To Paracorr or not to Paracorr?

[pez_espada]

This is the question. I have probably been spoiled by the very well corrected fields of both my long refractors and my 10x50 Fujinons that I seemingly cannot take the coma showing at the edges of my new 130PDS Newtonian at F5. I bought this as an entry level reflector to learn collimation and especially to team up with my binoculars as a wide field telescope for estimating magnitudes of bright variables. As you may know, very often you need to compare your variable star with stars of known magnitudes (called comparison stars) in the same field of view. So one wishes to have an instrument able to provide wide fields (such as small refractor or reflector), but obviously with a well corrected field for both curvature (especially in fast refractors) and/or coma (especially in fast reflectors).  My 5.1" Newt is showing its ugly coma in my ES eyepieces (but less in my TV Plössls).  I also think that, because of coma I am not detecting faint magnitude stars due to a low perceived contrast.

Should I invest in a Paracorr for such an instrument? i.e. the Paracorr would cost more than twice the small reflector!  would the corrector "clean" the image enough and provide more contrast as to show me stars close to the theoretically limit magnitude for its aperture? or the CO in the design (secondary is allegedly 46mm) is too much? (I heard this small Newtonian is optimized for AP and not too much for visual). I am aware that if I purchase a Paracorr/coma corrector I would be able to plug it in other larger fast reflectors I may have in the future but at the moment this little guy is the only Newtonian I have living among two long focus refractors....

Many thanks and your opinions and insights are very welcome

C

[jetstream]

33 minutes ago, pez_espada said:

Many thanks and your opinions and insights are very welcome

You might want to look for a scope with less CO, 35% is getting right up there. A Paracorr will increase perceived contrast (fo me anyway) but the large CO might not be your friend here. 30% CO is my max, 25% and less preferred and my best scopes are 21% or less.

Edited January 25, 2020 by jetstream
46mm to % CO

[Peter Drew]

A degree of your problem could be due to the eyepieces.  TV eyepieces are probably better corrected than the ES equivalents, specially if the ES's are wider field than the TV's.  The Paracorr is also optimised for TV eyepieces, particularly the wide field ones.    🙂

[pez_espada]

10 minutes ago, Peter Drew said:

A degree of your problem could be due to the eyepieces.  TV eyepieces are probably better corrected than the ES equivalents, specially if the ES's are wider field than the TV's.  The Paracorr is also optimised for TV eyepieces, particularly the wide field ones.    🙂

I though that Paracorrs even when designed with TVs Ethos and Naglers in mind, would work fine for whichever wide AFOV eyepieces reasonably well corrected for off-axis astigmatism and other aberrations at the eyepiece's border. And I heard that ES are reasonably good eyepieces (good value) for not having the green lettering on them.. The ES eyepieces do work flawlessly over the whole field of view in my F11 and F13 refractors! 

[John]

F/5 is very much more demanding on eyepiece correction than F/11 or F/13.

Some ES eyepieces are well corrected in faster scopes though. Which ES eyepieces do you use ?

You may be seeing astigmatism (eyepiece created) coma (scope created) or a mixture of both.

 

[pez_espada]

2 hours ago, jetstream said:

You might want to look for a scope with less CO, 35% is getting right up there. A Paracorr will increase perceived contrast (fo me anyway) but the large CO might not be your friend here. 30% CO is my max, 25% and less preferred and my best scopes are 21% or less.

It seems, or at least I think I remember reading something along these lines,  that in the reflector world the size of secondary mirrors scale inversely proportional to the focal length, which is  independently of the focal ratio , meaning that, for instance a 5" F6 would need a proportionally larger secondary than a 10"F6 scope (?).  I have seen from a popular brand a 6"F6 with a 46mm secondary (~30% of the primary),  and from the same manufacturer an 8" F6 with a 50mm secondary (25% of the primary). Both, they claim , optimized for visual..

If that's true then for a wide-field Newtonian with good contrast (i.e. proportionally smaller secondary) one necessarily hits a trade-off region, as the larger the scope so it can have  a proportionally smaller CO, the larger the focal distance that negates wide fields...  would that be a sound reasoning?

Maybe my best bet is to go the fast achromat refractor route?

Edited January 25, 2020 by pez_espada

[pez_espada]

Just now, John said:

F/5 is very much more demanding on eyepiece correction than F/11 or F/13.

Some ES eyepieces are well corrected in faster scopes though. Which ES eyepieces do you use ?

 

John, I use at the moment an ES68 16 and an ES68 24. But I was thinking in getting some from the 82 series for my wide-field endeavors after reading about them for years and hoping they are any good..

[jetstream]

29 minutes ago, pez_espada said:

It seems, or at least I think I remember reading something along these lines,  that in the reflector world the size of secondary mirrors scale inversely proportional to the focal length, which is  independently of the focal ratio , meaning that, for instance a 5" F6 would need a proportionally larger secondary than a 10"F6 scope (?).  I have seen from a popular brand a 6"F6 with a 46mm secondary (~30% of the primary),  and from the same manufacturer an 8" F6 with a 50mm secondary (25% of the primary). Both, they claim , optimized for visual..

If that's true then for a wide-field Newtonian with good contrast (i.e. proportionally smaller secondary) one necessarily hits a trade-off region, as the larger the scope so it can have  a proportionally smaller CO, the larger the focal distance that negates wide fields...  would that be a sound reasoning?

Maybe my best bet is to go the fast achromat refractor route?

This calculator from Mr Bartels will show you exactly whats happening re CO IMHO.  Yes the larger the aperture the smaller the CO you can use,fully illuminated field considered. No experience with achromats but I do like triplet APO's.

http://www2.arnes.si/~gljsentvid10/diagonal.htm

[pez_espada]

5 minutes ago, jetstream said:

This calculator from Mr Bartels will show you exactly whats happening re CO IMHO.  Yes the larger the aperture the smaller the CO you can use,fully illuminated field considered. No experience with achromats but I do like triplet APO's.

http://www2.arnes.si/~gljsentvid10/diagonal.htm

Gerry thank you for pointing me to this!

[pez_espada]

2 minutes ago, pez_espada said:

Gerry thank you for pointing me to this!

Looking at Bartels' web got me thinking that he uses himself  some not so large and very fast Newtonians (F3.3!) which would have (proportionally) huge  secondary mirrors. Ergo the contrast in these instruments should suffer for visual. However Mr. Bartels use them happily  (I guess with Paracorr and Ethos) for visual rich-field astronomy. So there's something I'm missing or maybe the role of CO on contrast is not that great as common knowledge implies?

[Don Pensack]

Don't worry about central obstruction.  It is unimportant for anything except lunar and planetary observing at high powers.

As for what you typically see at the edge of the field in ultrawide eyepieces at f/5:

Coma (from the mirror--needs a coma corrector to solve)

Astigmatism (all ES eyepieces have this except the 92s, but it can be in the optics as well)

Field curvature (causes slight defocus, making other aberrations look worse)  This can be in the eyepiece or the scope if the focal length is short enough (say, under a meter).

Lateral chromatic aberration (sometimes referred to as chromatic smear, looking a bit like a prism has spread the colors)

Edge of field brightening (EOFB) wherein the center of the field looks like it has better contrast than the edge

Edge of field vignetting, wherein the edge appears to be darker than the center of the field

Of these, astigmatism is the worst, and if combined with coma, you get stars at the edge that look like seagulls.  

But all f/5 scopes have visible coma.  Only the central 2.75mm of field (remember, 2" is 50.8mm) is essentially coma-free, so:

--can you accept 2" eyepieces?  Because all currently-available coma correctors are 2".

--If you cannot afford the TeleVue Paracorr II, I also recommend the Explore Scientific HRCC, which is a lot less.

