View Full Version here: : Paracorr
15-05-2007, 09:12 AM
I used a Paracorr for the first time last night in my 10" f/5 dob. I was expecting a subtle improvement after reading about this device but the improvement was very obvious. The edge of field was cleaned up in both the 22 Nagler and Pentax XW14, to the point where I now have no thoughts of selling the XW14mm. I put the XW in after using the Nagler at Paracorr setting 5 and the view was not good, but changing to setting 1 fixed that.
It's hard to draw conclusions after just one observing session, but the Paracorr looks like it will be staying in my eyepiece collection.
I'm curious to hear what setting others use with the XW14 and Paracorr?
15-05-2007, 12:31 PM
Its interesting in that the paracorr is designed for coma correction, but it does things other than that. The xw14 wouldnt be seeing coma in that scope and if the pracorr makes an improvement it would appear that it could be flattening the field.
15-05-2007, 01:06 PM
I'm not sure of my ability to separate the types of aberration or whether there are other factors that come into play, which is why I stuck to a generic "cleaned up the EOF" description. The EOF in the XW14 did look comatic to me though, so I suspect there's coma correction and field flattening occurring. I used the wishing well cluster as my main test subject as it has a nice spread of medium brightness stars across the field.
15-05-2007, 10:09 PM
Here's a line from the Paracorr page on the TV website (under 'astrophotography' but presumably applies visually as well).
"Edge-of-field sharpness is further enhanced since Paracorr also acts as a field-flattener."
16-05-2007, 07:45 AM
I also found that the paracorr does an excellent job of cleaning up the EOF in both the 14mm and 20mm Pentax XW's using #1 or #2 setting. Setting 1 is better but requires me to collimate the mirror very low to reach focus. The paracorr also cleans up the EOF nicely in both the 12mm and 17mm Nagler T4's. The dominant aberration in both the 14mm and 20mm Pentax XW's is field curvature. While the paracorr is not specifically designed to fix this, it does a remarkable job on it.
The 14mm and 20mm Pentax XW's don't show the same degree of field curvature in a larger scope with a longer focal length but the paracorr still improves them.
16-05-2007, 08:54 AM
I've been waiting for your input John, but I see in your ethos thread you have a good excuse for being a little tardy - you've been off enjoying yourself too much :) . Performace with the Paracorr certainly casts a new light on ownership of the XW14 and 20. Now that I can stop hyperventilating over EOF imperfections I feel I have a near perfect eyepiece set (Paracorr, Nagler 22, XW's 14 &10). It's convenient that the 14 and 22 just happen to use the two extreme Paracorr settings - no need to look at settings in the dark. Brighter nebulosity looks quite stunning through the XW14.
Thanks for your info too Morton.
18-05-2007, 10:50 PM
I have a Q that doesn`t relate to the original post, I`m hoping someone can kindly answer.
Does the Paracorr have a 2inch filter thread on the bottom???
18-05-2007, 11:33 PM
Starlane, just went and had a look for you and the Paracorr does have a 2" filter thread.
19-05-2007, 10:57 AM
Very interested to read your post. Did you have any balance issues using the Parracorr with the 22 mm?
I've been considering a Parracorr for a while now as I'm becoming less and less tolerant of EOF distortions.
19-05-2007, 11:43 AM
Hi Mark, I didn't have any balance problems with the Nagler 22 and Paracorr, but the tube wasn't pointed at lower altitudes either. I never use the altitude springs as I don't like to apply unnecessary friction to the bearing. I keep a stack of 3 or 4 speaker magnets on the eyepiece rack that can be easily applied to the rear of the scope if necessary, so I don't run into any balance problems anyway.
I've only had one chance to use the Paracorr so far, but it's very nice to be able to leave the scope still for longer and see a sharp image as an object moves across the field. I suspect the Paracorr is only worth having if you are using premium eyepieces, with astigmatism overwhelming other aberrations in cheaper eyepieces. As soon as I get a clear night I'll test that theory out with my Andrews 30mm UWA.
19-05-2007, 03:04 PM
very interesting thread. I'm very interested too about your further tests with the Paracorr. I've been toying with the idea of getting one, but haven't made the plunge yet, still wondering if it's the only option, or something else available too (that doesn't magnify the image like a Paracorr?). Maybe the Paracorr is the only game in town?
