Explore Scientific Coma Corrector - First Light review
This is my report for the first light review of the ES Coma Corrector. I had a good 2 hours with it tonight to test it out in ultra wide field eyepieces. Due to clouds to the west and a rising 95% moon to the east I had to focus mostly towards the top 70 degrees of the night sky. Luckily Orion was right over head and served as a good yard stick. Its bright stars, and bright doubles allowed for easy comparisons. I focused testing on 3 bright binaries in Orion. Rigel, Mintaka A and Mintaka C, and Nair al Saif. I also used the Trapezium. Trapezium stars observed were the A,B,C,D,E and F stars.
Testing was done in my 12" F5 Skywatcher Flex-Tube Dobsonian.
First thing to note. I found in-focus to be a real issue. Probably the biggest foreseeable problem with this corrector. Now, if you have used or own a Skywatcher dobsonian you know that the focuser is not a low profile arrangement it actually sticks out quite far. Thankfully, due to the the ingenious design of the flex tube I could easily just adjust the height of the upper telescope assembly effectively lowering it towards the primary and creating in-focus travel. For the effective setting for 2 of the eyepieces tested I needed to lower my UTA by about 1.5 INCHES to achieve focus. Also to note my primary mirror has upgraded springs and my mirror is pushed forward as well. This is not a problem for Skywatcher users but I would suggest a solid tube dob users take this into consideration.
First eyepiece. The 30MM Explore Scientific 82 degree's.
A fantastic eyepiece for sweeping the Milky Way, large star clusters or taking in the Carina Nebula or the LMC and SMC. It provides a true 1.5 degree field. Enough to take in the entire M42 complex with room to spare. Without a corrector to my relatively young eyes of 28yo I can see coma noticeably developing from about 40 degrees with progressively worst coma to the edge. I was not really sensitive to it but what I found did bother me the "fish bowl" effect while panning. It is a bit nauseating if I am honest. So much so I got the 24mm ES 68 degree eyepiece to escape it.
Inserting this heavy eyepiece into the corrector was easy enough the triple thumb screws and compression ring gripped that big b*****d tight. No chance of it letting go. I used the bright binaries as my testing targets. My theory behind it as since they were bright and well spaced binaries they could be split at a relatively lower power of ~60x. In the inner 20 degree's of eyepiece fields of view is what some call the "sweet spot". Essentially this is the area of the field of view that is diffraction limited. As you extend out of that area the coma effects the sharpness of the field getting worst at a linear rate as you move outwards. People who are not sensitive to coma may not detect it as easily. They may not notice the field deteriorating till 40 degree or more depending on the magnification, focal length, aperture and target object, if they notice it at all. Anyway, my idea is that in this "sweet spot" or diffraction limited area, the binary stars are easily split and as the field deteriorates as the stars drift to the field they become harder and harder to split to the point the distortion of the stars approaching the edge of field make attempting to split doubles impractical. It worked out to be a pretty good technique for me to figure out the best settings. There were no manuals that came with the corrector and since its such a new produce there are no online guides or tutorials. I literally had to eyeball the location of the settings on the corrector and compare the fields of stars and compare the diffraction limited field between the settings. When I say settings its just unscrew the eyepieces adapter. A lot like a twist up eye cup you might find on an eyepiece. On the sides are these little lines to help you measure what setting your at. These little lines seem to be about 1mm apart. Every 5 or so lines there is a larger line. For the ease of explanation I will call these spaces between the larger lines "bars".
