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Equipment Discussions >> Reflectors

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Atl
professor emeritus
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Reged: 04/13/12

coma corrector advice
      #5678548 - 02/13/13 11:05 PM

I got my collimation dialed in pretty good on my f4.6 12.5" dob. On some eyepieces I have coma at the edge of field. Will a coma corrector help with this? I know this may seem obvious but some of my eyepieces show almost no coma while others show a bit. I was making sure that it would work in this case.

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Jon Isaacs
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Re: coma corrector advice new [Re: Atl]
      #5678867 - 02/14/13 07:05 AM

Quote:

I got my collimation dialed in pretty good on my f4.6 12.5" dob. On some eyepieces I have coma at the edge of field. Will a coma corrector help with this? I know this may seem obvious but some of my eyepieces show almost no coma while others show a bit. I was making sure that it would work in this case.




A couple of questions:

What eyepieces are you using? Are you able to distinguish between off-axis astigmatism in the eyepiece and coma from the mirror. In my experience, unless you are using eyepieces on the order of the Naglers, Ethos's or the clones, astigmatism from the eyepiece will overwhelm the coma..

Jon


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Atl
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Reged: 04/13/12

Re: coma corrector advice new [Re: Jon Isaacs]
      #5678939 - 02/14/13 08:17 AM

The eyepieces that show the least coma are my Baader Hyperions (10mm and 13mm)...the ones that show the most are my 25 to 32mm Meade and Celestron plossls. I have one (very cheap) Agena wide angle 9mm that shows very little. The only place I see coma on any of them is at the edge of the viewing field. No Televues in my collection. The coma seems linked to the focal length of the eyepiece actually. By my calculations my scope is about f4.6 to 4.8.

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Zoomit
sage


Reged: 12/04/06

Loc: Tehachapi, CA
Re: coma corrector advice new [Re: Atl]
      #5678953 - 02/14/13 08:34 AM

There's an ongoing thread about coma and magnification that might be insightful

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5662033/page...


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Achernar
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: coma corrector advice new [Re: Atl]
      #5679157 - 02/14/13 10:50 AM

Yes, a coma corrector will eliminate or greatly reduce the coma inherit to the main optics. It will also remove or reduce the off-axis astimatism that may be present. However, it will not correct on-axis astigmatism or otherwise fix defective or misaligned optics. If there is astigmatism in the eyepiece, it will become glaringly apparent with a coma corrector in place. Astigmatism in eyepieces shows up as stars streteched into short lines along the edges of the field of view.

That said, if you have an F/5 or faster Newtonian or Dobsonian, a coma corrector will greatly improve the views, especially with the wide and ultrawide angle eyepieces. What you should do is try one out in person if possible with the sort of eyepieces you have now, or plan to buy in the near future to see if the improvement is worthwhile to you. You can get a used original Paracorr on the used market for 200 bucks or less, or you can buy one from other sources as well. If you want a coma corrector, there are alternatives to paying 500 bucks for one, even though the new Paracorr is admittedly better for very fast telescopes than the older models. Try one out, chances are you will see the improvement is worth the cost of purchasing one.

Taras


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Atl
professor emeritus
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Reged: 04/13/12

Re: coma corrector advice new [Re: Achernar]
      #5687451 - 02/18/13 05:56 PM

Despite the conflicting responses I bought the High Point Scientific coma corrector. Had my first viewing session at 4am this morning. Most of the eyepieces in which I saw coma cleared. Only one really cheap one by Meade still exhibited any. Every other eyepiece showed near perfect views. The viewing list was M81, M82, NGC3077, M97, M108, M109, NGC3448, NGC2403, M101, along with a small assortment of faint NGCs near M101, and IC2574 I think. I saw others but could not pinpoint their location well enough to get the names. There are a huge number of galaxies out by the big dipper it seems.

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Achernar
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Reged: 02/25/06

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Re: coma corrector advice new [Re: Atl]
      #5687519 - 02/18/13 06:38 PM

There's tons of galaxies accessible to amatuer telescopes in Ursa Major, of which I have yet to see even a third of the brighter ones. As for the eyepiece through which the views still has any coma, chances are you were seeing astigmatism around the edges. That is something no coma corrector can cure. I'm glad you got much better views with the coma corrector, personally I never leave the house without mine. Personally, I abhor the horrid star images I see without one through an F/4 or F/4.5 Newtonian or Dobsonian. It seems that coma corrector or not, you had a very nice time hoppin' galaxies.......

Taras


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GlennLeDrew
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Re: coma corrector advice new [Re: Achernar]
      #5687556 - 02/18/13 06:59 PM

A coma corrector is optimized for a certain spacing between the corrector and the focus. Might it be the case that those eyepieces still exhibiting coma do not have their field stop at the correct location? Or could the dominant aberration be eyepiece astigmatism?

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Atl
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Reged: 04/13/12

Re: coma corrector advice new [Re: Achernar]
      #5687568 - 02/18/13 07:02 PM

I also viewed Saturn and left the coma corrector in. Everything seemed be clear. The Cassini division stood out fine. Does a coma corrector affect planetary views. Is there a time when it should be really taken out?

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MikeBOKC
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Re: coma corrector advice new [Re: Atl]
      #5687737 - 02/18/13 08:30 PM

Well I only recently added a scope that requires (or at least will benefit from) coma correction, but I did a lot of reading here beforehand and reached the following conclusions:

-- The coma corrector corrects coma where it exists, meaning the outer edges of the field of view. It seems to not have much of an impact on-axis (in the center of the field of view) so that makes planetary observing pretty much a wash with or without the corrector, since it is rare for anyone to position Jupiter or Saturn at the edge of the field of view.

