Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Cats & Casses

Pages: 1 | 2 | 3 | (show all)
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Does "seeing" have an effect on sct collimation? new [Re: drollere]
      #6176252 - 11/04/13 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
DesertRat
Post Laureate
*****

Reged: 06/18/06

Loc: Valley of the Sun
Re: Does "seeing" have an effect on sct collimation? new [Re: azure1961p]
      #6176529 - 11/04/13 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
PGW Steve
Pooh-Bah


Reged: 10/03/06

Loc: Winnipeg, Canada
Re: Does "seeing" have an effect on sct collimation? new [Re: DesertRat]
      #6176621 - 11/04/13 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??


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Does "seeing" have an effect on sct collimation? new [Re: azure1961p]
      #6176734 - 11/05/13 02:50 AM

Quote:

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Does "seeing" have an effect on sct collimation? new [Re: drollere]
      #6176767 - 11/05/13 04:04 AM

Quote:

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Namlak
member


Reged: 06/14/12

Loc: Northridge, CA
Re: Does "seeing" have an effect on sct collimation? new [Re: freestar8n]
      #6181962 - 11/07/13 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
17 registered and 26 anonymous users are browsing this forum.

Moderator:  Cotts, Starman27, kkokkolis 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1706

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics