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What does it take to actually use a coma corrector on a dob?

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#1 kurtfriedrich

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Posted 09 September 2021 - 11:54 AM

It seems the paracorrs like the TeleVue just slide in to the focuser and then the eyepieces go into that, sounds easy enough, but I saw a comment about eyepieces being used with them that implied there is some complication?  Maybe it has to do with losing parafocal characteristic for a set of eyepieces?  I see one TelVue model has 5 settings (A ... E) what is that for?

 

Is there more to using these than just putting them in?

 

thanks


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#2 ngc7319_20

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Posted 09 September 2021 - 12:13 PM

To get optimal performance, the focal plane of the eyepiece must be at a certain position relative to the ParaCorr lenses.  The complication is that different eyepieces have different focal plane positions relative to the 2" seat.  So you can't just drop a 2" eyepiece in the ParaCorr and assume it is optimized.  Hence TeleVue has divided its eyepieces into classes (A ... H) according to where the focal plane is inside the eyepiece.  There is a table here.  And the newer ParaCorrs have an adjustment you set (A ... H) according to which eyepiece you are using.

 

The other occasional issue, is that the ParaCorr needs at least 14mm of so-called in-travel on the focuser.  That is, the scope's focus position must be at least 14mm above the focuser, when it is racked fully inward.  Most scopes will have this.  But now and then you get one that does not have the 14mm, and ParaCorr will not work.


Edited by ngc7319_20, 09 September 2021 - 12:21 PM.

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#3 kurtfriedrich

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Posted 09 September 2021 - 12:24 PM

AH!  that makes sense.  Thanks.  If I buy a brand X paracorr, that doesn't have these presets, can I just slide the eyepiece up and down to the correct location and visually tell what looks good?



#4 Astrojensen

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Posted 09 September 2021 - 12:30 PM

If I buy a brand X paracorr, that doesn't have these presets, can I just slide the eyepiece up and down to the correct location and visually tell what looks good?

Yes. This is also how you use the Paracorr with non-Televue eyepieces. 

 

 

Clear skies!

Thomas, Denmark


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#5 ngc7319_20

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Posted 09 September 2021 - 12:38 PM

AH!  that makes sense.  Thanks.  If I buy a brand X paracorr, that doesn't have these presets, can I just slide the eyepiece up and down to the correct location and visually tell what looks good?

Yes, exactly.  You can look at the field center and field edges, and find what position you prefer.  Sometimes there is spherical aberration at the field center when the eyepiece distance is off in one direction, and coma at the field edge when it is off in the other direction.  Just find what looks good to you and go with that.


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#6 MeridianStarGazer

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Posted 09 September 2021 - 01:10 PM

The eyepiece must be at the right position inside the coma corrector, and the coma corrector must be at the right position inside the focuser. You can't just turn one knob till perfect, but have to adjust 2 variables. Also differs for each eyepiece.

Get it wrong and the coma is still greatly improved but you may see spherical aberration on axis at high power.


The Paracorr 2 has a twist top so you can adjust eyepiece position easier and see what letters each is between. With the GSO, you have to put a pencil line on your barrel once you find the perfect location.
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#7 eyeoftexas

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Posted 10 September 2021 - 09:44 PM

There is a long thread listing the Paracorr II settings for many, many non-Televue eyepieces:

 

https://www.cloudyni...-brands-of-eps/

 

I have a P2 and use it with ES 82 eyepieces and it works great, zero coma once you have the correct setting.



#8 Starman1

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Posted 11 September 2021 - 09:34 AM

It seems the paracorrs like the TeleVue just slide in to the focuser and then the eyepieces go into that, sounds easy enough, but I saw a comment about eyepieces being used with them that implied there is some complication?  Maybe it has to do with losing parafocal characteristic for a set of eyepieces?  I see one TelVue model has 5 settings (A ... E) what is that for?

 

Is there more to using these than just putting them in?

 

thanks

In addition to all the points made by others, one of the advantages to the Paracorr's adjustable top is that once you have just one eyepiece whose setting you know,

all other eyepieces can have their settings determined by dropping the eyepiece into the Paracorr and focusing using the Tunable top of the Paracorr.

