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Equipment Discussions >> Reflectors

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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: coma corrector advice new [Re: Atl]
      #5691073 - 02/20/13 02:53 PM

The High Point Coma corrector looks to be the same as the Astro-Tech one designed by Roger Ceragioli, also sold as the Altair Astro and the GSO, who manufacture it. It does not come ready for visual use and requires extra spacing to take the back focus to 75mm (+/-10mm). I have written a user guide link fixed.

It works well, but would be really good if more information on its optical performance could be made available.

Edited by cjc (02/21/13 10:51 AM)


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Atl
professor emeritus
*****

Reged: 04/13/12

Re: coma corrector advice new [Re: cjc]
      #5691736 - 02/20/13 08:49 PM

According to High Point you just put it in. It is adjusted for f4.5. Your link is dead.

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: coma corrector advice [Re: Atl]
      #5691762 - 02/20/13 09:04 PM

Well, like the Astrotech, it will require some "fiddling" to get the eyepieces all properly placed in relation to the lens in the coma corrector.
Best coma correction will occur with the coma corrector at only one place in the optical cone coming from the primary, and with the focal planes of the eyepieces at a set distance from the lens.
This is accomplishable with parfocalizing rings, so it's not all that complicated. When you're done, all your eyepieces will be par-focal (if they can get to an optimum position), and coma correction will be at the best deliverable by the design of the corrector.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: coma corrector advice new [Re: Atl]
      #5692663 - 02/21/13 12:34 PM

Quote:

According to High Point you just put it in. It is adjusted for f4.5. Your link is dead.




Back focus is independent of focal ratio. If it is the same as the Astro-Tech, GSO, etc, then additional spacing will be needed. I have fixed the link above.


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daveyfitz
member


Reged: 01/02/13

Loc: Utah
Re: coma corrector advice new [Re: Starman1]
      #5694508 - 02/22/13 11:39 AM

Quote:

The coma correctors on the market are designed to have a set distance from the eyepiece's focal plane and the lens in the CC.




Don:
Reading through Chris' user guide, link fixed he says the distance is around 75mm, and added a 19mm spacer to his CC (and that was just to reach 66mm).

If that spacing is typical, and we wanted to get to the optimum (75mm), using your method instead of putting spacers into the CC, we would be putting a parafocalizing ring at (19 + 9) 28mm.

That seems like a looooong way out for a parafocalizing ring.

Do I misunderstand something?


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: coma corrector advice new [Re: daveyfitz]
      #5694564 - 02/22/13 12:04 PM

Quote:

Quote:

The coma correctors on the market are designed to have a set distance from the eyepiece's focal plane and the lens in the CC.




Don:
Reading through Chris' user guide, link fixed he says the distance is around 75mm, and added a 19mm spacer to his CC (and that was just to reach 66mm).

If that spacing is typical, and we wanted to get to the optimum (75mm), using your method instead of putting spacers into the CC, we would be putting a parafocalizing ring at (19 + 9) 28mm.

That seems like a looooong way out for a parafocalizing ring.

Do I misunderstand something?



Eyepieces have their focal planes anywhere from the very bottom of the barrel to way up inside the eyepiece. Each of these will require a dramatic amount of refocusing if used in a telescope without a coma corrector, and a dramatic re-spacing of the eyepiece-to-lens distance to optimize the eyepiece.

In order to get all those focal planes at the correct distance from the lens, it may be necessary to use:
--parfocalizing rings
--barrel extenders on the eyepiece
--drawtube extension tubes (unlikely, but possible)
--low-profile 1.25" adapters (if the 1.25" EP has it's focal plane way up inside)
--high profile (e.g. TeleVue Top Hat) 1.25" adapters (if the focal plane is near the bottom of the eyepiece)

If a particular eyepiece needs to come 28mm out of the CC to hit the sweet spot, a barrel extender on the eyepiece would probably do it. The TeleVue barrel extender, for instance, is 30.5mm long.
It's also possible that a 2" eyepiece might need an extension tube, though these these tend to pull the eyepieces back 50mm or more, so unlikely.

Spacing the eyepiece OUT is fairly easy. It's when an eyepiece needs to get closer to the lens that the difficulty arises. For example, the 14 and 17.3 Delos need to be closer to the Paracorr lens than the top of the Paracorr. Fortunately, there are some "drop-down" adapters available in the market that work perfectly for those. In the same way, the 31 Nagler and 21 Ethos needed a closer position to the Paracorr lens than the in-most setting of the original Paracorr, which is one of the reasons we got a Paracorr II.

The Coma correctors without telescoping tops simply utilize spacers to accomplish the same thing. And in the process of adjusting the eyepiece-to-lens distance, we also parfocalize our entire eyepiece collection. That's not a bad thing at all.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: coma corrector advice new [Re: Starman1]
      #5695053 - 02/22/13 04:03 PM

Don is right in that eyepieces can have their focal point almost anywhere, the Ethos range comes to mind. However many are parfocal or are nearly so like the Televue Plossls up to 32mm. My eyepieces are nearly parfocal, partly because I have rejected those that are too far out...

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daveyfitz
member


Reged: 01/02/13

Loc: Utah
Re: coma corrector advice new [Re: cjc]
      #5695076 - 02/22/13 04:15 PM

Quote:

Don is right in that eyepieces can have their focal point almost anywhere, the Ethos range comes to mind. However many are parfocal or are nearly so like the Televue Plossls up to 32mm. My eyepieces are nearly parfocal, partly because I have rejected those that are too far out...




So, what's the best way to determine the focal point?


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: coma corrector advice new [Re: daveyfitz]
      #5695146 - 02/22/13 04:53 PM

Quote:

Quote:

Don is right in that eyepieces can have their focal point almost anywhere, the Ethos range comes to mind. However many are parfocal or are nearly so like the Televue Plossls up to 32mm. My eyepieces are nearly parfocal, partly because I have rejected those that are too far out...




So, what's the best way to determine the focal point?



Take a simple eyepiece in which, when you look in the bottom, you can see a ring or iris representing the focal plane. Measure the distance from that ring to the bottom of the eyepiece and then measure the length of the insertion barrel. The difference is the distance the focal plane is above or below the "shoulder" of the eyepiece.

Put the eyepiece in a scope and focus. measure where the focuser drawtube is relative to the stop of the focuser. Insert eyepiece two and refocus. If the focuser moved in (whatever the measurement is), the focal plane in eyepiece 2 is higher in the eyepiece than eyepiece 1. If the focuser moved out, the focal plane in the second eyepiece is lower in the barrel than eyepiece 1.
Keep track of the extra inward and outward travel for each eyepiece.
By comparing that movement to the distance-from-shoulder derived from eyepiece 1, you can get the relative position of the focal plane in every eyepiece.
The differences are what you have to accommodate when you use a coma corrector.
Once you have even one eyepiece with the focal plane the correct distance from the coma corrector lens, every other eyepiece can have its optimum position determined by merely sliding the eyepiece in and out of the coma corrector until it is in focus (not using the focuser). You don't really have to measure them. But be prepared for some different positions of the eyepieces with spacers, extenders, and par-focalizing rings.
And if you get a new eyepiece later, you can easily determine its correct spacing the same way.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: coma corrector advice new [Re: daveyfitz]
      #5696048 - 02/23/13 04:25 AM

Quote:

So, what's the best way to determine the focal point?




Do it in daylight on a distant object or on something bright like the moon. Tissue paper over the focuser will show you the true focal point. Note the movement up or down for your eyepiece to the nearest mm or so. This movement does not depend on the distance of the object or the scope you are using. A refractor with a focusing scale is covenient.


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