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Would Delos benefit from a Paracorr at f/4?

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#1 Blueox4

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Posted 05 July 2020 - 02:54 PM

I’ve got a few Delos eyepieces, 6mm, 10mm, 14mm and 17.3mm along with the TV in-travel adapter so the 14mm and 17.3mm can ride at the same spot on the Paracorr2. My question is will I really see coma with the Delos 72 degree AFOV in my soon to be here 16” f/4Dob? If so, would it be beneficial to just leave the Paracorr in the focuser and use it for all Delos focal lengths? I thought coma was only and issue in longer focal length eyepieces. I am about to buy a Paracorr2 so I have one when my Dob gets here but I guess I’m trying to talk myself out of it thinking I may not need one with the Delos 72 degree AFOV.



#2 junomike

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Posted 05 July 2020 - 02:59 PM

The 14mm will benefit IMO, however the 17.3mm is a mess without a Coma Corrector.

With an F4 OTA a Coma Corrector is almost required unless using a UHC/OIII type filter which hides the Stars (shapes).



#3 havasman

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Posted 05 July 2020 - 03:08 PM

of course



#4 areyoukiddingme

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Posted 05 July 2020 - 03:14 PM

I'd say definitely yes.

 

I've found I'm rather sensitive to coma. Recently I've been getting the odd night of good seeing for planets from my balcony using an 8" F7.

 

Swapping back and forth comparing eyepieces and doing so with and without P2, I'vd settled on my barlowed zoom with the P2.

 

I can't see any degradation of performance on axis, and I get a sharp view all the way across the field. When I view without the P2, the edges of the field are just a tiny bit softer for details than the center. Mostly I notice it as a sharpening of detail near the center 30-40% of the field.

 

If I'm seeing this at 200x or so at F7, I'm going to see it at F4 at just about any magnification.

 



#5 Blueox4

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Posted 05 July 2020 - 03:36 PM

Thanks all! Paracorr2 on the way! 


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#6 Brian Carter

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Posted 05 July 2020 - 10:17 PM

If you have a paracorr and a Newtonian telescope, there is never a reason to take it out of the focuser.

 

Coma is more than just fan shaped stars, it’s a general destruction of everything you see.  Even if your mirror is perfect, the diffraction limited field is measured in millimeters, meaning that unless an object is exactly in the center of the field, your views will be about the same as a trashy coulter, or worse. 
 

If you have a paracorr, always use it, regardless of the eyepiece. 


Edited by Brian Carter, 05 July 2020 - 10:17 PM.

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#7 Miranda2525

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Posted 06 July 2020 - 12:20 AM

What he said above. It will help for every eyepiece you own.



#8 clivemilne

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Posted 06 July 2020 - 04:56 AM

I didn't find coma objectionable at f5.... until I tried a Paracorr 2.

 

There is no going back.


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#9 sixela

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Posted 06 July 2020 - 05:32 AM

Quite. For my solar 130mm f/5 (on which I used to use a 17mm Delos) I spent some money to get an ES HRCC as well -- couldn't live with the coma since I had a Paracorr on the other scope I have.


Edited by sixela, 06 July 2020 - 05:32 AM.


#10 havasman

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Posted 06 July 2020 - 12:06 PM

Do check it now & then to see if it needs cleaning. When I eventually checked mine the 1st time I was shocked to see how grimy it had become. It cleaned up well via standard ep cleaning process and now I check it every once in a while.



#11 Jon Isaacs

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Posted 06 July 2020 - 05:25 PM

If you have a paracorr and a Newtonian telescope, there is never a reason to take it out of the focuser.

 

 

I purchased my first Paracorr about 20 years ago, the 1.25inch. Since then, I've owned them all.

 

I will remove the Paracorr to achieve a wider field or to achieve a larger exit pupil. It doesn't happen a lot but it happens.. mostly when using H-Beta orbO-lll filters that knock stars down about 3 magnitudes and the goal is to see faint Nebulae and not sharp stars..

 

The 31 mm Nagler at F/4.4 provides a 7.1 mm exit pupil, at F/4.06, a 7.6 mm exit pupil, a nearly perfect match for my dark adapted pupil.

 

Otherwise, the Paracorr 2 pretty much lives in the focusers.

 

Jon


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#12 hakann

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Posted 06 July 2020 - 05:52 PM

Clevemilne,

I also compared it on/off in Zambuto 12, 14” f/4.5 - 5 etc and Kennedy 28” f/4 etc and in clubs 18” at f/4.5 and we that was around scopes never saw it.
Ok we might saw in the very end a hair coma but on/off did not change image. ( If it was to the negative side.
This is all know experiance ( for some.

This is a common case on a topic and its personal, I prefer not to has it on.
I was at Oz Sky some years ago and a experianced observer there Andrew Murell did not use it and scope was f/4 - 4.5.
Why are not people like Lowrey use it ?
We was in deep of Oregon some years ago and a scope was a 18” f/4 and owner did not has one ( this a well know observing couple ) but I brought my P2, but no one did like it on - so did we made a misstake ( we shore know math or read it and we shore can see coma or bad image )
One observer write in S&T used my Nikon HW 17/14 and prefered it less the P2 ( as me ) and he used the 17 one full night at OSP ( f/4 ) and love that EP.

We might not be as sensetive as others ( or good observers ) Ok, but we do observe and don’t use a CC, and if image in big would be that better we shore had it on - as this ain’t about spend 500 bucks since most of us has one and I has both Sips and the P2 plus a own made focuser developed to work with the P2 lens.