So, I started at the lowest position. 1 bar. Immediately I discovered I did not have enough in-focus. Not to worry. Just lower the UTA towards the primary mirror. I started at about half an inch. I was able to reach focus now. But, no real difference. The star drifting indicated the sweet spot was still small. I raised / unscrewed the corrector to 2 bar. Not much of a difference again. Moved to bar 3. Could not achieve focus again. I lowered the UTA by another half inch. Focus achieved. I did notice at this point the sweet spot widening. Things looked promising. This process continued to 5 and 1/2 bars. To achieve focus again I had to lower the UTA by another half inch. I also recollimated to adjust any alignment errors introduced by the adjustments I did to the UTA. At this point the field of view was genuinely amazing. Stars were truly pin point from edge to edge - save for the last couple degrees. The diffraction limited sweet spot was enormous. Upon close inspection it seemed to be at least 70+ degree's across with the rest being very well corrected indeed. I was staggered with how sharp the field was. And further more how flat the field was. The fish bowl effect that annoyed me so much was completely eliminated. Where the stars would normally be a washed out distorted mess, the binary stars could still be split with ease. Stars held their round shape from one side of the field to the other. While the binaries were separated the entire way.
However, I did notice something. In the outer 15% of the field I noticed the sky darkened. Could this be vignetting? Further testing is required.
Now, moving on to the second eyepiece the 14mm ES 100. Probably my favorite eyepiece. But, I was annoyed by the outer field so much that bought a Delos 14mm to escape it. I was willing to accept a smaller field of view if it was better corrected. I have to say that Delos 14 is damn good. It may be the finest mid power eyepiece I have looked through. Not my favorite just maybe the finest. To start testing this eyepiece I reset everything back to its start position. Telescope and corrector, then recollimated just in case. What happened for the next 20 min was the same routine as the 30mm. Eventually I got to about the same position and recollimated again to remove any introduced alignment errors. I focused on the trapezium in Orion. Stars A,B,C,D E and F were easily discernible at about 120x. I moved the trapezium to the far left and watched it drift through to the far side of the field to the right. What I saw was amazing. The diffraction limited field was once again seemed enormous. 80+ degree's were sharp pinpoint stars. The other 20% were still very well corrected. No complaints there. At no point during the drift did the E and F stars of the trapezium were not visible. Even at the extreme edge as they disappeared they were still there. Is this what its like to own a 13mm Ethos? I digress.
The last eyepiece I had to try was the 9MM ES 100. Now once again I reset everything and collimated. In my time testing it for about 40 min. I never really found the right setting. At least not between the 1st bar and the 5th. It was very strange what was happening. Normally without correction you would have a small field of diffraction limited objects. It would progressively get worst with coma. In this case with the corrector, once again the sweet spot was much larger. But, in an area of the field between 50% to 70% from the center the view would slightly defocus. Just slightly. But enough to change the objects in that section to no longer diffraction limited. But, then from 70% outwards. BAM! Sharp pin point stars to the edge. I have no idea why it is doing this. I tried the 9MM without the corrector. Its just a sweet spot in the middle that progressively gets worst with coma. This needs more testing and I may add more information to this review when I figure out the best setting.
I bought a 8mm Delos and 10mm XW to make up for the failings of the 9mm ES in its outer fields. Brilliant eyepieces. But, not my favorite. If I can figure out this 9MM. I will probably sell the 8mm and 14mm Delos as well the Pentax XW. Might pick up the ES 5.5mm 100 in their place.
If I can't figure this out I might still sell them with the ES 9mm and pick up an Ethos instead.
Interesting report Adrian.
Had to convert inches to millimetres…1.5" = 38mm. Went Metric while I was in 5th grade
It's interesting that the CC needs quite a bit of in focus like the ES Focal Extenders do, unlike traditional barlows.
Luckily my 10" Newt is set up for imaging, not that I will ever put a camera on it, so I have about 60-100mm in focus to play with. With the TV CC I have to add a 35mm extension tube to get focus, with a Feather Touch 2.5" focuser racked all the way out.
I ordered an ES CC which I hope will get here in a couple of days and can compare to my TV CC type I.
Lack of instructions??? Suppose you just make your own up. I going to have a guess and try it like the TV CC with par-focal rings on the eyepiece and start with the ES CC screwed all the way out. Start with the eyepiece with the longest out focus, ES 16 or 28mm 68º in the CC, and move the helical inwards until it shows pin point stars all the way across….
It's going to be a bit of mucking around. Sounds like these CC will be good in f4 scopes i.e. imaging scopes.
Hope to have the thing by the weekend.
Matt
Had some more time with it under the stars last night.
Once again the ES 14mm and 30mm were suburb. Still haven't figured out the best position for the 9mm. It is quite possible I have found the best position and I'm not satisfied with it. It is also possible that the coma corrector has removed the coma and flattened the field and has now revealed an eyepiece aberration I could not see before as a coma corrector does not actually correct the eyepiece itself.
I just got my ES CC in the post.
Adrian have you tried the 9mm in your refractor? I've sold my three ES 100's as I didn't use them, so can't remember what the edge was like in my refractor. I do remember the 14 and 20 being pinpoint all the way across.
Matt
I just got my ES CC in the post.
Adrian have you tried the 9mm in your refractor? I've sold my three ES 100's as I didn't use them, so can't remember what the edge was like in my refractor. I do remember the 14 and 20 being pinpoint all the way across.
Matt
OK. So this evening I had another go with the coma corrector. This time I changed my approach. I lowered the truss poles only about an inch and kept the focuser draw tube all the way in. I used the eyepiece adapter to obtain focus like a helical focuser. I happy to report this worked exceedingly well. Once again the 30mm and 14mm were exquisite. Pinpoint flat fields to the edge. The 9mm improved a lot with this method as well.
I believe this may be the way it is supposed to be used. Might replace my focuser with a nice Moonlight arrangement next month with a lower profile. I may not have to lower my UTA at all then.
Have spent 4 hours or so with the ES CC.
Scope is Orion Optics UK 10" f 4.8.
Feather Touch 2" focuser with 2.5" draw tube.
Eyepieces: ES 16 20 24 28 and 34mm 68º and XW 10 7.
Target for viewing the Southern Beehive and the Jewel Box.
With no CC the ES 68's start to be a mess around 60% out except the 20mm which is more like 80%.
Build on the CC is excellent, obviously made for 2" 100º eyepieces as there is no 1.25'' adaptor supplied.
With no instructions I followed what I do for using a CC with par focal rings, start with the ES CC screwed all the way out and focused til the centre stars were in focus and screwed the CC to til the outer stars were in focus, which meant the centre wasn't. Refocus and repeat til happy. For the 68's they all appeared pretty good at 20mm out in the ES CC. Nice stars virtually all the way across save the last 5% or less. In my f12 refractor the stars are pinpoint til 98% of the way out. The last bit is field curvature in the eyepiece as I see the same with the TV CC.
The XW's don't need the CC at all. With the CC the view gets worse!
So I'm happy with the ES CC, pinpoint stars 95% out is in my book, transforming a reflector into almost refractor status.
It's a joy to cruise the milky way with the ES 34mm and the ES CC.
Photo attached is the ES CC in the FT focuser….a lot of back focus on this scope! An ES 20mm 100º eyepiece would stick out a long way from the focuser with this arrangement!
Matt
A couple points:
1) The 30mm ES 82 degree DOES have a small amount of vignetting. It's hard to see, though.
2) The 9mm ES 100 is styled after the 8mm Ethos. The 8mm Ethos, if used as a 2" eyepiece, has it's perfect coma correction setting when pulled about 1/2" out a Paracorr. It may be the same with the ES 9. Here is how you would test:
--set the coma corrector for perfect correction on the 14.
--exchange the 14 with the 9mm.
--focus the eyepiece with the top of the coma corrector only--do NOT use the focuser. If you run out of travel, and I'll bet you will, pull the eyepiece out until it is in focus. THAT is the proper setting for the 9mm.
--you may need a 2" barrel extender and a parfocalizing ring to allow you to automatically insert the eyepiece to that point in the future.
Just note that the coma corrector will parfocalize all your eyepieces when it is set correctly. You won't have to touch the focuser knob at all except maybe for the very finest of focusing. I find the actual amount of focuser travel, once the eyepieces are parfocalized by moving the top up and down, is about 1mm tops.
A couple points:
1) The 30mm ES 82 degree DOES have a small amount of vignetting. It's hard to see, though.
