Tele Vue Paracorr 2 - The search for an almost perfect view

[HollyHound]

I've had the Bresser 10" f/5 Dobsonian for well over a month now, and despite the weather not always being kind, I have managed to get a good few hours with it. To go with it, I initially invested in some (hopefully) decent 70 degree eyepieces... Pentax XW30, 20, 14, 10 and 7. The views with such a large aperture (compared to my 5" SCT or Mak) have been awesome and I'm really enjoying the ability to quickly take the dob outside. Views of M31 and M42 are so much better than those I originally had last year in my smaller Mak, and I'm looking forward to hunting down some of the other targets with this scope.

However, I'd found that the XW20 and XW14 suffered noticeably from field curvature, and so stars at the edge of the field of view were not sharp (more of a large out of focus blob), in both the dob and my 80mm f/10 refractor. It was possible to re-focus for the stars at the edge, but then the centre goes out focus... not ideal. I'd also noticed in the XW30 that stars near the edge, although reasonably sharp, where not always "well shaped". Initially I considered this was astigmatism, so have a Dioptrx 0.75 fitted to this eyepiece and it seemed to help, but still not perfect.

In order to assess if the XW14 field curvature could be improved upon, I obtained a Delos 14 and this does indeed retain sharpness to the edge, but now it was clear that there was still a remaining aberration, as all stars from about half the field of view outwards appear to "ice cream cone shaped"... after investigation, I now know this is coma!

I found this reference very useful to confirm what I was looking at... http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

Perhaps I'm being too perfectionist, but I see any aberration that can be reduced/eliminated as worthwhile fixing, so yesterday I received the Tele Vue Paracorr Type 2 (thanks again @FLO). I'd read numerous threads about all the different types of coma corrector and consistently this came out as the easiest to use and of good quality.

Last night, the dob was put out and the skies cleared for a good number of hours and so I was able to test for myself if this is the missing link for the (almost) perfect view 🤔

I've learnt that it takes a good 30mins to an hour for this size of scope to cool properly and whilst cooling it was obvious that the stars shapes were all over the place. Finally around 11pm, I popped the Delos 14 in the scope, without the Paracorr and used M42 and Rigel as my initial test targets. This eyepiece has excellent contrast and sharpness across the field and M42 was very impressive, but as stars moved outwards the coma was evident, particularly on bright stars (such as Rigel).

I then inserted the Paracorr into the focuser, which involved removing the Clicklock extension adaptor I have been using (as the long nosepiece of the Paracorr wouldn't go fully in) and replacing it with the extension tube supplied with the scope. I replaced the 1.25" adaptor with my Clicklock adaptor and popped the Delos back in. TeleVue advise using the "A" setting for this eyepiece (when not using an "in travel" adaptor), so I set it to that. The tunable top has a lovely smooth and easily adjustable action and can be locked at anytime very easily.

Focussed back on the targets and I saw no noticeable decrease in contrast, brightness or sharpness in the centre of field. Looking at stars towards the edge though, was unbelievable... they were sharp, had no coma or astigmatic shape and just looked as they did at the centre. I spent a good few minutes panning stars around in the view, but they just remained as sharp stars no matter where they are in the field of view, except for perhaps a (very) small change just before they disappear out of view. To say this was a revelation, would be a massive understatement... it was what I hoped and expected, but still remarkable to see 😃

I then locked the focuser, and followed the advise (I think it was @Don Pensack, thanks) on how to adjust the Paracorr for using other (non Tele Vue) eyepieces. Inserted the Pentax XW14 and just turned the tunable top until stars at the centre where totally sharp/focussed again. This indicated approx the "C" setting, so I locked the Paracorr here and had a look around again. Another revelation... again no coma, but also stars remained in focus over more of the field area. They still got a little blurry in the outer 20/30% of the field (to my eyes) but much better. I also tried the trick of focussing for half way across and it was very usable. I still much prefer the total sharpness of the Delos for this 14mm focal length and after extensive testing, I found (almost) no discernible difference in contrast, brightness or visibility for M42 between either eyepiece.

