Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
The Lumicon has a full-aperture lens, so it won't vignette, but adjusting the eyepiece focal plane to lens distance will require the use of spacers, because there is no adjustment there. The Lumicon was designed as a photographic accessory where the distance from the camera's focal plane to the lens is set once and forgotten. Unfortunately, different eyepieces have their focal planes in different spots and some method of adjusting the lens to focal plane distance is needed.
Because of the lens design (it's a Ross), there will be more spherical aberration on axis, though somewhat better coma correction at the edge of the field.
But note: even if not perfect, applying coma correction, whether dialed in or not, to the image in a short focal ratio newtonian will result in better images than without it.
For visual use, nothing beats the Paracorr. But if you're correcting the image for imaging, the Lumicon, Baader, and TeleVue will all work wonders. And all are better than nothing.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
V.A.
sage
Reged: 05/14/05
Posts: 281
Loc: Ontario, Canada
|
|
what image degradation will happen if the 55mm distance between the paracorr lens and the eyepiece field stop is not maintained?, if there are eyepieces with focal planes that will increase or decrease the 55mm distance beyond the 55 +\- 4mm tolerance coma correction won't be optimal, but what about other abberiations, such as spherical abberiation?
--------------------
10" binocular telescope
Oberwerk 7x50 Mariner
tv 24pans , plossl's and 12mm t4s
6.5 mag. dark skies
|
Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
Coma correction will be less, and spherical aberration will be added. However, a less-than-optimum setting still works OK. Nagler's own 31mm Type 5 requires a setting 1/8" farther in than the Paracorr can be set, yet the correction is still superior to the eyepiece by itself. What small amount of spherical aberration that is added is a small price to pay for such a significant amount of coma reduction and some field flattening.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
backwoody
scholastic sledgehammer
Reged: 01/08/07
Posts: 903
Loc: Idaho USA
|
|
I recently purchased a Paracorr for my f4.8 dob, and really appreciate this thread. I'm working to find the best settings with various EPs, with no problem for most of them. But I am having a hard time finding field stop data for some, and thus, can't run the calculations -- although I will likely discover the best Paracorr settings by trial-and-error.
Anyone have personal knowledge of Paracorr settings for Vixen LVW 17mm and Pentax XF 12 and 8.5mm? 
Thanks in advance,
--------------------
woody
a parsec farther out...
12.5" f/4.8 custom truss dob, EQ platform
|
Denimsky
super member
Reged: 01/21/07
Posts: 180
|
|
Thank you Lawrance.
With your spreadsheet, I could calculate the tunable top positions for my eyepieces.
Pentax XW 7: setting 1
Pentax XW 20: setting 1
Speers-Waler 5-8 variable: setting 1
Pentax XL 40: setting 3.
--------------------
DH
Edited by Denimsky (01/27/08 12:06 PM)
|
Pete Laky
journeyman
Reged: 02/11/08
Posts: 8
|
|
Looking for a sanity check...
I worked through the spreadsheet but want to check out the results (might be locked up). I get setting 5 for a TV new model paracorr with the stock 1.25" adaptor and my 14mm Meade 5k UWA?
Took the advice on filters, ended up using my O-III filter laid over the top of the eyepiece...need to acquire some 2" filters...
Thanks in advance...
|
V.A.
sage
Reged: 05/14/05
Posts: 281
Loc: Ontario, Canada
|
|
i was looking at the parrcorr about that issue your having with the 31mmt5 not reaching it's optimal position. there's a ridge on top of the tunable top that the eyepiece rests on, it's about .2" tall, right above the brass ring. it could be machined down by .125" to bring the 31t5 to it's optimal position. this would make the #5 setting to #6 and you would loose the #1 position (or do i have the numbers backwards?) , but it seems like it will work fine and it's a very simple modification.
--------------------
10" binocular telescope
Oberwerk 7x50 Mariner
tv 24pans , plossl's and 12mm t4s
6.5 mag. dark skies
|
Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
Yes, the lip of the tunable top could be machined down.
I have eyepieces that use 1-5 now. I'd have to extend the tunable top's slot to allow more movement up to make that modification, though.
The correction may not be perfect, but it is fine for the low power use in my f/5 scope. Were my dob larger and faster (generating more magnification for the 31 and more coma to correct), position 5 might not be adequate. But, though it is 1/8" away from the perfect position, it certainly works OK for that eyepiece.
I have a machinist friend who might be able to make a tunable top for me with the necessary travel and shape, and I'd love to get rid of the compression ring and have a nice snug fit instead with a 1/4" nylon screw eyepiece tightener.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
V.A.
sage
Reged: 05/14/05
Posts: 281
Loc: Ontario, Canada
|
|
i think i may have overlooked the antireflection threads inside the parrcarr, it seems like the eyepiece barrel might hit the threads before it will go down any further, so the modifications are a little more extensive than i first thought, the antireflection threads would need to be machined down a little as well.
on a diffrient note, i noticed when holding the parrcorr in front of my eyes and looking through it ,the parrcorr seems to have a positive focal lenght, i can actually place an eyepiece in front and it will form an image. how does it extend the focal lenght by 1.15x when it appears like it would act as a focal reducer? there must be some optical principals i'm unware of.
--------------------
10" binocular telescope
Oberwerk 7x50 Mariner
tv 24pans , plossl's and 12mm t4s
6.5 mag. dark skies
|
Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
Well, in the aggregate, it has to perform like a slightly negative lens in order to have a positive magnification effect.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
Mike Foreman
member
Reged: 04/06/07
Posts: 41
Loc: Texas
|
|
Does anyone know the in focus of a Paracorr?