I told it before, best ever was a non CC scope ( build less a CC ) with a Lomo/Sitall 25” at f/4.5 and this did wipe all other Newtons away I ever used ( fast/slow, big/small or with a a CC or not ! )

This is of course just my own thoughts and not Any recommendations.

Edited by hakann, 06 July 2020 - 05:55 PM.


#13 Kunama

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Posted 07 July 2020 - 03:46 AM

Only ornithologists like fast Newtonians sans coma correction.....

My F3.54 primary is married to my SIPS, I only use Delos eyepieces with them and I dislike seagulls...


Edited by Kunama, 07 July 2020 - 03:48 AM.

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#14 clivemilne

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Posted 07 July 2020 - 04:11 AM

Well.... at F4, the geometric size of the comatic plume at the field edge (100 degree eyepiece)

subtends an apparent size of 45 arc minutes (17' RMS) ... Ergo, your field stars are going to 

appear in the eyepiece around the same size as the gibbous moon does naked eye. 

 

  Each to their own, I guess..


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#15 hakann

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Posted 07 July 2020 - 07:18 AM

Ok it sounds as the stars/object will look like the moon ;-)

I better install my CC then.



#16 Jon Isaacs

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Posted 07 July 2020 - 08:43 PM

I also compared it on/off in Zambuto 12, 14” f/4.5 - 5 etc and Kennedy 28” f/4 etc and in clubs 18” at f/4.5 and we that was around scopes never saw it.
Ok we might saw in the very end a hair coma but on/off did not change image. ( If it was to the negative side.
This is all know experiance ( for some.)

 

 

If you put a bright star like Porrima at the edge of the field of the 13 mm Ethos, are you saying it is a perfect round pair of with disks with no sign of coma?

 

That's a simple test that easily shows coma.

 

Jon



#17 hakann

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Posted 08 July 2020 - 10:57 AM

Jon,

 

We tested this in the clubs 18" f/4.5 at E13 and 10 mm from edge to edge.

Could we see coma, maybe but if so it was at the very end of the edge and no one did ever care.

Was it worth it to install the P2 and we did also try get that as perfect ( fine tune ) as possible.

This is all know to many, but I think it's not that odd as the very well observers I told about here don't care and they has optics at high end and pricy at up at 40K or up to a 100.

So I'm far alone on this.

When I hear it's a 'must' on 500 USD Skywatchers with EP at 25 USD each I think it's a way-way overreact. ( that's why I even write this ) and that's my point of it.

Some I has observe with is mainly on smaller faint object's - Yes but I also got a example of a Kennedy f/4 at 28" that owner did prefer to use both 17 & 14 mm Nikon HW less the P2, and he don't use it in his Ethos/Delos lower number than 17 mm either.

I had a 3 BLY object in it last year, and no CC of course, so if this is crule coma at f/4 at 2-3 mm pupil why did we not use the P2.

-Yes, it might be the case that we are not that observes, and I'm Ok with that as it's personal in the end.

 

What I do care about is contrast in the optic's.

I might mentioned the 25" Lomo and my Zeiss 2060S bino, that has huge coma in the end but few on the market ( no coma ) are even close on the optic contrast.

I know what I use/chose.

More glass in front of eye is not what I chose.


Edited by hakann, 08 July 2020 - 11:24 AM.


#18 Jon Isaacs

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Posted 09 July 2020 - 05:07 AM

Jon,

 

We tested this in the clubs 18" f/4.5 at E13 and 10 mm from edge to edge.

Could we see coma, maybe but if so it was at the very end of the edge and no one did ever care.

Was it worth it to install the P2 and we did also try get that as perfect ( fine tune ) as possible.

This is all know to many, but I think it's not that odd as the very well observers I told about here don't care and they has optics at high end and pricy at up at 40K or up to a 100.

So I'm far alone on this.

When I hear it's a 'must' on 500 USD Skywatchers with EP at 25 USD each I think it's a way-way overreact. ( that's why I even write this ) and that's my point of it.

Some I has observe with is mainly on smaller faint object's - Yes but I also got a example of a Kennedy f/4 at 28" that owner did prefer to use both 17 & 14 mm Nikon HW less the P2, and he don't use it in his Ethos/Delos lower number than 17 mm either.

I had a 3 BLY object in it last year, and no CC of course, so if this is crule coma at f/4 at 2-3 mm pupil why did we not use the P2.

-Yes, it might be the case that we are not that observes, and I'm Ok with that as it's personal in the end.

 

What I do care about is contrast in the optic's.

I might mentioned the 25" Lomo and my Zeiss 2060S bino, that has huge coma in the end but few on the market ( no coma ) are even close on the optic contrast.

I know what I use/chose.

More glass in front of eye is not what I chose.

 

Reality:

 

At F/4, there's massive coma at the edge of the field at F/4.

 

Put a bright star at the edge of the field. It's not just a little aberrated, it can't be missed.

 

I've had my 12.5 inch F/4.06 for 20 years. Coma is inherent in a parabolic mirror, a perfect parabola, has the same coma as a less perfect parabola.

 

People with driven mounts who focus on small objects, particularly nebulous objects and galaxies, aren't going to see it.

 

Look at M7.. you can't miss it. And M7 is an object well suited for large scopes, lots to see.

 

If you're OK with serious off-axis aberrations, that's your choice.

 

F/4.. 1.4 mm diffraction limited field.  That's what coma does to a perfect mirror.

 

I sure see it.

 

Jon

 

Jon


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