The XW20 performed much better, this time the field curvature was very slight indeed and only noticeable in the last 10% of the view. I was able to focus a little to the middle and all looked sharp enough, not perfect but very usable. I guess if someone was a little younger, they would have no issue at all. I ended up focusing for the centre and just accepting a small amount blurriness at the very edge, unless concentrating intently on the target at centre.

The XW30 worked perfectly, this time I had to reset the tunable top to the "G" setting and the view was very wide and sharp across the field.

I'd read that this Paracorr increases the focal length slightly, and it did indeed, but not so much as to feel anything was missing. Given the increased quality across the field, I'm really not sure I would use the scope without it now. However, I will do further testing using my other focal lengths to make sure it's suitable for all eyepieces and targets.

Given I really didn't know what to expect in terms of improvement (other than other reports/reviews), I was very impressed indeed with the Paracorr. I'm sure other coma correctors will perform as well, but the convenience of the tunable top makes this a pretty much permanent installation in my dob now. Just a shame it's so expensive, but I can confirm (for me at least) it really does make a big difference 👍

As a final footnote, whilst viewing M42 with both Delos 14 and XW14 (magnification approx 100x), I finally got a very clear view of the E and F stars in/around the Trapezium, the very first time I've been able to see anything other than the main four. There was very little difference between the eyepieces, although I did consider the Delos showed them (very) slightly more clearly. Perhaps it was the aperture, cold/clear sky or Bortle 4/5 conditions around here, but it was delightful to see them and they required no extra effort at all... a lovely way to round off a few hours with a clear sky at last 😃

Edited December 20, 2020 by HollyHound
Added photo

[John]

Interesting report.

Being a perfectionist can be quite expensive can't it ?

Another thought springs to mind - should we place much credibility on any reports on eyepiece performance that are based on their use in newtonians where a Paracorr or other suitable coma corrector is not being used ?

That one has bothered me for some time

 

 

Edited December 20, 2020 by John

[HollyHound]

23 minutes ago, John said:

Being a perfectionist can be quite expensive can't it ?

I've found the same with audio gear... I spent many months years ago getting to a point where I was happy with my headphone/amp setup, but once I got to the point where it was as good as I felt it could then go, I have been using that same equipment for years now very enjoyably... there's always a sweet spot between performance/price and beyond that it's very much diminishing returns. It's good to acquire the gear you need/want, but then it must be used effectively and that's more important.

23 minutes ago, John said:

Another thought springs to mind - should we place much credibility on any reports on eyepiece performance that are based on their use in newtonians where a Paracorr or other suitable coma corrector is not being used ?

That one has bothered me for some time

Based on what I discovered last night, I have to say that I feel any reports of edge behaviour for eyepieces in a (faster) Newtonian without a coma corrector, are likely going to be limited by visible coma!

Now it may be that some see it more easily than others, but to me, once the field curvature and astigmatism were solved with the eyepieces I now have, then coma was very much evident and (again to me) unacceptable. Again, this is my (relatively limited) experience and I'm sure others will other views on this, but I would classify the view I had last night as pretty much the same as I get from my 80mm ED f/10 refractor now... almost flawless from edge to edge 👍

Of course, this is with 70 degree FoV eyepieces. I do have a DeLite 13mm and 18.2mm (62 degree FoV) so will give them a try too and see if the coma is as evident without the Paracorr, before I let them go🤔

Edited December 20, 2020 by HollyHound

[John]

You would laugh out loud at what I call audio gear, believe me !

I must be lucky that coma is not something that I have found bothersome even with my Ethos eyepieces and even when I used to have an F/4.8 10 inch newtonian

I feel as if I should go back through my reviews though and add the health warning that "no coma correction was used with the newtonians involved".

Your experiences echo what @Piero has been posting recently though so there must be something in it

 

[HollyHound]

21 minutes ago, John said:

You would laugh out loud at what I call audio gear, believe me !