With the 26nT5 I can focus without it but can't focus with it.
I am trying to calculate how much to trim off my truss tubes.
BTW I am using an 18" f:5.
--------------------
TMB80ss; Nexstar 5SE
C8; Meade 10 SCT
18" f:5
|
walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2419
Loc: Doylestown, PA
|
|
An update on using a N12T4 with the barrel extension.
Note: I have an older Paracorr with the tunable top.
Check this procedure using the technique of starting with an EP that has a known setting. Then without moving the focuser, change to the N12T4 and adjust the Paracorr until focused.
When the barrel extension is installed on a N12T4 the extension will bottom out inside the Paracorr and not on the tunable top. This means that the tunable top's setting is meaningless if one just inserts the N12T4 into the Paracorr.
Through experimentation I discovered how to get the proper Paracorr setting on the N12T4 with the barrel extension.
1) The starting setting of the Paracorr is really unimportant but I usually have it at setting three, the mid-setting.
2) Insert the N12T4 into the Paracorr until the barrel extension bottoms in the Paracorr.
3) Tighten the thumb screw to lock the N12T4 to the tunable top.
4) Loosen the Paracorr tunable top lock.
5) Rotate the tunable top to the right (extending the top outward) one setting (or move to the next setting on the left). From setting 3 to setting 2 for example.
6) Lock the tunable top.
The Pararcorr is now set for the N12T4.
--------------------
Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
Edited by walt r (06/13/08 02:45 PM)
|
Mike Foreman
member
Reged: 04/06/07
Posts: 41
Loc: Texas
|
|
Thanks Walt.
So if I am understanding correctly once the Paracorr is set relative to the telescope then it is a matter of getting the eyepiece set to the Paracorr?
--------------------
TMB80ss; Nexstar 5SE
C8; Meade 10 SCT
18" f:5
|
walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2419
Loc: Doylestown, PA
|
|
Thats correct Mike. The Paracorr specs are a set distance from the Paracorr lens to the focal plane in the EP. It is the EP's focal plane to external dimensions that differ from EP to EP.
Translated, the Paracorr lens is to be at a fixed distance inside the original focal plane of the primary mirror.
--------------------
Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
|
Starkler
professor emeritus
Reged: 11/04/05
Posts: 691
Loc: Australia, Melbourne
|
|
Quote:
Thanks Walt.
So if I am understanding correctly once the Paracorr is set relative to the telescope then it is a matter of getting the eyepiece set to the Paracorr?
Once you have the correct position for one eyepiece sorted and focused, you can lock your focuser and focus other eyepieces with the tunable top and it will be correct 
--------------------
Geoff
15" SDM truss dob | Vixen r130sf | GSO 10" dob
|
Astrolabe
member
Reged: 07/08/07
Posts: 82
|
|
Thanks to everyone for the work done in the relation to the Paracorr settings. I recently bought a paracorr but I have no problems with the settings as all my eyepieces are made by Televue.
An observing mate wants to try the Paracorr with his Meade 5000 32mm - 2" Plossl and his 21mm Orion Stratus. I haven't any more information on these eyepieces so I cannot use the spreadsheet. I will be gratefull if someone could tell me the paracorr settings for these two eyepieces.
Thanks in advance and regards
George (astrolabe)
Sydney Australia
|
Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
Quote:
Thanks to everyone for the work done in the relation to the Paracorr settings. I recently bought a paracorr but I have no problems with the settings as all my eyepieces are made by Televue.
An observing mate wants to try the Paracorr with his Meade 5000 32mm - 2" Plossl and his 21mm Orion Stratus. I haven't any more information on these eyepieces so I cannot use the spreadsheet. I will be grateful if someone could tell me the paracorr settings for these two eyepieces.
Thanks in advance and regards
George (astrolabe)
Sydney Australia
It's easy, mate.
Set the Paracorr for one of your TeleVue eyepieces and focus.
Remove your eyepiece and install his. Refocus using the tunable top and look at the setting. That's the setting for that eyepiece. Write it down. REmove the eyepiece and install the other one. Focus using the tunable top. Note the setting. You can do this for a whole box of unknowns.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
Astrolabe
member
Reged: 07/08/07
Posts: 82
|
|
Don
Your opinion and advice is always valued and appreciated, thank you for the prompt reply.
Best regards
George (astrolabe)
|
cyberhedz
member
Reged: 07/29/08
Posts: 93
Loc: 38.16N, 76.28W
|
|
since we have a bunch of paracorr users here, it seems rather fitting to ask, even if only an opinion, I'm getting a 12" dob f/4.9 and was wondering how necessary paracorr was. i do plan to get an antares 1.6 barlow and some hyperions EPs with it and plan to save up for a 2x powermate and an ethos.
--------------------
Arrakis, Dune, homeworld to the greatest treasure in the universe, and the never ending struggle to defend it.
-Celestron 15X70 skymaster binocs-
-Celestron PowerSeeker 127EQ- (trading in for lightbridge 12" this weekend)
-Celestron 2x barlow-
Christmas wishlist:
-Celestron CPC 1100-
-Antares 1.6x barlow-
-Baader UHC filter-
-Baader Skyglow & moon filter-
-lumicon O-III filter-
-DGM Optics NPB filter-
-Complete Hyperion set-
|
Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10957
Loc: Los Angeles
|
|
Varies. I found coma severe enough at f/5 to require a Paracorr. Other observers don't use one at f/4.5.
Here's a question: Do you want the stars at the edge to be as tight as the center? You'll need a Paracorr.
Don't care about the edge? Probably won't need one.
Don't know the answer?
Look first, buy later. There's no shortage of Paracorrs.
--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 