Coma corrector and back focus

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Coma corrector and back focus

#1

Post by turboscrew »

I've been thinking of getting a coma corrector for my 12" F/4 scope (F = 1200mm), but what kind?
And what's back focus? I really didn't find an understandable description. Google hasn't been my friend on this.
At the moment I can visually focus only if I use 50 mm extension tube, and I was under the impression that ES HR2 I'd need another 35 mm extender?

There is also Baader MPCC Mark III, but it seems hard to use it for visual and 2" eyepieces.
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Re: Coma corrector and back focus

#2

Post by Piet Le Roux »

The Tele Vue Paracor II does a very good job at reducing comma, it needs about 3/4"(19mm) in-focus, the opposite of back focus, meaning you would need shorten your focus distance. with back focus you would increase your focus distance. Normally with correctors you need in focus not back focus.
Main Equipment : 15" Obsession F4.5 Classic, Tele Vue 7&13mm Nagler, Tele Vue 2" 27mm Panoptic, Tele Vue Big Barlow, Tele Vue Paracorr II : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal, Baarder Hyperion MK III 24-8mm zoom : Camera Fuji XT100 : Into my third year and its just getting more interesting!
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Re: Coma corrector and back focus

#3

Post by turboscrew »

Piet Le Roux wrote:
Sun Jul 26, 2020 9:35 pm
The Tele Vue Paracor II does a very good job at reducing comma, it needs about 3/4"(19mm) in-focus, the opposite of back focus, meaning you would need shorten your focus distance. with back focus you would increase your focus distance. Normally with correctors you need in focus not back focus.
So with that, I could use 30 mm extension instead of current 50 mm extension?
TeleVue is just quite expensive.
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Re: Coma corrector and back focus

#4

Post by Piet Le Roux »

Unfortunately yes they are very expensive, I am glad I bought mine a few years back when the South African Rand were in better shape then now!
Main Equipment : 15" Obsession F4.5 Classic, Tele Vue 7&13mm Nagler, Tele Vue 2" 27mm Panoptic, Tele Vue Big Barlow, Tele Vue Paracorr II : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal, Baarder Hyperion MK III 24-8mm zoom : Camera Fuji XT100 : Into my third year and its just getting more interesting!
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Re: Coma corrector and back focus

#5

Post by Mooninite »

Ngl I’ve owned the cheaper GSO coma corrector with my dob 1270mm and it was a huge pain to use, I couldn’t really figure it out even when asking for help nothing worked and I sold it.

Found that maybe I don’t actually need one. But that’s my two cents
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Re: Coma corrector and back focus

#6

Post by yobbo89 »

I don't recommend a badder mk, paracor or a skywatcher aplanatic f4 corrector, but you're doing visual? So probs one with a turn top adjuster "helix focuser". Which the paracor comes with.
scopes :gso/bintel f4 12"trust tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
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extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel

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Re: Coma corrector and back focus

#7

Post by KathyNS »

The Paracorr doesn't really have any back focus reqirements. It sits in the focuser where an eyepiece would sit. (There's a complication on my scope, which needs a 35mm extension for eyepieces. The Paracorr is too long to sit in a 35mm extension. So I made a 35mm parfocal ring instead and used it without the extension.)

The Paracorr creates a 57mm back focus from the back of the optical unit to the focal plane. For imaging, the T-adapter consumes 2mm, leaving the regulation 55mm for the camera and T-ring. For visual use, a helical eyepiece adapter positions the eyepiece focal plane at the 57mm position. Coma is only properly controlled at the 57mm image plane, hence the need for precise positioning.
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Re: Coma corrector and back focus

#8

Post by turboscrew »

yobbo89 wrote:
Mon Jul 27, 2020 10:23 am
I don't recommend a badder mk, paracor or a skywatcher aplanatic f4 corrector, but you're doing visual? So probs one with a turn top adjuster "helix focuser". Which the paracor comes with.
I have a funny feeling that photographing comes in later...
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Re: Coma corrector and back focus

#9

Post by turboscrew »

KathyNS wrote:
Mon Jul 27, 2020 11:41 am
The Paracorr doesn't really have any back focus reqirements. It sits in the focuser where an eyepiece would sit. (There's a complication on my scope, which needs a 35mm extension for eyepieces. The Paracorr is too long to sit in a 35mm extension. So I made a 35mm parfocal ring instead and used it without the extension.)

The Paracorr creates a 57mm back focus from the back of the optical unit to the focal plane. For imaging, the T-adapter consumes 2mm, leaving the regulation 55mm for the camera and T-ring. For visual use, a helical eyepiece adapter positions the eyepiece focal plane at the 57mm position. Coma is only properly controlled at the 57mm image plane, hence the need for precise positioning.
I read this twice, but seems I have to read it a couple of times more...
First, I don't seem to get what "back focus" is.
Now that I checked out what "parfocal ring" is, your text starts to open gradually. :lol:

[edit] Ah, you defined back focus here: "from the back of the optical unit to the focal plane" (back = towards the observer)?

