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Old 28-11-2018, 01:46 PM
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Precise Collimation

I have been chasing my tail on this one for some time now, which has to some extent due my use of CCD inspector in pursuit of "collimation Nirvana"

I've not found it yet, as when I point my system to a different star field (literally a few arc minutes away) , CCDinspector often tells me my collimation has changed by a poofteenth.

Here is a link to results thus far

But I'd be curious to hear how others have nailed down their systems....and *really* interested if you are using a 35mm format or larger sensor.
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Old 28-11-2018, 02:08 PM
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You mentioned you aligned your secondary with a glatter's concentric pattern. Was this to center it only or did you do the tilt as well? Wouldn't the TAK scope with the center spot be more accurate?
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Old 28-11-2018, 02:24 PM
glend (Glen)
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Is this a GSO RC? If so I doubt collimation nirvana can ever be achieved. You can get it close, but if you want collimation nirvana - buy a good APO.
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Old 28-11-2018, 02:25 PM
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Quote:
Originally Posted by multiweb View Post
You mentioned you aligned your secondary with a glatter's concentric pattern. Was this to center it only or did you do the tilt as well? Wouldn't the TAK scope with the center spot be more accurate?
The secondary centre spot as viewed with the Tak scope and laser spot agreed..but the laser is more telling when you look at the beamís reflection back to the laser...small errors are decidedly more detectable that with the Tak alone.
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Old 28-11-2018, 02:31 PM
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Quote:
Originally Posted by Peter Ward View Post
The secondary centre spot as viewed with the Tak scope and laser spot agreed..but the laser is more telling when you look at the beam’s reflection back to the laser...small errors are decidedly more detectable that with the Tak alone.
Fair enough. So you move the secondary spider until the center spot and the laser coincide then adjust the tilt to have a perfect concentric reflection back on to the primary? Do you have enough rings spaced enough to do that given the short path and the secondary is convexe?
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Old 28-11-2018, 02:37 PM
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Quote:
Originally Posted by glend View Post
Is this a GSO RC? If so I doubt collimation nirvana can ever be achieved. You can get it close, but if you want collimation nirvana - buy a good APO.
Nup, itís a cheap GSO knock-off.

APO? hummm....think I might have one kicking about somewhere in the shed.

Seriously though, the scope Iím using has no, zero, nada, nil flexure issues. All sky variation is around 1/10th of an arc second, which may simply be refraction or a CCDinspector artifact. With A KAF8300 sensor results were indeed perfect...but the KAF16803 random, and to be fair very tiny, variations have me puzzled.
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Old 28-11-2018, 02:43 PM
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Originally Posted by multiweb View Post
Fair enough. So you move the secondary spider until the center spot and the laser coincide then adjust the tilt to have a perfect concentric reflection back on to the primary? Do you have enough rings spaced enough to do that given the short path and the secondary is convexe?
Oh crap no...the spider on my scope is set at the factory and should never be touched.

The seconday mirrorís center spot can be aligned using the Taka or Laser...but itís not a point, itís a spot...hence Iíve found the laser reflecting back on itself does a better job that guestimating the spot centration.
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Old 28-11-2018, 02:51 PM
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Quote:
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Oh crap no...the spider on my scope is set at the factory and should never be touched.

The seconday mirrorís center spot can be aligned using the Taka or Laser...but itís not a point, itís a spot...hence Iíve found the laser reflecting back on itself does a better job that guestimating the spot centration.
Oh wow, set at the factory? Well that makes sense now that you can rely on the reflection.
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Old 28-11-2018, 03:28 PM
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Quote:
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Nup, it’s a cheap GSO knock-off.
There's something cheaper and nastier than a GSO RC? Really?

Surely Alluna doesn't make sonotube RC's?
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Old 28-11-2018, 03:42 PM
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There's something cheaper and nastier than a GSO RC? Really?

Surely Alluna doesn't make sonotube RC's?
I'm not knocking GSO...they are good value for money, but also built to a price.

Alluna are actually now making "closed Carbon Tube RC telescopes
Splash-water-proof and dustproof, nitrogen flooding,
Zerodur mirrors, extremely stable" from memory....an eye watering 100k Euro
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Old 28-11-2018, 03:57 PM
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With the amount of collimation shift that CCDInspector is picking up I’d largely put it down to error in software.

