324 replies to this topic

[Lawrence Sayre]

OK guys, I really hate to do this to you, but I just changed my spreadsheet once again to fix a glitch, so if you downloaded a copy of my "final" version prior to my 8:39 PM today revision time as listed in my above post with the attached spreadsheet, please go to that same post above and download it from there once again.

One interesting thing I discovered is that in its final (I hope) form, I can now definitively conclude that the spreadsheet/chart output which TeleVue has posted on its website is in fact for the much older version of the Visual Paracorr which came with the much lower profile 1.25" to 2" adapter (which is 0.16" high). Therefore in its current form, the TeleVue chart on their website is not usable for 1.25" eyepieces in conjunction with the higher profile adapter models (which I believe may have came about when the compression ring was added to the adapter). This makes sense, as the chart has been around for a good while, and the compression ring adapter is the new kid on the block so to speak.

[walt r]

Don,
The Spreadsheet I posted does show the optimum setting even if its negative or larger than 5.
walt

[Starman1]

Lawrence,
I have a question about your spreadsheet:
On the TeleVue site, TeleVue shows the setting for the 1-1/4" Type 6 Naglers as setting 4. It also shows that the field stop for each of these eyepieces is 0.25" below (toward the field lens end) the eyepiece "shoulder" (where the inserted eyepiece sits).
Do I enter this as a positve (as TV shows on their site) in your calculator, or as a negative?

[Starman1]

Don,
The Spreadsheet I posted does show the optimum setting even if its negative or larger than 5.
walt

Walt,
Your calculator showed the proper setting for the Type 6 Naglers as 4.11 with the new adapter. My empirical test done with the 13 showed setting 4 as the best setting.
So there's a general agreement between your formula and what I see at the eyepiece for these 1-1/4" Naglers.

Lawrence's calculator showed setting 2 or 1 (depending on how the field stop distance from the shoulder is input), which didn't match with what I see.

Your formula is easy to use for an eyepiece whose field stop position can be determined. But many new eyepieces don't have such information available. The old empirical method would seem to be the only choice in those cases.

However, on the 2" Naglers, the results from your calculator are diffierent than TeleVue's claims, but I understand why. For example, I get a setting of greater than 6 for my 31 Nagler, which makes sense, I guess. The field stop of that eyepiece is 0.38" ABOVE the shoulder, which means the field stop-to-lens distance is 58.65mm in setting 5. Since the optimum is 55mm, the 31 Nagler should go 3.65mm further into the barrel to achieve the optimum setting (i.e.an imaginary setting 6). Setting 5 is just the best the Paracorr can do for that eyepiece.

I like the calculator. It works.

[Starman1]

One more point about Walt's calculator:
The divisor should be 0.125 instead of 0.12
It defines the difference between each setting in inches, and I measured 0.125" between settings, not 0.12".
The results are the same, so I'm being persnickety.

[walt r]

Don,
I forgot that you got the 0.125 measurement for the new style ParaCorr. I have the old style and measured a difference of 0.48 inch from setting 1 to 5. That's 0.12 inch per setting.

My best guess is that there are some dimensional differences between the old and new style ParaCorrs beside the diffence in the 1.25 to 2" ep adapter.

walt

[Lawrence Sayre]

Lawrence,
I have a question about your spreadsheet:
On the TeleVue site, TeleVue shows the setting for the 1-1/4" Type 6 Naglers as setting 4. It also shows that the field stop for each of these eyepieces is 0.25" below (toward the field lens end) the eyepiece "shoulder" (where the inserted eyepiece sits).
Do I enter this as a positve (as TV shows on their site) in your calculator, or as a negative?

Don,

Below is positive, and above is negative. This is the same as TeleVue's convention.

[Starman1]

Lawrence,
Then something isn't working in your calculator.
Example:
55mm (2.165") is the optimum field stop-to-lens distance on the Paracorr.

The depth of the Paracorr is 49mm (1.929") and the height of the 1-1/4" adapter is .379". That would be 2.308" at setting 5 (all the way in). A setting of 4 (+0.125") would yield 2.433".

The Type 6 Naglers have a field stop 0.25" into the 1-1/4" lower tube, so I subtract that from 2.433" to yield 2.183", only 0.018" from the perfect location.

TeleVue's website says position 4 is optimum for the Type 6 Naglers.

Your calculator derives a setting of 2 for those eyepieces. Walt's derives a setting of 4, like TeleVue's recommendation.

[Lawrence Sayre]

Starman,

The distances are what they are, and all of this is really quite simple (being nothing but straightforward distance addition and subtraction). I've already explained that the chart on the TeleVue website dates to the older pre-compression ring Paracorr with the low profile adapter. For this adapter it gives the proper setting of position 4 for all of the 1.25" eyepieces with the field stop 0.25" below the focuser shoulder. This is a natural outcome of my finally having done it right.

The conclusion which I have reached is that for the newer model Paracorr and adapter the optimal 1.25" TeleVue eyepiece setting is in fact setting 2 (for all of their 0.25" below the shoulder FP position 1.25" eyepieces). The reason for this is straight forward. Add 1/4" to the height of the adapter, and the FP of the 1.25" eyepiece is thereby moved 1/4" farther away from the 55mm optimal. To counter this, the Paracorr must be screwed in 2 full notches to make it 1/4" shorter. Of course none of the 2" eyepieces are affected by any of this.

[Starman1]

Lawrence,
If you read my post, you will see I used the dimension from the newest adapter, with the .379" thickness to show that the Type 6 eyepieces optimally use setting 4 of the Paracorr, which is what TeleVue suggests. My Paracorr's instructions even have a cross-section of the Paracorr pictured, showing the new adapter.
Simple arithmetic shows setting 4 is correct for these eyepieces with the new adapter.
My empirical test also showed setting 4 produced the best correction.
Hence, there is an error somewhere in the calculator if it suggests setting 2 is optimum.