--do you have quite a bit of in-travel of the focuser left in the scope?  The Paracorr needs about 14mm of in-travel at the focuser compared to where the eyepieces focus without it, and the HRCC needs 32mm of in-travel at the focuser to get to a focus position.  The good news is that a coma corrector will allow all your eyepieces to focus at the same point of focuser travel, so you won't need any additional in-travel except to accommodate your near-sighted friends, and that's only a few millimeters at most.

You could be where I was many years ago.  I saw horrible coma, and got a coma corrector to eliminate it, which it did, only to show me my low power eyepiece had strong field curvature (it got replaced).  Once coma is removed, all the other aberrations become apparent.  Alas, eyepieces without astigmatism at f/5 are few and far between.  Only 2 from ES, none from Baader, 49 from TeleVue, none from Olivon/Barsta, a few from Nikon, none from Vixen, none from Pentax, none from APM, etc.  But, zero astigmatism may not be a necessity if coma is gone and astigmatism is very small.  But if you want refractor-like star images to the edge, prepare to pay.. As we say: Wide-field, well-corrected at the edge, low price--pick any two!

[Louis D]

If your budget is tight, get the Revelation/GSO coma corrector and add a 25mm extension between the optical section and the eyepiece holder.  As long as your eyepieces focus with 5mm of the shoulder, it will correct over 95% of the coma in my experience, often far more.  It needs about 11mm of in-focus, so most focusers can accommodate it.

[Piero]

Personally, I would upgrade the telescope rather than getting a coma corrector to be used with that telescope. 

Then, yeah, at F5 I would use a coma corrector, especially if using eyepieces with 70 Deg AFOV or more. Not everyone, and here on SGL very few, would though. 

[pez_espada]

54 minutes ago, Don Pensack said:

Don't worry about central obstruction.  It is unimportant for anything except lunar and planetary observing at high powers.

As for what you typically see at the edge of the field in ultrawide eyepieces at f/5:

Coma (from the mirror--needs a coma corrector to solve)

Astigmatism (all ES eyepieces have this except the 92s, but it can be in the optics as well)

Field curvature (causes slight defocus, making other aberrations look worse)  This can be in the eyepiece or the scope if the focal length is short enough (say, under a meter).

Lateral chromatic aberration (sometimes referred to as chromatic smear, looking a bit like a prism has spread the colors)

Edge of field brightening (EOFB) wherein the center of the field looks like it has better contrast than the edge

Edge of field vignetting, wherein the edge appears to be darker than the center of the field

Of these, astigmatism is the worst, and if combined with coma, you get stars at the edge that look like seagulls.  

But all f/5 scopes have visible coma.  Only the central 2.75mm of field (remember, 2" is 50.8mm) is essentially coma-free, so:

--can you accept 2" eyepieces?  Because all currently-available coma correctors are 2".

--If you cannot afford the TeleVue Paracorr II, I also recommend the Explore Scientific HRCC, which is a lot less.

--do you have quite a bit of in-travel of the focuser left in the scope?  The Paracorr needs about 14mm of in-travel at the focuser compared to where the eyepieces focus without it, and the HRCC needs 32mm of in-travel at the focuser to get to a focus position.  The good news is that a coma corrector will allow all your eyepieces to focus at the same point of focuser travel, so you won't need any additional in-travel except to accommodate your near-sighted friends, and that's only a few millimeters at most.

You could be where I was many years ago.  I saw horrible coma, and got a coma corrector to eliminate it, which it did, only to show me my low power eyepiece had strong field curvature (it got replaced).  Once coma is removed, all the other aberrations become apparent.  Alas, eyepieces without astigmatism at f/5 are few and far between.  Only 2 from ES, none from Baader, 49 from TeleVue, none from Olivon/Barsta, a few from Nikon, none from Vixen, none from Pentax, none from APM, etc.  But, zero astigmatism may not be a necessity if coma is gone and astigmatism is very small.  But if you want refractor-like star images to the edge, prepare to pay.. As we say: Wide-field, well-corrected at the edge, low price--pick any two!