Whatever setting is derived is the correct setting for that eyepiece.

You can do a whole set of eyepieces that way.

In use, in the field, you pre-dial the letter that corresponds to the setting for that eyepiece, and when you insert the eyepiece it will be nearly in focus.

Your total focuser adjustment after finding the settings for every eyepiece may be as little as +/-1mm.

 

If you have one TeleVue eyepiece, you can look at their chart, set the Paracorr top for that eyepiece, and focus the scope.

That puts the Paracorr in the right position in the light cone from the primary mirror.  Any/all other eyepieces would have their positions identified by focusing using the Paracorr's top.

 

If you don't have a TeleVue eyepiece, there is another way to find the right position for the Paracorr in the light cone:

Set the top to "E", and place translucent scotch tape across the 2" opening of the Paracorr.

Point the scope at the Moon and focus the Moon on the tape (no eyepiece).  When the Moon is in focus, you have found the optimum position for the Paracorr.

After that, without moving the focuser at all, focus each eyepiece with the Paracorr's Tunable top and write down the setting you identify for each eyepiece.

Those will be the letters you dial in for every eyepiece before you insert them.

 

IF the tunable top of your Paracorr moves smoothly enough, you can actually focus using the tunable top, but pre-dialing the letters and using the fine focus on the scope

is probably easier.

 

If you share the view with someone who is nearsighted and is not wearing glasses, have them dial the tunable top of the Paracorr to focus the eyepiece for their eye.

Then return the eyepiece to the letter setting you've determined is best and focus with the fine focus knob on the scope to return the Paracorr's top to the right setting for your eyes.

 

One point: having no spherical error in your eye is important when setting up the Paracorr, so if you use the tape trick, wear your glasses to find focus.

Don't remove your glasses if you need correction.  You can determine the settings of the Paracorr's top later without glasses if you observe alone, but that initial

setting should be done with glasses on if you wear glasses to correct a spherical error in the eye.


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#9 Lentini

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Posted 11 September 2021 - 06:55 PM

That all sounds so complicated. I have a SIPS permanently installed. It’s just there… I don’t have to do anything. 



#10 Starman1

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Posted 12 September 2021 - 01:46 AM

That all sounds so complicated. I have a SIPS permanently installed. It’s just there… I don’t have to do anything.

You set up the SIPS the same way, with tape across the focuser, and the caveat about your vision is the same.
The difference is the tunable top is doing the job of the focuser and eyepieces requiring maximum in focus can be used without glasses by near sighted people.
One thing that makes nearly every coma corrector easier to use is to parfocalize all your eyepieces with the one that requires the most in focus. Then, no adjustment for eyepiece focal plane is necessary.
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#11 CHASLX200

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Posted 12 September 2021 - 05:54 AM

I liked the first version the best. No having to adjust anything just stick it in and enjoy.



#12 Jon Isaacs

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Posted 14 September 2021 - 05:25 AM

AH!  that makes sense.  Thanks.  If I buy a brand X paracorr, that doesn't have these presets, can I just slide the eyepiece up and down to the correct location and visually tell what looks good?

 

In theory you can do it that way but it practice, it's not so easy.  It will look pretty good even if it's not properly spaced but getting the good view will take some fiddling.  The Paracorr simplifies everything because once you determine the appropriate focuser position, you are basically home free.  The Tunable Top is used for coarse focus and the regular focuser for fine focus.  

 

To find the proper focuser position, you only need one TeleVue eyepiece or any eyepiece with a known setting. 

 

The SIPs has the same optics as the Paracorr 2 but it is setup so the focuser is actually the Tunable Top.  It has the advantage that once properly setup, you can use any eyepiece without thinking.  The downside is that it's a package, focuser and coma corrector and it's not cheap.  It can't be used with other focusers and swapping between scopes is not so easy.  You either have to swap focusers or you have to have multiple SIPS focusers.  And if you want to view without the Paracorr, that is not so easy either.

 

Myself, I have 5 Dobs, one Paracorr 2 works with them all, all of them have good focusers that I see no need to replace.