2) The 9mm ES 100 is styled after the 8mm Ethos. The 8mm Ethos, if used as a 2" eyepiece, has it's perfect coma correction setting when pulled about 1/2" out a Paracorr. It may be the same with the ES 9. Here is how you would test:
--set the coma corrector for perfect correction on the 14.
--exchange the 14 with the 9mm.
--focus the eyepiece with the top of the coma corrector only--do NOT use the focuser. If you run out of travel, and I'll bet you will, pull the eyepiece out until it is in focus. THAT is the proper setting for the 9mm.
--you may need a 2" barrel extender and a parfocalizing ring to allow you to automatically insert the eyepiece to that point in the future.
Just note that the coma corrector will parfocalize all your eyepieces when it is set correctly. You won't have to touch the focuser knob at all except maybe for the very finest of focusing. I find the actual amount of focuser travel, once the eyepieces are parfocalized by moving the top up and down, is about 1mm tops.
Things I have noted and agree with. The 30mm and 14mm are sensibly parafocal. Only a very slight adjustment is required. Your right about the 30mm with the vignetting. Tiny amount. Its starts at about the outer 10% I would say. The field is improved so much I don't even care about it though.
One problem your hypothesis for the 9mm Don. From the 14mm's position of best focus. The 9mm comes to focus turning the top of the corrector inwards. Not outwards. Or turn top left for in focus and right for out focus.
The top of the HR Coma corrector goes OUT (out travel) with counter-clockwise motion (often called "to the left"), and IN (in travel) with clock-wise motion (often called "to the right").
Are you saying the 14 requires more OUT travel (counter clockwise) to get it to focus than the 9mm does?
Once I get some clear skies, I'll be able to test this myself (except the 9x100 ES hasn't been available for quite a while), but I'm curious what you've found. Because if the 9 and 14 are like the Ethos 8 and 13. the 9 should require a lot of counter-clockwise movement of the HRCC to get it to focus after the 14 is in focus. If it requires clockwise motion to get it to focus, then the 9 and 14 x 100 are constructed differently than the Ethos. Yet, Xrays show them identical in configuration to the equivalent Ethos.
The top of the HR Coma corrector goes OUT (out travel) with counter-clockwise motion (often called "to the left"), and IN (in travel) with clock-wise motion (often called "to the right").
Are you saying the 14 requires more OUT travel (counter clockwise) to get it to focus than the 9mm does?
Once I get some clear skies, I'll be able to test this myself (except the 9x100 ES hasn't been available for quite a while), but I'm curious what you've found. Because if the 9 and 14 are like the Ethos 8 and 13. the 9 should require a lot of counter-clockwise movement of the HRCC to get it to focus after the 14 is in focus. If it requires clockwise motion to get it to focus, then the 9 and 14 x 100 are constructed differently than the Ethos. Yet, Xrays show them identical in configuration to the equivalent Ethos.
Weather permitting. I'll confirm or deny this tonight. I'll take exact measurement on in focus and out focus requirements of all 3 relative to each other.
Edit: Actually now that you mention it. I turned the top LEFT when I moved from the 14mm to the 9mm to get focus. But, it was not much. Couple of millimeters over all. I think.
OK. Another round of testing. Don you might find this interesting.
I started by lowering my truss pole ~35 mm to make sure I have enough in focus. I had the tunable top at the lowest position to start at essentially 0mm. The 30mm ES came into focus at ~9mm-10mm from the lowest setting. The ES 14mm 100 came to best focus at about ~14mm. The 9mm came into best focus at about ~15mm-16mm. Still a bit disappointed with the 9mm if I'm honest. I was expecting a better field. Maybe its because I am comparing it to the 14mm. Which is sensibly a perfect 100 degree field of correction.
I put the 8mm Delos in for a test. That came into focus just at what I estimated to be 1mm. Completely flat a pinpoint as well. But it was a Delos at F/5. What does one expect?
I tried the Pentax 10XW. Couldn't get it to come to focus. It needs more in focus then 8mm Delos. Didn't even try the 14mm Delos as it needs even more in focus then the Pentax.