19-05-2007, 03:42 PM
there are other options, that I haven't seriously investigated, but which are cheaper and don't magnify. My brief impression was that if you have only one eyepiece to correct for, seriously consider one of the other coma correctors. If you want to use a number of eyepieces with corrector, get the Paracorr as it is the only one that allows convenient adjustment of the distance between the eyepiece and the correcting lens assembly. Also the Paracorr corrects the field curvature issue in the XW14&20 which is a big deal if you want to use those eyepieces - I don't know if the others do that.
I don't mind the 15% magnification increase as I was considering swapping my 22mm Nagler for a 17mm - now I have an 18.7mm Nagler when used with the Paracorr. The 22mm also has 2mm more eye relief than the 17mm, so I'm happy how things have worked out.
19-05-2007, 06:24 PM
Although I don't own a Paracorr (yet), I think it is the only option that is convenient to use in the field, i.e. you don't need to unscrew something each time you change eyepieces (as you would with the MPCC, for example). As Tony said, there are four(?) different height settings with the Paracorr, and you need to figure out the optimum position for each eyepiece you own, but this isn't really a big deal.
All things considered, I think a slight magnification increase is aceptable for the improvment in performance. I'm sure if it was possible to make a similar item with no magnification, Al Nagler would have done it!
19-05-2007, 07:59 PM
Thanks for that Tony.
20-05-2007, 08:59 AM
Thanks for all that Tony. Hmmm...now where's that piggy bank?
22-05-2007, 07:43 AM
Thanks everyone for commenting. I probably wouldn't use the Paracorr just for one eyepiece, but possibly for the XW14 XW20 and a couple of others too, all longer FL's. While I find the XW14 fine (very slight curvature), the XW20 performance is great in some scopes but in my smaller newts (FL = 900mm to 1000mm), field curvature is noticeable. In longer FL's it's not a big issue at all for me. From what you say, the Paracorr will noticeably improve things.
Having to disassemble would be a pain, so while I had thought of the MPCC, it looks seriously like the Paracorr is the best option,
22-05-2007, 09:31 AM
Nick , there is the Lumicon2" Photo-Visiual Coma Corrector for $345 at Bintel, and it doesn't magnify. I know a guy in Adelaide who owns an 18" F4.5 and uses one and he says it works really well.
23-05-2007, 11:26 AM
I'll look into the Lumicon too. This is cheaper than the Paracorr I think?
24-05-2007, 02:00 AM
It won't work properly for visual I dont think. It is more an imaging unit. The advantage of the Televue one is that it is adjustable and you use all five of those adjustment positions depending on the eyepiece used. This is why Televue make both a visual and photographic Paracorr.
I have the Televue Visual and it is superb.
27-05-2007, 08:51 PM
I'll give the other options a miss and look at the Paracorr. The flexibility and ease of use as well as optical performance is important. When you're tired and you want to drag the scope out, you want life as easy as possible!!!
Cheers and clear skies,
18-06-2007, 08:00 AM
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.:lol:
18-06-2007, 08:57 AM
Thanks for the usefull info Don. If anyone out there has a Paracorr visual and wants to sell it, feel free to contact me :D
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?
18-06-2007, 09:14 AM
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 ?
18-06-2007, 01:01 PM
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.
The Paracorr is very tempting...
20-06-2007, 07:54 AM
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! :D
03-07-2007, 06:56 PM
My visual Paracorr arrived today. :D
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! :thumbsup:
03-07-2007, 08:43 PM
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.
03-07-2007, 10:15 PM
Great news, Rod.
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.
04-07-2007, 04:58 AM
That's fantastic news, Rod! I can't wait to look through it next time we get together.
04-07-2007, 10:00 AM
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.
04-07-2007, 10:42 AM
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.
04-07-2007, 01:39 PM
G'day there Tony,
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.
04-07-2007, 02:50 PM
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.
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.
04-07-2007, 03:05 PM
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.
04-07-2007, 03:20 PM
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!
Clear Skies all,
04-07-2007, 10:34 PM
Thanks Tony and John,
Were you simply after wider FOV Tony or did your eyes not "match" with the 27?
05-07-2007, 09:11 AM
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.
05-07-2007, 04:31 PM
I'm getting lost in the detail again here but if the mirrors are properly configured, why are they necessary at all?
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.
05-07-2007, 06:10 PM
So is it a defect in the shape of the mirror that is corrected for?
No Colhut it is not a defect, it is just very hard to make an mirror system that is fast and without coma, just think $$$$$$$.