Great info here Don, thanks. I have the SW 130PDS which has a 2" focuser.  Which ES and TV eypieces you would recommend then at F5? As you say, probably I wouldn't need perfect astigmatism correction right at the edge! I am happy with the level of correction my Fujinons 10x50 provides (or my F11 refractor for that matter). Also, in the practice what does offer the Paracorr over the ES HRCC?

Thanks,

C

[Ships and Stars]

I've the ES HR coma corrector, works a treat on my eyepieces, especially the 21 Ethos, in a f4 dob. Bought mine second-hand for around £160-180 (can't recall exactly), couldn't find a single used Paracorr 2 online after months of searching - which speaks volumes about how good those are too! The ES has a tunable top similar to the paracorr, so no faffing around with extra extensions and spacers. For a 130PDS I don't know if it makes sense though? 

[pez_espada]

13 minutes ago, Ships and Stars said:

I've the ES HR coma corrector, works a treat on my eyepieces, especially the 21 Ethos, in a f4 dob. Bought mine second-hand for around £160-180 (can't recall exactly), couldn't find a single used Paracorr 2 online after months of searching - which speaks volumes about how good those are too! The ES has a tunable top similar to the paracorr, so no faffing around with extra extensions and spacers. For a 130PDS I don't know if it makes sense though? 

You guys  think it makes not sense because  maybe not worthy to buy a coma corrector costing more than twice than the Chinese scope needed to be corrected?   I have seen some folks with eyepiece collections costing many times more than the scopes they are used on.  But again, I am not sure that this would be like buying the latest generation low noise cold camera to shot mag 14 carbon stars with your ShortTube 80. On the other hand,  I have heard this 130PDS is used by some guys here for AP,  and I guess some of them plug a CC  into this scope for this application.. why wouldn't one plug a CC for visual then?  I'm not interested now in obtaining an larger/more worthy Newtonian for the kind of observation I pursue at the moment (visual observation of brighter variable stars ).  Probably some day I will get something like am 8" but I don't think that would happen any time soon. 

 Maybe I should consider a fast refractor, but I would be struggling with CA instead of coma (not enough money for an APO). However I m not sure what would be worst (or less bad) for visual variables, coma of the fast reflector or false color  (and field curvature) of a fast refractor.

[Ships and Stars]

One thing I don't miss is chromatic aberration and all of that! I'll take coma any day over false colour. Go for the coma corrector then and a good widefield eyepiece 👍 

[jetstream]

48 minutes ago, pez_espada said:

why wouldn't one plug a CC for visual then? 

It will work nicely, try to find a used CC like S&S did- the ES or possibly a Baader .

 

3 hours ago, pez_espada said:

So there's something I'm missing or maybe the role of CO on contrast is not that great as common knowledge implies?

He runs relatively small CO for the aperture and with a PCII in specialized little dobs- big ones now too. Lockwood likes 12"+ aperture for the ultra fast newtonians because you can reduce the CO, at least when I talked with him. I have used 37% CO in my 200mm f3.8 but much prefer 31.5% and I observe many of the things he does. There are theories, ideas and then what we each prefer- and I know what I prefer.

Point sources such as stars do behave differently than extended objects, but one thing is certain- to view near the edge of the FOV you must control coma and astig IMHO.

Have you heard of Suiters "wobbly stack"?

[Charles Kirk]

To make comparisons between variable stars and comparison stars in the same field you would need to have 100% field illumination for the circle containing the two stars. For that reason you would want a larger central obstruction than you would in a reflector optimised for observing fine details on the planets and/or moon.

Personally I would not be interested in purchasing a coma corrector that cost twice as much as the telescope that it was going to be used with.