 

If you are using a coma corrector like the GSO that does not have a Tunable Top, the easiest thing to do is parfocalize your eyepieces.  That way, once you determine the proper spacing for one of them, that's the proper spacing for all of them.

 

Jon



#13 Dave Bush

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Posted 14 September 2021 - 02:37 PM

Well, you could always drop about $100 on a GSO coma corrector and you might just find like I and other have that it's pretty much plug-n-play.  I get excellent correction with any eyepiece (at least any I've tried in it, and that's quite a few) without any adjusting or fiddling.

 

If you don't like it you can always sell it and not lose that much.  


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#14 MeridianStarGazer

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Posted 14 September 2021 - 04:16 PM

Some people worry about coma corrector transmission but are not as worried about a barlow. The barlow has fewer surfaces, but not many fewer. My eyes have haze in them.


The main thing holding me back from getting a CC is the tighter collimation tolerances. Also need to rack the focuser in, which means needing that travel.

Edited by MeridianStarGazer, 14 September 2021 - 04:17 PM.


#15 Tiago Ferreira

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Posted 14 September 2021 - 04:19 PM

Parfocalazing eyepieces and cameras made me forget that i have a coma corrector inside the focuser.

#16 Jon Isaacs

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Posted 15 September 2021 - 12:38 AM

Well, you could always drop about $100 on a GSO coma corrector and you might just find like I and other have that it's pretty much plug-n-play.  I get excellent correction with any eyepiece (at least any I've tried in it, and that's quite a few) without any adjusting or fiddling.

 

If you don't like it you can always sell it and not lose that much.  

 

I have one..

 

Head out to dark skies.  Sharp focus a bright star in the center of the field.  Shift it to the edge.  

 

There's coma to be seen.

 

Jon



#17 Starman1

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Posted 15 September 2021 - 01:09 AM

I have one..

Head out to dark skies. Sharp focus a bright star in the center of the field. Shift it to the edge.

There's coma to be seen.

Jon

Is that with the eyepiece at the optimum distance from the lens, or simply using it as a "drop-in"?

#18 Jon Isaacs

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Posted 15 September 2021 - 02:08 AM

Is that with the eyepiece at the optimum distance from the lens, or simply using it as a "drop-in"?

That would be using it as a "drop in", not paying attention to optimum spacing. 

 

Jon



#19 Dave Bush

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Posted 15 September 2021 - 04:57 AM

I have one..

 

Head out to dark skies.  Sharp focus a bright star in the center of the field.  Shift it to the edge.  

 

There's coma to be seen.

 

Jon

It doesn’t seem to work for everyone but my point is that for the price it’s worth trying. And if you don’t get perfect elimination of coma then you can go through the process of setting the optimal spacing.  Even if you have to do that it’s still a better deal than paying 3-4x more for a Paracorr in my opinion. 



#20 ButterFly

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Posted 15 September 2021 - 11:49 AM

You set up the SIPS the same way, with tape across the focuser, and the caveat about your vision is the same.
The difference is the tunable top is doing the job of the focuser and eyepieces requiring maximum in focus can be used without glasses by near sighted people.
One thing that makes nearly every coma corrector easier to use is to parfocalize all your eyepieces with the one that requires the most in focus. Then, no adjustment for eyepiece focal plane is necessary.

That caveat is one of the reasons I decided against a SIPS.  Once in a blue Moon, I take my glasses off for some target, mainly planets at high power.  I'm not going to recalibrate a SIPS at that point.  At high power, it can be a lot of focal shift.

 

I do also observe with several different glasses.  They are not all corrected the same because they changed over the years.  Tuning out the coma (and over/undercorrection due to mirror cooling) with small tweaks is fairly easy after doing it for a little while.  The touchstone is always whether the view looks better.  I strongly recommend a tunable top for anyone with glasses.  As soon as I get to the parking lot after an eye exam, I routinely think aloud, "NOOOO, 2 was better!"  A measurement that determines whether I can read a Stop Sign at 20 feet isn't always sufficient for whether tiny pinpoints of light stay that way.



#21 tommm

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Posted 15 September 2021 - 12:21 PM

 

 

If you don't have a TeleVue eyepiece, there is another way to find the right position for the Paracorr in the light cone:

Set the top to "E", and place translucent scotch tape across the 2" opening of the Paracorr.