By the way, if the newer adapter is thicker than the older adapter, that would raise the eyepiece away from the lens, necessitating a closer setting of the Paracorr with the thicker adapter. If the thinner adapter used setting 4, the new adapter would require a setting 6 in order to get close enough to the lens. So your contention that the older, thinner adapter used setting 4 and the newer thicker one setting 2 is illogical.
Remember, setting 1 is the farthest from the lens.

Rereading your post, I see we are talking about the same direction, and that you have merely reversed the order of the settings. 5 is closest, and 1 is farthest, not the other way around.

[Lawrence Sayre]

Starman, is not 5 the highest (longest) setting, and 1 the lowest (shortest) setting, or am I looking at the Paracorr completely backwards? If the eyepiece is lifted 1/4" "more" away from the Paracorr by the new adapter, then to erase this 1/4" increase in the distance between their respective focal planes the Paracorr itself must contract in length by 1/4".

Obviously the TeleVue website chart of Paracorr settings can only be correct for one of the two possible adapters. Does anyone recall this chart suddenly changing at some point?

[Lawrence Sayre]

I sold mine awhile back. Will anyone who currently owns a Paracorr with a tunable top tell me which numeral (setting) corresponds to the shortest overall length of the Paracorr, and which corresponds to the greatest overall length?

[Starman1]

Lawrence,
5 is the lowest setting, with the tunable top screwed in to make the Paracorr shortest.
1 is the highest setting, with the tunable top screwed out to make the Paracorr longest.
Since the screw doesn't move, the leftmost slot position is 1 and the righthand slot position is 5. The slot tilts uphill toward the right, allowing the tunable top to come in (down) as a higher number is dialed.
Don

[Starman1]

Or, another way:
5-- 0"
4-- +1/8"
3-- +1/4"
2-- +3/8"
1-- +1/2"

[Lawrence Sayre]

Don, somehow I don't intuitively get that from looking at this photo:

Paracorr with tunable top

And how can anyone explain that the spreadsheet gives correct output for 2" eyepieces, while not working for 1.25" eyepieces? The laws of optics are the same for both, so the math which works for one must work for the other, no?

[astrodon]

Lawrence-
In the photo of the Paracorr with the tunable top you can see that setting 5 is the lowest, for in order to reach that setting the tunable top with the slot in it must descend in order for the set screw to reach that level.
Hope I helped to clear this up, I know it is hard to get just by looking at the image,
~Don

[Lawrence Sayre]

OK, I finally realize that my intuitive view of the 5 being higher in the picture than the 1 was all wet, and I now trust that 5 is the lowest and 1 is the highest setting. Obviously the slide lever has to go up for the tunable top to go down, and visa versa. Thank you Don and Don!!! With that to go on, I've re-worked my spreadsheet once again, and I'm attaching it right here. I'm also now willing to believe that the TeleVue website chart is using the taller adapter for its 1.25" eyepieces. Please kick the tires with this spreadsheet, and let me know if it gives correct tunable top settings across the board.

Attached Files

•  1768777-Paracorr Setting Calculator.xls   47KB   630 downloads

[Starman1]

Lawrence,
It worked for my 31 Nagler and my Type 6 Naglers.
It didn't work for the 17 Nagler, with a field stop to shoulder dimension of -0.03"
The spreadsheet erroneously gives a setting of 5 instead of 3.

[Starman1]

Lawrence,
I accidentally included the adapter height for a 2" eyepiece.
Doh!
Your calculator works fine.
Don

[Lawrence Sayre]

Thanks!!!

Whew, you had me worried there for a bit. I was beginning to wonder if I would ever get it right. A sanity tester for sure.

[ubernator]

for the 30mm 1rpd, setting 3, works suprisingly well. When I later aquired the real deal 30mm widescan III, suprisingly, it wasnt as well correctable with the paracorr, possibly because it has a wider field stop. I kept both eyepieces. Since I also aquired a nice f10 OTA, I use the WSIII with it, and my most recently aquired 31mm nagler with the dob, with or without the paracorr. I have the older photo/visual model, and the vignetting bothers me, so I will likely add the visual model in the future, and keep the other for my 8"f4 for imaging.

my 2c in the 8.8 UWA discussion, I used it in the lowest setting (5) with the included 1.25 adapter in 1.25 mode, like suggested for the 9mm nagler and it worked very well. I dont think I tried it with the 14mm yet, which I aquired more recently.

[Starman1]

I went back and looked for my records:
14mm UWA (used as 2")--setting 1
8.8mm UWA (used as 2")--setting 5

The 31 Nagler would be better corrected at setting 6. The fact there is no setting 6 shows the Paracorr was designed long before the 31 Nagler. I find it "tolerable" with setting 5, however.

[Zooball]

Thanks Don,

You just saved me a lot of work. I have both of those EPs and a brand new Paracorr PCV-2000 still snug in the box waiting for dark skies.

[sixela]

I woke up early this morning and I've been attempting to make a spreadsheet to calculate the ideal Paracorr tunable top setting based solely upon the field stop location of the eyepiece, and although in its present state it appears to give correct answers for all of the 2" TeleVue eyepieces, it gives incorrect answers for all of the 1.25" TeleVue eyepieces.

Probably means you haven't accounted for the extra 9.6mm caused by the Paracorr 2"-1.25" adapter.

Edit: never mind - late to the party...

[bkushner]

What about Ethos?