324 replies to this topic

[Starman1]

The position of the Ethos' focal plane is only 0.02" different than the Type 6 Naglers. That means setting 4 would be correct--the Nagler Type 6s WITH the adapter, and the Ethos WITHOUT an adapter.
I don't believe you could get the Ethos close enough to the Paracorr lens if used with a 1-1/4" adapter.

[sixela]

Can't. In 1.25" mode in a 2"-1.25" adapter, it doesn't insert with the 1.25" barrel completely seated in the adapter, because the 2" barrel contacts the adapter.

When used this way, the focal plane is much too far from a Paracorr lens, even with a zero-length adapter.

Ethos: 2" mode, top fully pulled out.

[Starman1]

It works fine in a TeleVue "High-Hat" 1-1/4" adapter, but, in general, the Ethos should be used as a 2" eyepiece in 2" focusers. And in the Paracorr too.

[Mr. Bill]

Anybody know offhand what the settings for the 14, 10.5 and 7 mm Pentax XLs is?

I say "is" because they are all parfocal.

[sixela]

It works fine in a TeleVue "High-Hat" 1-1/4" adapter,

That doesn't fix the issue either, because the reason it works is because the high-hat adds extra length.

[Mr. Bill]

It works fine in a TeleVue "High-Hat" 1-1/4" adapter, but, in general, the Ethos should be used as a 2" eyepiece in 2" focusers. And in the Paracorr too.

That's what TV's Ethos instruction sheet plainly says....

[Starkler]

Anybody know offhand what the settings for the 14, 10.5 and 7 mm Pentax XLs is?

I say "is" because they are all parfocal.

The Pentaxs appear to perform best with the tunable top set to the highest setting (which requires the max amount of focuser in-travel). I had to cut 15mm from my truss poles to accommodate but its worth it

I find the paracorr works wonderfully as a field flattener, nicely mitigating the often criticised field curvature in the 14 and 21mm pentaxs

A note of caution! A friend handed me an OIII filter to try on my 31nagler. Dont try it as it hits the paracorr lens

[Starman1]

A note of caution! A friend handed me an OIII filter to try on my 31nagler. Dont try it as it hits the paracorr lens

Ouch.
Note: 2" filters should be threaded on the bottom of the Paracorr for safety's (and convenience's) sake.
And don't use 1-1/4" filters on the bottom of the dual 1-1/4"/2" eyepieces when used as 2" eyepieces either. The filter will hit the lens if a lower setting is used.
If you use a Paracorr, it is safer to use all 2" filters and thread them on the bottom of the Paracorr.

[Olivier Biot]

For that reason I'm designing a 2" filter wheel that goes right below the ParaCorr.

[Shawn H]

Lumicon sells thier coma correcter for about 100 euros less than TV's paracorr in France, I'd love to hear if anyone has done a side by side comparison!

Shawn Haggerty Avignon, France

[Starman1]

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.

[V.A.]

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?

[Starman1]

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.

[backwoody]

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,

[Denimsky]

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.

[Pete Laky]

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.]

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.

[Starman1]

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.

[V.A.]

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.

[Starman1]

Well, in the aggregate, it has to perform like a slightly negative lens in order to have a positive magnification effect.

[Mike Foreman]

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.

[walt r]

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.

[Mike Foreman]

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?

[walt r]

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.

[Starkler]

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