The Paracorr is adjustable for the proper distance between the field stop of the eyepiece and the lens in the corrector. This distance determines optimum coma correction.
The Lumicon or Baader are simpler lenses, and are not adjustable, though a judicious use of spacer rings could work OK. They were originally designed for photographic coma correction--a less stringent application than visual use (note that TeleVue completely redesigned the Paracorr when it became obvious more people were using it visually than photographically).
I thought I would be bothered by the 15% increase in focal length and its concomitant loss of field of view, but it turned out to be an "on-paper" issue more than an "in-field" issue. It improved the images so much, I completely forgot about its magnification factor, as will you.
The coma-free field of an f/5 scope id .022mm x f/ratio cubed = 2.75mm.
The field stop of my 5mm Nagler is 7mm, and I can see coma when I don't use the Paracorr. The 14XW has a field stop of 17.6mm, so certainly coma would be visible and obvious in that eyepiece. With a Paracorr, the coma-free field at f/5 expands to 16.5mm, so a trace of coma might still be noticeable on bigger eyepieces, but not the 14.
Hope the info helps.
P.S. label the setting stops on the Paracorr 1-5 with a label maker. Put a small label on the side of each eyepiece with its appropriate setting. You'll never have to remember the proper settings again. As for picking the right setting, start all the way out, focus, and check the edge-of-field images. Dial it in one setting and refocus, comparing the edge. And so on. Whichever setting works best on the edge-of-field images, that's the one you make a label for.
My eyepieces use every setting from 1-5, so the labels on the eyepieces are nearly essential, because I have CRS.
Don
Thanks for the usefull info Don. If anyone out there has a Paracorr visual and wants to sell it, feel free to contact me
A question for John. You mentioned earlier in this thread you had to move your mirror to get focus? Hope I understand correctly, but I understood your comment to mean you didn't have enough out travel. Is that correct and is that what the Paracorr does?
A question for John. You mentioned earlier in this thread you had to move your mirror to get focus? Hope I understand correctly, but I understood your comment to mean you didn't have enough out travel. Is that correct and is that what the Paracorr does?
CS
Nick
Hi Nick,
I confused myself in that previous post.
In fact I have to collimate the scope with the mirror higher in its cell, not lower, due to insufficient focuser IN travel when using the Paracorr on setting #1 with both the 14mm and 20mm Pentax XW's, which is where they work best. As you adjust the paracorr towards the lowest numbers it requires more in travel. Unfortunately, you can't compensate for this with an extension tube like you can when you have insufficient out travel. It doesn't need the mirror pushed up too far and you should be fine. There are fixes anyway, if it won't reach focus with some of your eyepieces. What focuser do you have ?
Ah, OK, that makes sense now. I presently have a range of typical rack and pinion types and also a couple of Chinese crayfords. I'm looking seriously at one really good focuser at the moment to finish one of my scopes, a 12.5" F6 Newt planet gobbler, and have looked at the Clement Focussers (EXPENSIVE!!!), and also possibly Feathertouch or similar. I don't know anyone who has used or seen a Clement though. Would be interested to know if anyone has? Seems to me with the Clement, you don't have to worry about a focusser tube getting into the light path if you position it close in.
Infocus travel shouldn't be an issue as I built the scope and can re-arrange things to some degree to accomodate. Had this problem some time ago with Binoviewing already, so the Pararcorr shouldn't be an issue with this previous experience.
Well, continuing on from the previous post, I went ahead and scored a Paracorr visual with tunable top off Astromart just now. Ended costing me AUD$340 landed here, and is (apparantly) in excellent condition, hardly used, 6 months old. Couldn't resist after what you fellows said (thanks John!).
Once I get it, I'll try and give you my impressions on how well it works. Will be primarily using it with Pentax 20XW and 14XW. Will also try it with the 30XW, although I must admit, this is excellent already. I'm hoping it will clean up most of the remaining coma in the 20XW anyway. Can't wait!
I have just squeezed in 20 minutes under the stars before dinner, and getting kids to bed.
Initial impression? UNBELIEVABLE! It has totally sharpened up the view through my 22 Panoptic, which was suffering from quite significant coma. Although the seeing is only average and the mirror is yet to cool, the views of Trumpler 14 and 16 , in the Eta Carinae complex, are the best I have seen through that eyepiece. Omega Cent has many more stars which were beyond visibility without the Paracorr. NGC 3532 is full of sharp bright stars across the entire FOV.
I suspect the Paracorr will never leave the Moonlite focuser again. I am an instant convert. I am so pleased to have bought this item. Quite simply, it is superb with my f/5 mirror.
I have just squeezed in 20 minutes under the stars before dinner, and getting kids to bed.
Initial impression? UNBELIEVABLE! It has totally sharpened up the view through my 22 Panoptic, which was suffering from quite significant coma. Although the seeing is only average and the mirror is yet to cool, the views of Trumpler 14 and 16 , in the Eta Carinae complex, are the best I have seen through that eyepiece. Omega Cent has many more stars which were beyond visibility without the Paracorr. NGC 3532 is full of sharp bright stars across the entire FOV.
I suspect the Paracorr will never leave the Moonlite focuser again. I am an instant convert. I am so pleased to have bought this item. Quite simply, it is superb with my f/5 mirror.
You gotta get one!