Don I'd be interested to see your results with the 8mm Ethos if you have one to test. Actually if you have a ES 9mm 120 I would be interested to see that performance as that would not have the same eyepiece design as the 9mm ES 100 right?
Well, the best way to use the HRCC is to optimize one eyepiece, whichever one that is. Play with the settings until you get the star images good up to as close to the field edge as possible. That will be your focuser's setting for all eyepieces.
Then, insert a second eyepiece and focus with the helical top of the HRCC. Don't move the focuser at all. Whatever setting of the HRCC top is required to achieve focus, THAT is the setting for the HRCC with that eyepiece.
And, repeat the above with every eyepiece in your collection. The helical top of the HRCC will make your entire eyepiece collection parfocal. There may be an eyepiece whose focus requires pulling it slightly out of the HRCC. Pull it out of the HRCC until it focuses. This may require an eyepiece barrel extender (like the 8 and 6 Ethos in the Paracorr when they're used as 2" eyepieces). There may be a couple eyepieces that require a more in-most setting of the HRCC than the top allows. For these, you have no choice but to refocus using the focuser. Overall, though, it's unlikely to need more than 2mm of focuser travel to optimize all eyepieces.
if you have the type of focuser that is a 2" focuser and uses a 1.25" adapter for 1.25" eyepieces, that is all you need to know.
If you have the type of focuser where either a 2" adapter or a 1.25" adapter is held in a ring on the top of the focuser drawtube (some Sky-Watchers have this), note that the ring on the focuser has slightly less than a full 2" inside diameter, so the coma corrector bottoms on the ring. This means a lot of in-travel of the focuser is needed to get to focus. If this is your focuser, hone the ring out to 2"+ so the HRCC, when inserted in the 2" adapter, bypasses the ring entirely and sits on the top lip of the 2" adapter. This would gain another 1.5" of in-travel if this is your focuser type.
If this is your focuser, hone the ring out to 2"+ so the HRCC, when inserted in the 2" adapter, bypasses the ring entirely and sits on the top lip of the 2" adapter. This would gain another 1.5" of in-travel if this is your focuser type.
This is my focuser. But, I don't understand what you mean by hone the ring out. Yeah my 2" adapter has that silly sub 2" ring that limits how far I can put the HRCC in.
The black ring that threads onto the top of the focuser, into which the 2" and 1.25" adapters fit, has a 1-15/16" inside diameter that prevents long 2" accessories, like barlows or coma correctors, from passing through until their shoulders rest on the top of the 2" adapter.
If the ring on the top of the focuser is machined out to 2" (and there is enough material to do so) inside diameter, then 2" accessories like the HRCC will insert much further and require much less in-travel of the focuser.
It can be done with sandpaper while you're sitting watching TV. It doesn't take much.
When the 2" accessory like the HRCC passes through the ring on the focuser, you have removed enough material.
This modification, by the way, will have no effect on the fitting of the 2" or 1.25" adapters in the ring on the focuser.
The black ring that threads onto the top of the focuser, into which the 2" and 1.25" adapters fit, has a 1-15/16" inside diameter that prevents long 2" accessories, like barlows or coma correctors, from passing through until their shoulders rest on the top of the 2" adapter.
If the ring on the top of the focuser is machined out to 2" (and there is enough material to do so) inside diameter, then 2" accessories like the HRCC will insert much further and require much less in-travel of the focuser.
It can be done with sandpaper while you're sitting watching TV. It doesn't take much.
When the 2" accessory like the HRCC passes through the ring on the focuser, you have removed enough material.
This modification, by the way, will have no effect on the fitting of the 2" or 1.25" adapters in the ring on the focuser.
I'll take a closer look at this tonight when I get home from work. If I'm honest though. I don't think any solution will involve me with sand paper grinding away at a metal pipe while in the living room in front of the television.
Explore Scientific Coma Corrector - First Light review
This needs more testing and I may add more information to this review when I figure out the best setting.
in my book, transforming a reflector into almost refractor status.