-- If it works for your scope to improve images at the edges, just leave it in the focuser all the time. As far as I can tell there a CC delivers only positives, and no negatives, in fast scopes.


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Brett Carlson
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Reged: 12/12/11

Loc: Rochester, NY
Re: coma corrector advice new [Re: Atl]
      #5687757 - 02/18/13 08:37 PM

I like the price of the High point coma corrector over the Paracorr 2 but I do like the tunability of the paracorr.

Is the high point coma corrector really that effective? It seems optimized for my f4.5.


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Atl
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Reged: 04/13/12

Re: coma corrector advice new [Re: Brett Carlson]
      #5687815 - 02/18/13 09:06 PM

My reading tells me that the parracor is tunable while High Point's is set. I am very close to f4.5 and it works great. I wasn't willing to spend more than the $350 I paid for the 12.5" dob. I would think you would get good results.

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Atl
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Reged: 04/13/12

Re: coma corrector advice new [Re: MikeBOKC]
      #5687823 - 02/18/13 09:10 PM

It didn't improve the planetary images...I just don't like pulling it out and putting it back a lot. If it doesn't hurt I would just as soon leave it in.

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Achernar
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: coma corrector advice new [Re: Atl]
      #5687832 - 02/18/13 09:15 PM

It seems to improve those too, because without one they are not nearly as sharp and clear as they are at the center of the field of view. At least, that was my experience with the Paracorrs I use.

Taras


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: coma corrector advice new [Re: Atl]
      #5687839 - 02/18/13 09:21 PM

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.
Try focusing with the eyepiece all the way ito the CC. Still see coma at the edge? Pull the eyepiece 1/8" out of the focuser, refocus and look at the edge again? Coma better? Repeat until you find the position of the eyepiece where coma is least. It might be with the eyepiece pulled out of the CC to a set distance. You might need to add parfocalizing rings to your eyepieces to adjust them all for your CC.
Now, without refocusing the scope at all, put the next eyepiece in and pull it out of the CC until the stars are in focus. That will be the correct setting for that eyepiece.
In the same manner, you can find the optimum position for all your eyepieces.
Your eyepieces will all be parfocal from that point on, and the amount of coma correction will be equal on all your eyepieces.

You may find one of your eyepieces needs to move in farther than the CC body allows in order to get to the perfect position. Don't worry about that eyepiece and just use it all the way in.

So as long as you have parfocalizing rings, you don't need a tunable top. The tunable top just does the same thing by moving the top up and down instead of adding parfocalizing rings on the eyepieces. But, using the rings, one could do the same procedure with the Paracorr and only use one setting of the tunable top. Some creative types are doing just that.
In my scope, by adjusting the top, all my eyepieces focus in a 1-2mm range of focuser travel.


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Atl
professor emeritus
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Reged: 04/13/12

Re: coma corrector advice new [Re: Starman1]
      #5687866 - 02/18/13 09:40 PM

That makes complete sense. Thanks for the input. So far duh moments have been coming regularly with this hobby. 2 mirrors and an eyepiece should be simple but the subtleties are many.

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Sean Puett
Carpal Tunnel
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Reged: 09/06/10

Loc: always cloudy, washington
Re: coma corrector advice new [Re: Atl]
      #5688132 - 02/19/13 12:33 AM

Quote:

I also viewed Saturn and left the coma corrector in. Everything seemed be clear. The Cassini division stood out fine. Does a coma corrector affect planetary views. Is there a time when it should be really taken out?



I take mine out when I am putting my scope away. I don't believe it negatively affects the planetary views but, that is just my opinion. I have heard other people say that they see a difference. Try it both ways and see what you think. That is all that matters anyway...


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Jon Isaacs
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Loc: San Diego and Boulevard, CA
Re: coma corrector advice new [Re: Atl]
      #5688532 - 02/19/13 09:21 AM

Quote:

Only one really cheap one by Meade still exhibited any.




Are you sure it was coma and not astigmatism in the eyepiece? Most eyepieces have problems in scopes fast enough to need a Paracorr.

Plantary viewing and the Paracorr. In a 14 inch f/4.5, the diffraction limited field is 2mm or about 4 arcminutes. That's an AFoV of 14 degrees at 200x, not a very big sweet spot.

Jon


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DeanS
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Reged: 07/12/05

Loc: Central Kentucky
Re: coma corrector advice new [Re: Jon Isaacs]
      #5688996 - 02/19/13 02:23 PM

I was offered the opportunity to try out a SIPS in my starmaster while at WSP. Should have known better as I would not give it back after that night. The first night Al Nagler came over while we tried an old original paracorr which did decent with the Ethos but he pointed out it was designed long before the ethos so was not optimized for them at all. The new SIPS is amazing, but then again it should be for the $$$$$$$$

Dean


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Atl
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Reged: 04/13/12

Re: coma corrector advice new [Re: Jon Isaacs]
      #5689903 - 02/19/13 10:47 PM

I am not sure. I am a newbie to this, but It helped with my better long focal length eyepieces. I have a 25mm Meade that it didn't do much for. My 10mm through 6mm eyepieces never needed it from the start.

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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
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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)
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Reged: 06/24/03

Loc: Los Angeles
Re: coma corrector advice new [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
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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
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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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