I must be lucky that coma is not something that I have found bothersome even with my Ethos eyepieces and even when I used to have an F/4.8 10 inch newtonian

I feel as if I should go back through my reviews though and add the health warning that "no coma correction was used with the newtonians involved".

Your experiences echo what @Piero has been posting recently though so there must be something in it

 

At the end of the day, it’s our own perceptions and experiences, and they can vary wildly, so not everyone will find coma an issue... or indeed audio quality!

I haven’t yet tried an Ethos and am actually firmly resisting it for now, in case I find the experience irresistible. I’m very happy with the views I’m getting now, I consider it the sweet spot for me 😀

I will try with and without the Paracorr over the next few weeks (weather permitting) to confirm my initial findings, but it was quite overwhelmingly better for me last night.

[Captain Scarlet]

I very recently had an identical experience with my new paracorr. It totally transformed my view of star-fields, especially open clusters and especially with my panoptic 35 which I decided to not sell as a result.

My understanding is that for Newtonian coma it’s not the eyepiece fov that chiefly matters, but how far from the centre of the Newtonian’s image circle you are. So the longer the eyepiece FL you are using, the more of that image you are looking at, further away from Its centre. High mag eyepieces won’t show much of the coma as they’re looking at the very centre of that field where there’s less coma.

I too regard it as an essential part of the system.

cheers, Magnus

[Louis D]

According to @Don Pensack, Newtonian coma grows linearly from center to edge, so the wider the apparent field of view at a given magnification, the more obvious coma becomes.  Doubling the magnification results in the same amount of coma because it is half as large, but magnified twice as much, so you're back where you started.

This is in contrast to field curvature where low powers and wide fields really do show more field curvature than higher powers with the same apparent field since it's not a linear relationship across the image circle.

[Louis D]

6 hours ago, HollyHound said:

Based on what I discovered last night, I have to say that I feel any reports of edge behaviour for eyepieces in a (faster) Newtonian without a coma corrector, are likely going to be limited by visible coma!

And by corollary, eyepiece reports in non-flat field refractors should not critique eyepiece field curvature since it is very difficult to separate it from that of the scope.  I use a TSFLAT2 in each of my refractors to come close to a flattened field.  Unless you spend all of your time observing objects on axis such as planets using a tracking mount, I can't understand how so many people can stand the strongly curved field of short ED and APO refractors.

[Ricochet]

3 hours ago, Louis D said:

I use a TSFLAT2 in each of my refractors to come close to a flattened field. 

Out of interest, how do you connect the flattener for use visually? Is it screwed to the nose of a 2" diagonal with extensions to get the correct distance? Do you have to have any form of adjustment on it to change that distance for eyepieces that have their focal planes at different heights?

[Ricochet]

11 hours ago, HollyHound said:

I then inserted the Paracorr into the focuser, which involved removing the Clicklock extension adaptor I have been using (as the long nosepiece of the Paracorr wouldn't go fully in) and replacing it with the extension tube supplied with the scope.

What part of the clicklock prevented the insertion of the paracorr? I can't think of anything on mine that would do that.If you mean some extension tube fitted between the clicklock and focuser, I think you could do away with that as with the paracorr you are almost at the inward limit of focuser travel.

[HollyHound]

3 hours ago, Ricochet said:

What part of the clicklock prevented the insertion of the paracorr? I can't think of anything on mine that would do that.If you mean some extension tube fitted between the clicklock and focuser, I think you could do away with that as with the paracorr you are almost at the inward limit of focuser travel.

In order to obtain focus visually on the Bresser 10", you need to use a 37.5mm extension tube ring (which they supply in the box). The small 2" clamp screws into this and then both 2" and 1.25" (with adaptor) eyepieces fit into that and can be focussed normally.

As I wanted to use a ClickLock (because I have them on other scopes), I bought this one... https://www.firstlightoptics.com/adapters/baader-2-inch-click-lock-cl-extension-47mm.html, figuring that the extra depth on this would be equivalent (approximately) to the extension ring, which I then removed. This all worked perfectly and there was still a reasonable bit of "in travel" available even when using the closest focussing eyepiece (the Delos I think).