The better the mirror the better the picture but the better the price and even then nobody can get rid of coma unless you individually could polish that all out (don't even know if that is possible).
Think about the light bouncing of the mirror.
In the middle it bounces up almost straight and at the edges it will be on an angle.
So the faster the mirror is the worse it gets.
Please anyone correct me if i'm wrong.
05-07-2007, 07:41 PM
Here is some more detail about coma: http://www.opticalmechanics.com/about_coma.htm
05-07-2007, 10:45 PM
So either use very expensive eyepieces or longer focal ratios - which means smalle FOV so wider (than 2") eyepieces which are prohibitively expensive...
06-07-2007, 07:10 AM
My understanding is that Coma in a Newtonian design is a result of the design. Just like chromatic aberation is a result of a refractor design, the light goes through a lens and you get dispersion of different wavelengths causing different focal lengths for each wavelength. You can correct for it (eg APO) but it is still there. The ParaCorr just helps to correct for the aberation caused by the Newt design, you can alter the design and maybe even get rid of it, but it wouldn't be a Newt, and would cost a lot more. On a photons per dollar basis, a Newt is probably the cheapest you can get!
06-07-2007, 08:03 AM
No its not a defect as such. No optical system regardless of its design or parameters is perfect. Some get very close and the more money you spend the closer they get. For instance even the best Ritchey Chretien Telescopes costing over $100K suffer from field field curvature. This is an aberation inherent in the design. Coma is an aberration inherent in any fast newtonian telescope, regardless of how perfect the mirror is. The worlds' best F4/1200mm focal length mirror will inherently have exactly the same amount of coma as the worlds worst F4/1200mm focal length mirror. What causes it is quite complex mathematically and optically and not worth going into here. Fact is it's there in every newtonian and the steeper the radius of curvature of the mirror the worse it is.
A paracorr is an optical device inserted into the focuser before the eyepiece and it corrects for coma.
26-07-2007, 08:06 AM
Use a label maker to label the settings of the Paracorr 1-5. Then, evaluate which setting of the Paracorr best corrects the edge of the field of each eyepiece. Start with the out-most setting, focus and look at the edge. Move the Paracorr in one setting, refocus, and compare the edge with the last image. There will be one setting that results in the best correction for that eyepiece.
Use your label maker to put a number on the side of the eyepiece for the appropriate Paracorr setting.
Then, when you're in the field, just match Paracorr number with eyepiece number--no memory or chart necessary.
I note that my eyepieces use all 5 of the Paracorr settings.
26-07-2007, 04:17 PM
Well I deliberated for an eternity but finally got a Paracorr for my Newt. I simply wasn't satisfied with what I was seeing.
I can only say it knocked my eyes out with the general improvement in viewing. In particular, open clusters such as NGC3532 take on new life. Clean, sharp stars right across the FOV and more surprisingly, tiny pinpoints of stars becoming apparent where they weren't before, making the view even more pleasing.
Clearly, this is one purchase I'll never regret.
26-07-2007, 06:27 PM
I wish everyone would stop posting "How happy they are with their Paracorr purchase" and related comments...
It makes me envious and will utlimately cause me to buy one too...
Just kidding...they sound like they are a helpful tool to extract the best performance from a reflector...
26-07-2007, 07:53 PM
My observations also, I seemed to be able to see stars that just weren't there if I took it out! Maybe I need to put in on my C11 to fix my collimation problem :mad2: (another thread).
26-07-2007, 10:59 PM
Are there different models of paracorr?
I keep seeing references to paracorrs "with a tunable top", is this a newer model? Better?
26-07-2007, 11:19 PM
I feel the same way about the Paracorr. It's just "glued in" to the focuser. I never want to go back to the "going to lightspeed" view of space my scope had before.
There are two different versions of the Paracorr: a photographic version without a tunable top (which has a smaller diameter lens appropriate for imaging), and a visual/photographic version with a tunable top (needed to allow control of lens to eyepiece distance) and a larger lens (to prevent vignetting with eyepieces having large field stops).
27-07-2007, 08:00 AM
[QUOTE=Don Pensack;239161]I feel the same way about the Paracorr. It's just "glued in" to the focuser. I never want to go back to the "going to lightspeed" view of space my scope had before.
Maybe you mentioned it before and I just forgot but what scope(s) do you use it in?
27-07-2007, 10:55 AM
Nicely put Don!
