Coma Correction

[Buzzard75]

I received an ES 30mm 82-series eyepiece for Christmas and have only had one opportunity to use it since. I used it in my Orion 12" dob. While viewing the Orion Nebula I noted that the stars towards the core of the Trapezium were fairly well focused, however, the ones towards the edge tended to be more stretched. I'm not an expert on the matter and not having really seen an example of it before, it's my understanding this is what coma looks like and is fairly common in fast scopes such as mine, which is an f/5. I didn't see any scattering of color so I don't believe it's chromatic aberration. The fast scope coupled with the wide field of view from the eyepiece just seems to be a recipe for disaster. I have a handful of other eyepieces, an ES 14mm 82-series included, and don't seem to have the issue with any other eyepiece. Has anyone else experienced the same issue with the combination of a large aperture f/5 and such a wide FOV, long focal length eyepiece? Am I misinterpreting what I'm seeing? Do I just need to buy a coma corrector and go on my merry way?

[Louis D]

Rack your focuser on either side of focus with a bright star near the edge of the 30mm ES-82.  If it stretches tangentially on one side of focus and radially on the other (either as ovals or lines perpendicular to each other), you're looking at astigmatism rather than coma.  If it fans out away from the center toward the edge on either side of focus, that's coma.

[tuckstar]

I have the 100° es with my 10" f4.7 and there is definitely some coma. It is inherent with this type of system. It is really just at the very outer edge of fov and you have to look to see it so doesn't both me to much. One day I'll get a paracor but no rush as it's a huge outlay.

[Buzzard75]

7 hours ago, Louis D said:

Rack your focuser on either side of focus with a bright star near the edge of the 30mm ES-82.  If it stretches tangentially on one side of focus and radially on the other (either as ovals or lines perpendicular to each other), you're looking at astigmatism rather than coma.  If it fans out away from the center toward the edge on either side of focus, that's coma.

Thank you! I was hoping someone would chime in with a method to try and determine the difference. Even with all my research, I still don't think I'm educated enough to be able to discern the difference just by a basic visual observation. I needed some way to test and determine what I'm actually seeing.

3 hours ago, tuckstar said:

I have the 100° es with my 10" f4.7 and there is definitely some coma. It is inherent with this type of system. It is really just at the very outer edge of fov and you have to look to see it so doesn't both me to much. One day I'll get a paracor but no rush as it's a huge outlay.

As it seems to be the nature of a fast Newtonian reflector and wide FOV, and based on what I THINK I'm seeing, I'm guessing it's coma. I'm going to try the method suggested by Louis and see if I can figure it out first before I go spending money on a coma corrector.

[Louis D]

If you don't want to spend a lot of money on a coma corrector, but still get most of your coma corrected on an f/5 scope, I'd do what I did and get the GSO CC which is sold in the UK as the Revelation or Altair Astro CC.  It is similar to the Paracorr I in that it has four elements, flattens the field, and corrects most of the coma.  It only magnifies by 10% instead of 15% like the Paracorr, so that is nicer.  It does require a bit of adjustment to get it working.  I added a 15mm or 20mm extension tube between the optical section and the eyepiece holder and then parfocalized any eyepieces focusing more than 5mm from the eyepiece shoulder.  Now, I just insert them in the CC and there's no need to adjust the spacing, I just observe.  Sure, there's a bit of leftover coma at the edge, but it's so minor, I barely notice it.  I'm OCD, but not so OCD that I'll spend hundreds more on a Paracorr II to correct the last of the coma.

[N3ptune]

hello @Buzzard75

You can look at a few website to identify the most common aberrations like this one, it has some representative pictures:

http://umich.edu/~lowbrows/reflections/2007/dscobel.27.html

Perhaps what you see is not really significant to say it's a true serious problem. I can live with a bit of coma with my newtonian F5, they are like that.

 

 

[Louis D]

I can definitely recommend the GSO CC after trying to live with f/6 levels of coma in my Newt last night.  After 5 minutes without it, I couldn't stand the indistinct stars and put it in the focuser.  WOW, what a difference.  It cleaned up 90% of the coma leaving very little to bother me the rest of the night.  Of course, I view exclusively with 68 to 92 degree AFOV eyepieces with excellent astigmatism correction, so I know the stars are supposed to be nearly pinpoint everywhere.  When they aren't, it really annoys me.

[Buzzard75]

Thanks for all the responses. The next time I get a chance to take it out, I'll try Louis' method and at our next club event I'll see if anyone already has a corrector I can try out. I've seen GSO, Explore Scientific, and Tele Vue all have their own coma corrector. I'm sure there are others. If that's what I need, what would be the benefit of getting a ridiculously expensive TV over the other two? Or is there a reason I may want to choose either the GSO or the ES besides saving some money?

[laowhoo]

On 1/31/2018 at 20:56, N3ptune said:

hello @Buzzard75

You can look at a few website to identify the most common aberrations like this one, it has some representative pictures:

http://umich.edu/~lowbrows/reflections/2007/dscobel.27.html

Perhaps what you see is not really significant to say it's a true serious problem. I can live with a bit of coma with my newtonian F5, they are like that.