I figured that the Paracorr might go in this and still have enough "in travel" to work fine, unfortunately however, this Clicklock adaptor (having a 2" nosepiece) won't of course allow the Paracorr to go more than half way inside. So I've now reverted to the supplied extension tube ring and clamp the Paracorr fully into that... given that I am now very likely to leave the Paracorr in for most (if not all) my observing, then being able to use ClickLock to swap 2"  eyepieces is not such a big deal, although I do have a ClickLock 2" to 1.25" adaptor inserted in the Paracorr to use for the majority of my eyepieces 👍

I may still obtain the normal HexaFoc fitting ClickLock... https://www.firstlightoptics.com/adapters/baader-clicklock-m68-clamp-for-hexafoc-focusers-bresser-omegon-and-explore-scientific.html, at some point to make quickly fitting the Paracorr easier, but this is very much a minor detail for now.

I'll have another play with the Paracorr and figure out how much inward travel I've got left now and see if maybe I can even do away with the extension tube ring, but likely it may put focus position out 🤔 I'd like to set it up so there is the minimum amount of focus tube extending into the OTA, as I have noticed when I de-focus a star that I can see the shape of that in the similar way to the secondary spider vane supports, and worry it may degrade image quality.

 

[Paz]

Nice report. I drew the same kind of conclusions and I don't observe with my reflector without a paracorr now. 

Personally, for some reason I find I notice the flat field benefits more than the coma benefits.

You need to seek out some party pieces to make the most of it. For me that means open star clusters that you can fill the whole field of view with, such as the double cluster, M44, and M45.

[HollyHound]

21 minutes ago, Paz said:

Nice report. I drew the same kind of conclusions and I don't observe with my reflector without a paracorr now. 

Personally, for some reason I find I notice the flat field benefits more than the coma benefits.

You need to seek out some party pieces to make the most of it. For me that means open star clusters that you can fill the whole field of view with, such as the double cluster, M44, and M45.

I really didn't expect the improvement to be so great, but it does indeed seem to really clean up the view for coma (it was very noticeable to me before) and helps with field curvature too.

It very noticeably improved the Pentax XW20 to where I can't really see field curvature until the very edge and this is a superb eyepiece now. The XW14 also improves, but the Delos 14 is flat field anyway with no other obvious downsides, to that's my first choice at that focal length.

I certainly intend to seek these targets out now and make the most of the improved view 👍

Edited December 21, 2020 by HollyHound

[Paz]

There is one other target that benefits but keep it a secret as it is not what big reflectors and a paracorr are supposed to be used for.

Check out the moon. Super flat/clean from top to bottom and side to side. 

[HollyHound]

1 minute ago, Paz said:

There is one other target that benefits but keep it a secret as it is not what big reflectors and a paracorr are supposed to be used for.

Check out the moon. Super flat/clean from top to bottom and side to side. 

Umm, not such a big secret here... I've already used the Bresser on the moon and loved it, so if the Paracorr improves further, then am in for a treat... I love the moon, hence why I have the StellaMira 80mm ED f/10 and Mak 127 already. I wasn't expecting a dob to be such fun on the moon but it was 😃

[Louis D]

8 hours ago, Ricochet said:

Out of interest, how do you connect the flattener for use visually? Is it screwed to the nose of a 2" diagonal with extensions to get the correct distance? Do you have to have any form of adjustment on it to change that distance for eyepieces that have their focal planes at different heights?

I use a GSO dielectric 2" diagonal that happens to have an SCT nose thread.  I replace the original nose barrel with a 15mm SCT to M48 thread adapter.  I then screw the TSFLAT2 into it.  The original nose barrel is about 15mm too long and over corrects field curvature as a result.  I discovered this method with my AT72ED (430mm FL).  I thought I would need to shorten it up on my new to me 90mm TS triplet APO (600mm FL), but it surprisingly works fine at this spacing as well.  Shortening it up didn't seem to improve matters, so I left it long to have more barrel in the focuser.  I may play with it more in the future.  I'm not taking photographs, after all.  I'm just trying to get rid of most of the annoying field curvature.  If 90%+ of it is gone, I'm happy.

