Coma correctors

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turboscrew
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Coma correctors

#1

Post by turboscrew »

How do coma correctors work? Is there only one right place a certain distance from the focal plane (or focal point in the optical axis)?

I wonder, because the set-up instructions of my ES HR CC say that first the CC's helical focuser needs to be set in its "neutral" position and then use the telescope's focuser to get the image on top of the helical focuser (using something like frosted tape on top of the helical focuser as a screen). And after that the telescope's focuser should be locked in place, and focusing the eyepieces is done only using the CCs helical focuser.

Tonight there's full moon, and very little clouds, so I got to (re-)do the setting up. Unlike trying in daytime with treetops half a kilometer away, this time all the eyepieces came to focus. It seems that half a kilometer is not nearly "optical infinity". But, this time the CC'c helical focuser needed to be screwed almost all in. No more extension tubes. Even the 26 mm Omegon SWAN 70° came to focus (that I saw around the huge secondary mirror shadow).

The reason I'm asking how CC works, is if there is an "alternative" way to set it up such that there'd be more room for focusing - or even play for tinkering with afocal photography. Or is there only one correct place where it works as it should? For prime focus photography, the helical focuser can be removed.

At least got enough "light treatment" tonight. (The full moon looked pretty bright through a 300 mm telescope.)
Couldn't do further tests, like with barlow, because it's already late (0:38 AM) and I got to wake up for work.
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12
Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 6 mm Baader Classic Ortho, 5 mm TV DeLite
Explore Scientific HR 2" coma corrector
Meade x3 1.25" Barlow
Some filters (#80A, ND-96, ND-09, UHC)

LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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Don Pensack
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Re: Coma correctors

#2

Post by Don Pensack »

Yes, to correct coma properly and place the new focal plane at the focus of the coma corrector lens, there is only one place within the light cone from the primary that is correct.

Your HRCC instructions tell you how to set the new focal plane properly, and to prevent it from moving, you focus with the helical top, which is essentially adjusting to bring the focal planes in the eyepieces coincident with the focal plane of the scope + HRCC.

The HRCC requires the most in travel of all the coma correctors--32mm--and it won't work in a lot of scopes. The TeleVue Paracorr requires 14mm, so works 18mm farther out than the HRCC.
That you could bring it to focus on the tape across the opening in the 13.5mm position of the top was a miracle. Bear in mind, as long as you achieved focus, it's fine, since all eyepieces will focus in exactly the same place in the travel of the focuser.

For afocal photography, you're simply shooting through the eyepiece, so that isn't a problem.
When you use the t-thread photo adapter that came with the unit, the helical top is removed, and that gets you to a lot closer to the lens to get to the "working distance" (as mentioned in the instructions).

Daytime objects are closer than infinity and require out travel at the focuser. That's why to focus on the tape using the Moon, which is at infinity distance.
Any alternative position of the HRCC will not correct coma correctly.
Astonomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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turboscrew
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Re: Coma correctors

#3

Post by turboscrew »

Don Pensack wrote: Tue Sep 21, 2021 10:07 pm Yes, to correct coma properly and place the new focal plane at the focus of the coma corrector lens, there is only one place within the light cone from the primary that is correct.

Your HRCC instructions tell you how to set the new focal plane properly, and to prevent it from moving, you focus with the helical top, which is essentially adjusting to bring the focal planes in the eyepieces coincident with the focal plane of the scope + HRCC.

The HRCC requires the most in travel of all the coma correctors--32mm--and it won't work in a lot of scopes. The TeleVue Paracorr requires 14mm, so works 18mm farther out than the HRCC.
That you could bring it to focus on the tape across the opening in the 13.5mm position of the top was a miracle. Bear in mind, as long as you achieved focus, it's fine, since all eyepieces will focus in exactly the same place in the travel of the focuser.

For afocal photography, you're simply shooting through the eyepiece, so that isn't a problem.
When you use the t-thread photo adapter that came with the unit, the helical top is removed, and that gets you to a lot closer to the lens to get to the "working distance" (as mentioned in the instructions).

Daytime objects are closer than infinity and require out travel at the focuser. That's why to focus on the tape using the Moon, which is at infinity distance.
Any alternative position of the HRCC will not correct coma correctly.
Thank you for confirming. That's what I suspected. Both the location of CC and that terrestrial targets are usually not far enough to be considered optically being in infinity.
The camera is then focused by right length extension tubes and rings only?
That is, the focuser shouldn't be touched after the CC setup is done?
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12
Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 6 mm Baader Classic Ortho, 5 mm TV DeLite
Explore Scientific HR 2" coma corrector
Meade x3 1.25" Barlow
Some filters (#80A, ND-96, ND-09, UHC)

LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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Don Pensack
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Re: Coma correctors

#4

Post by Don Pensack »

At least with the HRCC, after initial set up at infinity, the focuser is not used.
As for how to set it up for daylight use, I am not clear whether that would be done at the focuser, or whether some other setting of the HRCC's top would be best.
I think it would be the helical top, but I could not tell you what that setting would be.
Astonomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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turboscrew
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Re: Coma correctors

#5

Post by turboscrew »

Don Pensack wrote: Mon Oct 04, 2021 9:15 pm At least with the HRCC, after initial set up at infinity, the focuser is not used.
As for how to set it up for daylight use, I am not clear whether that would be done at the focuser, or whether some other setting of the HRCC's top would be best.
I think it would be the helical top, but I could not tell you what that setting would be.
I rather meant that no wonder setting it up in daytime didn't work out. I couldn't bring some eyepieces to focus at all. With moon, all eyepieces came to focus, but some barely.

I'm wondering, if HRCC is set up, and the telescope's focuser is locked, and the helical focuser of HRCC is removed, how do I focus the camera? Just getting the right length extensions?
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12
Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 6 mm Baader Classic Ortho, 5 mm TV DeLite
Explore Scientific HR 2" coma corrector
Meade x3 1.25" Barlow
Some filters (#80A, ND-96, ND-09, UHC)

LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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Don Pensack
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Re: Coma correctors

#6

Post by Don Pensack »

You would lock the focuser and use spacers to get the camera as close as possible to focused, then do any final mm or 2 of focusing using the focuser.
A 1-2mm movement of the coma corrector away from its optimum position shouldn't introduce any visible coma.
Astonomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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turboscrew
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Re: Coma correctors

#7

Post by turboscrew »

Don Pensack wrote: Tue Oct 05, 2021 9:15 pm You would lock the focuser and use spacers to get the camera as close as possible to focused, then do any final mm or 2 of focusing using the focuser.
A 1-2mm movement of the coma corrector away from its optimum position shouldn't introduce any visible coma.
Truckload of thanks! Now that became clear - finally.
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12
Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 6 mm Baader Classic Ortho, 5 mm TV DeLite
Explore Scientific HR 2" coma corrector
Meade x3 1.25" Barlow
Some filters (#80A, ND-96, ND-09, UHC)

LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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