 

 

 

Excellent link. And another (page you might have seen floating around, borrowed from Suiter's Star Testing) re: primary aberrations

[Buzzard75]

Yes, thank you for the links and the pictures. Reading Louis' method and seeing the graphic Laowhoo had posted is exactly what I had pictured in my mind for an astigmatism check. I'm fairly certain what I was seeing was coma, but I have yet to confirm. Haven't had too many clear nights. Just trying to envision it in my head I seem to recall they were bunched up on one side and then stretching outward just like in the focused image of coma so I'm fairly confident. Hopefully I'll get a chance to get it out this weekend, but it's not looking like the weather wants to cooperate.

[Bart]

I have an F4.5 16” and an F5 10” and always use wide angle EPs. Coma is a problem. One day I bought the ES cc from the US for smallish money. One problem. I had to move the primary on the 16” about an inch closer to the secondary. Bit of a pain, but there you go. Could never justify the Parracor cost. 

 

Barry

[Louis D]

16 hours ago, Bart said:

I have an F4.5 16” and an F5 10” and always use wide angle EPs. Coma is a problem. One day I bought the ES cc from the US for smallish money. One problem. I had to move the primary on the 16” about an inch closer to the secondary. Bit of a pain, but there you go. Could never justify the Parracor cost. 

 

Barry

That's one advantage to the GSO CC I forgot to mention.  I think it only needs about 10mm or so of in-focus.  I was surprised I could get it to focus in my Dob which is set to give primary focus about one inch above the most inward focuser travel position of the low profile focus.

[25585]

I did get a Paracorr 2 last year. The tunable top is what sold it to me. Usually its on G setting for my 35 Panoptic, but can be used for other eps as well - less hassle. It would be a reason for a 41 Panoptic. I would like to try it with a Nagler 31mm some day.

[Don Pensack]

It sounds like coma.

Newtonian optics produce coma, but coma is a linearly-increasing aberration from the center.

So the wider the apparent field of view at a given focal length of eyepiece, the more coma will be seen since the field is extending farther out into the field of the scope.  In a very real sense, coma correctors made 100° eyepieces possible.  And it's the same at all powers as long as the apparent field is the same.  The star image at the edge has a size to it, so it is magnified when the magnification increases.  A 15mm 82° eyepiece would have a field 1/2 as wide as your 30mm, but also have twice the power.  1/2 the size of comatic star image times 2X the magnification = the same apparent size.  I've tried using my 3.7mm eyepiece without a coma corrector, and it displays the same coma at the edge as the 21mm with the same apparent field.

There are 2 coma correctors with helical tops, TeleVue and Explore Scientific.  The advantage of the helical top is that all your eyepieces will come to focus at the same place in the focuser travel--the top is turned to focus the eyepiece so the coma corrector stays at the same place in the light cone from the mirror.  Essentially, the coma corrector parfocalizes your eyepiece set.  Now, if your eyepieces are already parfocal, you'll do almost no focusing and almost no adjusting of the top of the coma corrector.  If that's the case, a non-adjustable coma corrector like the GSO could work for you.  You only have to optimize the position of one eyepiece and all the other eyepieces will follow.

The Explore Scientific HRCC can also be used as a fine focuser once it's set up.  Note that the Paracorr requires 14mm of in travel of the focuser versus eyepieces in order to get to focus (TV says you should have 19mm of in-travel to allow for near sightedness to come to focus) and the ES HRCC requires 32mm of in-travel.  Most scopes have enough in travel for the TV, but not every scope has enough in-travel for the HRCC.

 

A note to sub dwarf: once the Paracorr is set at G for your 35 Panoptic, you can adjust it for every other eyepiece of every brand by merely focusing using the tunable top.  That way, each and every eyepiece will be brought to its optimum coma correction position relative to the lens.  Pretty easy, I'd say--you don't even have to know the correct setting for any other eyepiece in advance.

[25585]

For their own eps, TV have a table showing which letter is best. 

Review ES v TV http://avt-astro.com/index.php/reviews/accessories/televue-paracorr-ii-explore-scientific-coma-correctors

[Buzzard75]

I was FINALLY able to get out last night. Unfortunately, I wasn't able to diagnose the problem. Not that it's really a huge problem. I believed it to be coma, but now I'm not 100% certain. I attempted the inside and outside focus on Sirius. On the inside focus, the star appears to bulge outward as you would expect with coma. On the outside focus it actually bulges inward toward the center of the field of view. I have no idea what would be causing it to do that. From what I understand, it should bulge to the outside with coma for both inside and outside focus. If it were astigmatism it would bulge inward and outward from center and then towards the sides on opposite sides of focus. I don't believe it's pinched optics either as the stars don't appear to be triangular. Sirius was perfectly round when in focus and I had the beautiful defraction spikes from the vanes. The stars in the Triangulum area of Orion were perfect little pin points and easily split. I'm at a bit of a loss right now.

I'm open to more suggestions.