I am glad that you got one and are enjoying it...I've heard many people say that faster than f/5 they are a must have but that even out to f/6 they show visble improvement. Someday I will have one too.
I've probably posted this diagram by Al Nagler before but I'll post it again for anyone who wants to brush up again on the improvements this optical set gives.
The graph plots the blur spot diameter ( in microns ) on the vertical axis against the distance from the centre of the focal plane on the horizontal axis
The yellow bar at the bottom of each graph is the Airy Disc radius ( below which you would call the performance diffraction limited ).
In the case of the 22 Panoptic at the focal plane of an F5 mirror which probably has a field stop of 25mm or 12.5mm radius, the blur spot due to coma is reduced from 25 micron to below 5 micron, essentially diffraction limited across the whole field! Note also that the vertical scale is different on some graphs.
For F4 mirrors the performance is even more remarkeable, reducing coma to below the diffraction limit over most of the field. With an F4 mirror, star images are 3 times as tight at the edge of a 25mm field , than an F5 'scope without the Paracor, and blur size only twice the Airy disc diameter The improvements on an F6 mirror are also significant.
I've probably posted this diagram by Al Nagler before but I'll post it again for anyone who wants to brush up again on the improvements this optical set gives.
The graph plots the blur spot diameter ( in microns ) on the vertical axis against the distance from the centre of the focal plane on the horizontal axis
The yellow bar at the bottom of each graph is the Airy Disc radius ( below which you would call the performance diffraction limited ).
In the case of the 22 Panoptic at the focal plane of an F5 mirror which probably has a field stop of 25mm or 12.5mm radius, the blur spot due to coma is reduced from 25 micron to below 5 micron, essentially diffraction limited across the whole field! Note also that the vertical scale is different on some graphs.
For F4 mirrors the performance is even more remarkeable, reducing coma to below the diffraction limit over most of the field. With an F4 mirror, star images are 3 times as tight at the edge of a 25mm field , than an F5 'scope without the Paracor, and blur size only twice the Airy disc diameter The improvements on an F6 mirror are also significant.
That is the first time that I've seen that diagram (at least paid attention to it) and am glad that you posted it (or posted it again)...That is great infomration.
I added a used 35 Pan to the collection recently and as expected the Paracorr gave a substantial improvement in the views. The Paracorr also brings the exit pupil down to 6mm and the 35 Pan uses one of the extreme settings which is convenient. I find the eye relief perfect on this eyepiece too.
I have the same three types of telescopes as you; what a coincidence!
When first using my 27 Pano the damn thing wouldn't focus - ran out of outfocus. Finally got it to work after recollimating with the mirror screwed in a bit. Only a couple of millimetres of outfocus left though.
What I'm wondering is; does the Paracorr move your focus point at all? Would hate to get one and find that the very EP I primarily bought it for couldn't achieve focus.
I have the same three types of telescopes as you; what a coincidence!
[SIZE=2]What I'm wondering is; does the Paracorr move your focus point at all? Would hate to get one and find that the very EP I primarily bought it for couldn't achieve focus.
Mark.[/QUOTE]
Hi Mark,
Yes it does and by a good way in some cases. It generally moves the focal point inwards, or at least it does with the eyepieces I own, Consequently, if your problem at the moment is insufficient out travel you shouldn't have a problem introducing a paracorr.
For insufficient out travel you can easily compensate by using an extension tube or pulling the eyepiece out a little and using a rubber o-ring or parfocalising ring as a stop on its barrell.
Hi Mark,
the GSO crayford on my scope has 2 thumbscrews; if the screw closest to the ota is wound out a few turns I get another several millimetres of out focus, which was enough to bring a 27 Pan to focus.
I haven't checked to see if the Paracorr changes the focus point, but I have no problem getting focus with/without the Paracorr using these eyepieces: XW14, Nagler 22, Panoptic 35. I sold my 27 Pan before buying the Paracorr.
I like all of my scopes, but I have had thoughts of trading the LX90 for a larger fork mounted go-to. I like the simplicity of the dob but I tend to look at stuff that's easy to find; If the object is out of the milky way and not near bright stars I have a bit of trouble.
Hi Rod, really glad to hear your experience and positive comments. Congrats.
I received mine about a week ago. Came from the US and is in "as new" condition, so really happy and relieved, BUT, the weather has been lousy, so haven't had a chance to try that bad boy yet! From everyones comments, can't wait....but looks like I'll have to for at least a few more days!
The 27 Pan was just a bit close to my 22 Nagler. The 27mm is a very nice eyepiece, a little easier with eye position than the 35mm and considerably lighter. I don't like to have expensive gear sitting around unused and the $ from a sale seem to get quickly reabsorbed with new purchases. A previous or pending sale helps to justify new purchases to the wife too.
Colhut, even if the mirrors are perfectly aligned, in a fast system (F5 and faster) you get abbarations (coma etc).
This is why a paracorr is used so the whole field of view in an eyepiece is pinpoint stars, not just the first 1/3 in the middle of the picture.
Colhut, even if the mirrors are perfectly aligned, in a fast system (F5 and faster) you get abbarations (coma etc).
This is why a paracorr is used so the whole field of view in an eyepiece is pinpoint stars, not just the first 1/3 in the middle of the picture.
So is it a defect in the shape of the mirror that is corrected for?
The improvements on an F6 mirror are also significant.