With my scope, the image (without eyepieces) formed on the inside of a cap put at the end of 50 mm extension tube.
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Re: Coma corrector and back focus

#10

Post by turboscrew »

I wonder if I understood this right: Paracorr wants an exact distance from the eyepiece's focus. ES HR2 is particular about primary focus? With Paracorr you use the telescope's focuser for focusing, with ES HR2 you use the corrector's helical focuser for focusing?
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Re: Coma corrector and back focus

#11

Post by Piet Le Roux »

This may help:
Pacacorr Type-2 Instructions.pdf
(567.87 KiB) Downloaded 3 times
Main Equipment : 15" Obsession F4.5 Classic, Tele Vue 7&13mm Nagler, Tele Vue 2" 27mm Panoptic, Tele Vue Big Barlow, Tele Vue Paracorr II : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal, Baarder Hyperion MK III 24-8mm zoom : Camera Fuji XT100 : Into my third year and its just getting more interesting!
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Re: Coma corrector and back focus

#12

Post by turboscrew »

That it sounds: for paracorr
Focus the telescope after properly setting the eyepiece position. Do not acci-dentally use the tuning feature for focussing.
For ES HR2:
When the moon appears to be sharp you can lock the focuser of your telescope –it is not needed any more.Now remove the tape and insert a eyepiece into the corrector and secure it with the locking screws. Focus the eyepiece with the helical focuser by rotating the eyepiece -without touching the telescopes focuser.
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Re: Coma corrector and back focus

#13

Post by Kanadalainen »

I like my gen I Paracorr, it works perfectly.

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Re: Coma corrector and back focus

#14

Post by JayTee »

Does anyone know at what focal ratio (FR) do you no longer notice the effects of coma?

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∞ Scopes: Celestron CPC1100 #2 Scope: 8" f/7.5 Dob AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scopes: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs
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Re: Coma corrector and back focus

#15

Post by helicon »

I think at around f/8 or above...
http://www.rfroyce.com/mirror_performance.htm
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Re: Coma corrector and back focus

#16

Post by JayTee »

Thanks for the article, Michael.

My 8" Newt is f/7.5 and I really don't notice a lot of coma at the edge of the FOV.

Cheers,
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∞ Scopes: Celestron CPC1100 #2 Scope: 8" f/7.5 Dob AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scopes: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs
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Re: Coma corrector and back focus

#17

Post by JayTee »

From the Bible of Telescope Optics (called Amateur Telescope Optics). This is what happens when you want a more in-depth answer. It's almost more than I can digest. https://www.telescope-optics.net/newton ... ations.htm

Image
this is just the first graphic of many on this subject.

If you read the linked page on this aberration, coma, (all the way to the bottom) you will see that the f/8 number is mostly correct.

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt
∞ Scopes: Celestron CPC1100 #2 Scope: 8" f/7.5 Dob AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scopes: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs
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Re: Coma corrector and back focus

#18

Post by notFritzArgelander »

JayTee wrote:
Wed Jul 29, 2020 4:15 am

....

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt
Let's discuss the blue line first. Coma increases with distance from the center of field. So the blue curve in the graph gives the distance in the image from the center of field where there is 3 arc minutes of visual coma as a function of the f ratio of the primary (the x axis).

The eye notices coma if it is more than 5 arc minutes when viewed from the eye. So the red line corresponds to when the eye can resolve the coma and it becomes obnoxious.

I use the ideas behind the graph to decide whether to use a coma corrector or not with an eyepiece. To use the graph (or the equations behind it) one needs to know the field stop of the eyepiece and the f ratio of the scope. Specifically I use a simpler method of knowing the FS (field stop) of the eyepiece and computing the following quantity:

Newtonian_limit = square-root(2*FS)

If that number is larger than the f ratio of the scope a coma corrector is needed. The need for a coma corrector depends on the eyepiece being used. There is no magical f ratio that is good for all eyepieces / magnifications. If your eyepiece DOESN'T have a field stop then use the size of the barrel. If you can't find the mfr's FS specification you can estimate the FS from

FS = (fl*AFOV/57.3)*(26/27.3)

Here's some worked examples:

Televue Panoptic 41mm fl, FS = 46mm. Newtonian_limit = square-root(2*FS) = 9.6. So this could be used without a coma corrector in a Newtonian at f9.6 or slower.

Televue Panoptic 19mm fl, FS = 21.30. Newtonian_limit = square-root(2*FS) = 6.5. So this could be used without a coma corrector in a Newtonian at f6.5 or slower.

Televue Plössl 11mm fl, FS = 7mm. Newtonian_limit = square-root(2*FS) = 4.3. So I could use this without a coma corrector in my f5 Z12 but things would get hairy around the edges in my f4 AT 8".

Higher magnification eyepieces have smaller field stops that block out the part of the FOV that has coma.

I no longer have the AT8 f4, but when I did a coma corrector was essential. I favored the TV Paracorr since it was so very easy to use. It has a built in helical focuser with markings so that if you know the setting for your eyepiece you can just dial it in and use the focuser on your scope. TV provides the settings in its specification page for eyepieces.

If you prefer a different brand of eyepiece, no worry. Just experiment.

The above formulas are pretty conservative. I find I could get by ignoring the coma with a 24mm Panoptic in my f5 Z12. At f4 I never am without the coma corrector.

PS If you need references to how/where I got these formulas, that's another post.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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Re: Coma corrector and back focus

#19

Post by notFritzArgelander »

Looking at my notes the formulas were derived as a simplification of the method in the link JayTee included. I wanted a simple algorithm that would give a yes/no answer as to whether I needed a coma corrector with a particular eyepiece and scope combination.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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Re: Coma corrector and back focus

#20

Post by turboscrew »

JayTee wrote:
Wed Jul 29, 2020 4:15 am
From the Bible of Telescope Optics (called Amateur Telescope Optics). This is what happens when you want a more in-depth answer. It's almost more than I can digest. https://www.telescope-optics.net/newton ... ations.htm

Image
this is just the first graphic of many on this subject.

If you read the linked page on this aberration, coma, (all the way to the bottom) you will see that the f/8 number is mostly correct.

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt
WHOA! Quite a document! Thanks!
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