Quote

My 5.1" Newt is showing its ugly coma in my ES eyepieces (but less in my TV Plössls).  I also think that, because of coma I am not detecting faint magnitude stars due to a low perceived contrast.

I would have thought that the difference between the two is more likely to be due to astigmatism.

[jetstream]

2 hours ago, Charles Kirk said:

To make comparisons between variable stars and comparison stars in the same field you would need to have 100% field illumination for the circle containing the two stars. For that reason you would want a larger central obstruction than you would in a reflector optimised for observing fine details on the planets and/or moon.

This is technically true- but what point source magntitude drop can be detected visually by an astronomer? ie .10mag, .20 mag etc?

[Charles Kirk]

https://www.assa.org.au/resources/variable-stars/observing-variable-stars/

Experienced visual observers can see 0.1 mag differences under ideal conditions, and 0.2 mag more typically.

 

Edited January 26, 2020 by Charles Kirk
Formatting

[pez_espada]

32 minutes ago, Charles Kirk said:

https://www.assa.org.au/resources/variable-stars/observing-variable-stars/

Experienced visual observers can see 0.1 mag differences under ideal conditions, and 0.2 mag more typically.

 

That's right, estimates down to  .1 mag are expected. And yes, field of view should be fully illuminated or at least symmetrically illuminated.

Edited January 26, 2020 by pez_espada

[jetstream]

1 hour ago, Charles Kirk said:

https://www.assa.org.au/resources/variable-stars/observing-variable-stars/

Experienced visual observers can see 0.1 mag differences under ideal conditions, and 0.2 mag more typically.

 

Interesting, this is something I've never done or considered but am puzzled from a statement earlier on in this site.

 

"How many variable stars are there?

Nobody knows. Seriously! Defining a star as variable requires at least two reliable magnitude measurements, at two different times, which disagree with each other. How much of a discrepancy qualifies as a "real variation" is a matter of opinion. Anything below 0.2 magnitudes is virtually undetectable to the human eye; and visual observers trying to follow a star with less than 0.4 magnitude variation are wasting their time. They should use a CCD or photometer instead."

[Charles Kirk]

Given a long enough timescale all stars are variable.

[Don Pensack]

On 25/01/2020 at 14:59, pez_espada said:

Great info here Don, thanks. I have the SW 130PDS which has a 2" focuser.  Which ES and TV eypieces you would recommend then at F5? As you say, probably I wouldn't need perfect astigmatism correction right at the edge! I am happy with the level of correction my Fujinons 10x50 provides (or my F11 refractor for that matter). Also, in the practice what does offer the Paracorr over the ES HRCC?

Thanks,

C

The Paracorr offers a top with a bit less wobble and slightly better correction of eyepiece field stops as large as 46mm and better correction at f/3-f/3.5.  Other than that, the HRCC would be fine IF you have sufficient in-travel of the focuser to accommodate it.

You should note that the visibility and appearance of coma is related to apparent field.  Use 50° eyepieces in your scope, and you likely won't notice coma.  Use 100° eyepieces in your scope and you will.  Though the linear size of coma changes at the edge of the field with true field (it gets smaller as the focal length of the eyepiece shrinks), the apparent size (what you see) of coma only changes with apparent field. 

Also, note that a short focal length scope will have inherent curvature in the focal plane that will mildly defocus eyepieces that have large field stop diameters.  Another way of looking at it is that coma is made worse in appearance by field curvature, so keeping apparent fields narrow will reduce the ability to see field curvature.

So, suggesting an apparent field wide enough to yield fairly large true fields, as well as avoiding defocus from field curvature, and having just about zero astigmatism, I think I would recommend TeleVue Delites.  You might find you don't need a coma corrector with them, at 62°, but a coma corrector would yield perfect stars from center to edge in them.  Also, they're all parfocal, so would use the exact same setting of the coma corrector for each +/- 0.5mm or so.