I've found this spacing works well for pretty much all of my eyepieces despite the fluctuation in focusing distance.  Again, this isn't photography, being in the ball park to eliminate 90%+ of the curvature is enough to move it from annoying to barely perceivable.  It's the same for my GSO CC, if 90%+ of the coma is gone, I'm good with it and don't feel a need to fine tune the distance for each eyepiece.  Only my 12mm TV NT4 focuses way off and had to be parfocalized (20mm below the shoulder) for both CC and FF.

[Ricochet]

4 hours ago, HollyHound said:

As I wanted to use a ClickLock (because I have them on other scopes), I bought this one... https://www.firstlightoptics.com/adapters/baader-2-inch-click-lock-cl-extension-47mm.html,

Ah, I was thinking you'd bought the hexafoc version and wondering how they had changed it for the paracorr not to fit. If you do go for the hexafoc version it takes up something like 31mm so there's a bit of a focus saving over the standard extension and clamp but I would think it would still protrude into the OTA a bit. You might also find it doesn't extend far enough outwards for use without the paracorr, but it doesn't sound like that would be an issue for you now. 

[Ricochet]

3 hours ago, Paz said:

There is one other target that benefits but keep it a secret as it is not what big reflectors and a paracorr are supposed to be used for.

Check out the moon. Super flat/clean from top to bottom and side to side. 

I'm just going to leave this here.

https://www.firstlightoptics.com/binoviewers/baader-2-inch-1-7x-glasspathcorrector-with-integrated-coma-corrector-for-newtonian-telescopes.html

🙂

[HollyHound]

1 minute ago, Ricochet said:

Ah, I was thinking you'd bought the hexafoc version and wondering how they had changed it for the paracorr not to fit. If you do go for the hexafoc version it takes up something like 31mm so there's a bit of a focus saving over the standard extension and clamp but I would think it would still protrude into the OTA a bit. You might also find it doesn't extend far enough outwards for use without the paracorr, but it doesn't sound like that would be an issue for you now. 

Thanks. Unfortunately the Hexafoc version hasn't been in stock for a while, so this worked well without the Paracorr, but not with 😬

[Ricochet]

1 hour ago, Louis D said:

I use a GSO dielectric 2" diagonal that happens to have an SCT nose thread.  I replace the original nose barrel with a 15mm SCT to M48 thread adapter.  I then screw the TSFLAT2 into it.  The original nose barrel is about 15mm too long and over corrects field curvature as a result.  I discovered this method with my AT72ED (430mm FL).  I thought I would need to shorten it up on my new to me 90mm TS triplet APO (600mm FL), but it surprisingly works fine at this spacing as well.  Shortening it up didn't seem to improve matters, so I left it long to have more barrel in the focuser.  I may play with it more in the future.  I'm not taking photographs, after all.  I'm just trying to get rid of most of the annoying field curvature.  If 90%+ of it is gone, I'm happy.

I've found this spacing works well for pretty much all of my eyepieces despite the fluctuation in focusing distance.  Again, this isn't photography, being in the ball park to eliminate 90%+ of the curvature is enough to move it from annoying to barely perceivable.  It's the same for my GSO CC, if 90%+ of the coma is gone, I'm good with it and don't feel a need to fine tune the distance for each eyepiece.  Only my 12mm TV NT4 focuses way off and had to be parfocalized (20mm below the shoulder) for both CC and FF.

Thanks, Louis. Great information for when the time comes to flatten a frac. 

[Ricochet]

6 minutes ago, HollyHound said:

Thanks. Unfortunately the Hexafoc version hasn't been in stock for a while, so this worked well without the Paracorr, but not with 😬

That doesn't surprise me. I suspect most of us have been hit by the recent supply issues. 

[Louis D]

It would be nice if Bresser also offered a low profile focuser option with a minimal secondary obstruction of around 20%.  The current setup has a 28% obstruction which is quite large for a visual Newtonian.