Point the scope at the Moon and focus the Moon on the tape (no eyepiece).  When the Moon is in focus, you have found the optimum position for the Paracorr.

After that, without moving the focuser at all, focus each eyepiece with the Paracorr's Tunable top and write down the setting you identify for each eyepiece.

Those will be the letters you dial in for every eyepiece before you insert them.

 

IF the tunable top of your Paracorr moves smoothly enough, you can actually focus using the tunable top, but pre-dialing the letters and using the fine focus on the scope

is probably easier.

 

That's what I do.  Once the correct position was ascertained I made a small Delrin block with thickness equal the distance between the focuser top and the Paracorr shoulder at that focuser position.  I keep it tethered to the focuser.  During scope set up, I set the Paracorr in the focuser and adjust it until the block is snug between the focuser and Paracorr shoulder, giving correct position of the Paracorr.  The rest of the night I just use the tunable top to focus, leaving the focuser locked in position.  I have tried improving focus with the fine focus knob occasionally, but I've not seen any improvement. Seems I can focus it fine just using the tunable top.

 

I can't tell you what letter position I use for various eyepieces because I've never looked.  And it's easy. In the dark I can just set the block on top the focuser by feel, crank the focuser in until the Paracorr hits the block, and lock the focuser. Done for the night.  If I forget and adjust the coarse focus during the night, the block is hanging right there, so I just use it to reset focuser position.

 

 


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#22 Starman1

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Posted 15 September 2021 - 12:29 PM

That's what I do.  Once the correct position was ascertained I made a small Delrin block with thickness equal the distance between the focuser top and the Paracorr shoulder at that focuser position.  I keep it tethered to the focuser.  During scope set up, I set the Paracorr in the focuser and adjust it until the block is snug between the focuser and Paracorr shoulder, giving correct position of the Paracorr.  The rest of the night I just use the tunable top to focus, leaving the focuser locked in position.  I have tried improving focus with the fine focus knob occasionally, but I've not seen any improvement. Seems I can focus it fine just using the tunable top.

 

I can't tell you what letter position I use for various eyepieces because I've never looked.  And it's easy. In the dark I can just set the block on top the focuser by feel, crank the focuser in until the Paracorr hits the block, and lock the focuser. Done for the night.  If I forget and adjust the coarse focus during the night, the block is hanging right there, so I just use it to reset focuser position.

Brilliant idea--the block spacer, I mean.  



#23 Jon Isaacs

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Posted 15 September 2021 - 12:40 PM

I have tried improving focus with the fine focus knob occasionally, but I've not seen any improvement. Seems I can focus it fine just using the tunable top.

 

 

For me  the Tunable Top works quite well as a focuser at low and moderate magnifications but for the planets and close doubles, the fine focus of a two speed is helpful.

 

At F/4.4, the depth of focus is 42 microns = 0.0016", less than half the thickness of a piece of typing paper. And I want to be dead center in those 42 microns.

 

Jon



#24 Speedy1985

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Posted 15 September 2021 - 03:38 PM

That's what I do.  Once the correct position was ascertained I made a small Delrin block with thickness equal the distance between the focuser top and the Paracorr shoulder at that focuser position.  I keep it tethered to the focuser.  During scope set up, I set the Paracorr in the focuser and adjust it until the block is snug between the focuser and Paracorr shoulder, giving correct position of the Paracorr.  The rest of the night I just use the tunable top to focus, leaving the focuser locked in position.  I have tried improving focus with the fine focus knob occasionally, but I've not seen any improvement. Seems I can focus it fine just using the tunable top.

 

I can't tell you what letter position I use for various eyepieces because I've never looked.  And it's easy. In the dark I can just set the block on top the focuser by feel, crank the focuser in until the Paracorr hits the block, and lock the focuser. Done for the night.  If I forget and adjust the coarse focus during the night, the block is hanging right there, so I just use it to reset focuser position.

Once I get a chance to get out again and get the optimal position set based on the scotch tape method, I'm stealing your slip in block idea!




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