views.


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174465 - 11/03/13 08:52 PM Attachment (4 downloads)

Quote:

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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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174503 - 11/03/13 09:13 PM

Quote:

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.


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Simon Alderman
member
*****

Reged: 08/22/12

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6174595 - 11/03/13 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!


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6174796 - 11/04/13 02:27 AM

Quote:

Quote:

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!


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174798 - 11/04/13 02:29 AM

Guy in a furry hat...

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

Great post.
Lovely sketch.


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174824 - 11/04/13 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.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6174829 - 11/04/13 03:41 AM

Quote:

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.


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6174867 - 11/04/13 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?


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6175277 - 11/04/13 11:34 AM

Quote:

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.


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6175317 - 11/04/13 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


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WesC
Carpal Tunnel
*****

Reged: 02/06/13

Loc: La Crescenta, CA
Re: Does "seeing" have an effect on sct collimation? new [Re: HowardK]
      #6175382 - 11/04/13 12:41 PM

...in his opinion.

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DesertRat
Postmaster
*****

Reged: 06/18/06

Loc: Valley of the Sun
Re: Does "seeing" have an effect on sct collimation? new [Re: Alph]
      #6175637 - 11/04/13 02:39 PM Attachment (9 downloads)

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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RossSackett
Carpal Tunnel
*****

Reged: 08/17/07

Loc: Memphis, TN
Re: Does "seeing" have an effect on sct collimation? new [Re: WesC]
      #6175648 - 11/04/13 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...

Edited by RossSackett (11/04/13 03:15 PM)


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Eric63
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Does "seeing" have an effect on sct collimation? new [Re: RossSackett]
      #6175853 - 11/04/13 04:34 PM

You guys just need a Mak!! 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!

(Sorry I couldn’t resist)


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drollere
Pooh-Bah
*****

Reged: 02/02/10

Loc: sebastopol, california
Re: Does "seeing" have an effect on sct collimation? new [Re: DesertRat]
      #6176157 - 11/04/13 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.


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