Atmospheric refraction and airmass are linked so you should be able to easily test whether the atmosphere is in play. Going from 40į to 90į on both sides of meridian with zero mechanical play should show a repeatable and calculatable collimation shift with respect to airmass. I think Maxim even gives an airmass value to each exposure in the FITS header, been a while since I looked.
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Old 28-11-2018, 04:37 PM
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Nothing wrong with a GSO RC, it just needs a bit more finessing than a scope 44k more expensive. That said, I feel your pain Peter. Still trying to eliminate issues with the AG12. The RC12 on the other hand is collecting data on a regular basis. Might buy a Harmer Wynne if the AG12 cannot be tamed.
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Old 28-11-2018, 05:26 PM
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Quote:
Originally Posted by Paul Haese View Post
Nothing wrong with a GSO RC, it just needs a bit more finessing than a scope 44k more expensive. That said, I feel your pain Peter. Still trying to eliminate issues with the AG12. The RC12 on the other hand is collecting data on a regular basis. Might buy a Harmer Wynne if the AG12 cannot be tamed.
Not pain Paul, more puzzlement. I am very happy with the data out to the edge of the 16803...with FWHMís under 1.9 arc sec.

I think Atmos is correct...CCDinspector is not perfect. I can literally change the focus 10 steps (about 1/1000th) in the same location and it gives me a different result.....as to why? A mystery so far.
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Old 28-11-2018, 05:45 PM
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I have never found CCD inspector terribly reliable. It can change only a few degrees away as you say. It can show different results either better or worse depending on elevation and not necessarily that the lower you go the worse the result.
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Old 28-11-2018, 05:59 PM
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Quote:
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I have never found CCD inspector terribly reliable. It can change only a few degrees away as you say. It can show different results either better or worse depending on elevation and not necessarily that the lower you go the worse the result.
Beat me to it !

I was going to add, I saw the same behaviour with my AP Riccardi Honders.

Hence more a case of understanding what CCDinspector is actually looking at, rather rather than blaming the instrument, which mystically goes from Nirvana to something less...
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Old 28-11-2018, 10:14 PM
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Quote:
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Beat me to it !

I was going to add, I saw the same behaviour with my AP Riccardi Honders.

Hence more a case of understanding what CCDinspector is actually looking at, rather rather than blaming the instrument, which mystically goes from Nirvana to something less...
With CCD inspector even changing exposure to show more stars can change the result its a blunt instrument but effective in giving you an idea where you are . I usually take a pic of something like the Jewel box and base it off that.
I do exactly what you have done using a laser to centre focuser and secondary then a star image to adjust the primary I just use the Tak scope as a second check . Been working well that way for 6yrs
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Old 30-11-2018, 10:31 AM
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Managed to do some more finessing of my RC's collimation last night

Seems I've finally achieved collimation Nirvana
(leastways according to CCDinspector)

It was literally 1/20th of a turn on one of the primary's adjustment bolts.

Min FWHM of 1.36"
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Old 30-11-2018, 11:59 AM
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Quote:
Originally Posted by Peter Ward View Post
Managed to do some more finessing of my RC's collimation last night

Seems I've finally achieved collimation Nirvana
(leastways according to CCDinspector)

It was literally 1/20th of a turn on one of the primary's adjustment bolts.

Min FWHM of 1.36"
Looks great it only takes a tiny tweak the only issue I find on the GSO is that there is a tiny bit of shift when locking the primary down unfortunately .
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Old 30-11-2018, 02:03 PM
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Impressive result Peter. Even more so if it holds.
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Old 30-11-2018, 07:27 PM
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Impressive result Peter. Even more so if it holds.
Well, I have to say I was amazed to see a FWHM of 1.36"....but have no doubts about the mechanical rigidity. I've never experienced meridian flip issues etc....the task now is to get some quality data

Hummm M42 is up!

Quote:
Originally Posted by Marke View Post
Looks great it only takes a tiny tweak the only issue I find on the GSO is that there is a tiny bit of shift when locking the primary down unfortunately .
Indeed. Tiny changes. As for the GSO....Really? I've not experienced that problem.
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