27-07-2007, 03:44 PM
It's an ugly fact of life that a single optical surface can't give you everything you want. There is no coma with a spherical mirror :thumbsup: , but you get spherical aberration:( , which you can correct with a schmidt camera correcting plate. A parabolic mirror gives a perfect on axis image:thumbsup: , but gets worse as you move away, so you need some correction to get rid of the coma. Same sort of thing with refractors--a single lens is hopeless, a doublet is better and a triplet (apo) is better still.
29-07-2007, 09:03 AM
Well after last nights viewing experience I'm the latest to jump onto the paracorr bandwagon. :D
I have one on loan along with 17 and 22mm naglers on a try before buying basis and in a nutshell, the paracorr cleaned up the EOF in every eyepiece I tried it with. :eyepop:
I have written before that I wasn't that impressed with naglers due to seeing astigmatic seagulls and flares in the outer field, well the paracorr cleans them right up :)
Heard about 14mm Pentax having field curvature? The paracorr nicely flattens the field and now shows sharp stars edge to edge even in my little 650mm focal length newt. :eyepop:
The 24 panoptic in the paracorr? Beautiful :thumbsup:
Am I going to buy it? Definately!
29-07-2007, 10:48 PM
Yeah, there is no turning back! And the better the optics the more important they are I find - how typical is that!
<off topic> Have you done a review of that fine scope of yours yet? i've been otherwise engaged for a while and not noticed.</off topic>
21-08-2007, 07:55 AM
Just to remind myself how valuable the Paracorr is, I decided to observe a couple of hours without it, last night.
I just goes to show how you can get used to the Paracorr and not appreciate the job it does in sharpening the stars up. I have used the Paracorr exclusively for 6 years now, and tend to not appreciate it's worth until I do not use it.
21-08-2007, 12:29 PM
I periodically do the same thing. Many people do not use theirs at high power, so I occasionally remove it when I'm using 5-9mm.
I immediately put it back in.
You might say I've gotten used to pin-point stars.
"Don't it always seem to go, that you don't know what you've got till it's gone"
21-08-2007, 01:51 PM
So, should I save my pennies to buy a Paracorr and use it with my el-cheapo eyepieces, rather than attempt to save several times that amount of money to buy a full suite of excellent (but expensive) eyepieces?
Could I indeed make a silk purse out of a sow's ear?
21-08-2007, 03:09 PM
I would say no, unless the predominate abberations that you see are due to coma or field curvature. A paracorr will not mitigate any other type of abberation.
I should have qualified this by stating that the eyepieces I tested with were televue and pentax. ie not cheapies.
21-08-2007, 03:55 PM
Thanks Geoff - I have my answer - it seemed to good to be true. However, I look forward to being able to check my eyepieces through a paracorr when I find myself set up beside some generous soul who can let me try theirs. :)
21-08-2007, 08:05 PM
Having just got a paracorr what can i say ... it works.
21-08-2007, 11:21 PM
It's often hard to know which is the predominant aberration in inexpensive widefield eyepieces when used in a short f/ratio scope. Coma is certainly there, but astigmatism is often severe. The traditional "seagulls", or "batwings" that people see as star shapes are definitely astigmatism-dominated. Eyepieces that are fully corrected for astigmatism will seemingly benefit more from coma correction, since the only aberration left is likely to be a tiny amount of field curvature, easily accommodated by a slight focusing change.
But some acquaintances who use inexpensive widefields say that coma correction has often made these eyepieces quite usable and tolerable when coma is removed, so perhaps no change of eyepieces would be required.
I had already gone the route of fully-corrected, expensive, eyepieces in order to eliminate other aberrations when I had an f/10 scope. When I converted to an f/4.9 scope, it was obvious coma was the dominating aberration, and a Paracorr fixed that right up.
I have used one inexpensive widefield (Speers-WALER) in my scope since obtaining the Paracorr, and it was very nice, so perhaps my friends are right that the Paracorr should come first.
The Paracorr provides a small amount of field flattening, coma correction, and a very slight amount of astigmatism reduction (due to the 15% barlow factor). Buying one Paracorr will be less expensive than replacing all your eyepieces.
Just some thoughts.
22-08-2007, 09:02 AM
Thanks Don, it sounds like a good investment, may well add some benefit now, and will probably be extremely useful at some stage of my likely upgrades to scope and eyepieces. OK, it's on the shopping/wish list.
22-08-2007, 10:06 AM
For those thinking about buying a paracorr, you will need up to an extra 15mm in-travel in your focuser.
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