how much back focus an EP needs

[Stonius]

Hey there;

How do you tell how much back focus an EP needs before you buy it? I don't think I've ever seen that spec published anywhere. I have some EP's (GSO Superviews, for example) that will focus on my 8" f'6, but not on the 16" f/4.5. Not a big deal since I got the Pentax's, but curious as to what's going on here and what to be aware of in future.

I've heard of this with people trying attach a camera to a scope designed for visual, but never with eyepieces.

Cheers

Markus

[Kunama]

Televue have a spec chart which gives the relative back focus requirement of their current EPs.
As for the other brands, the only chart I have seen was made up by Tamiji Homma on Cloudy Nights some time ago. He tested and gave relative focus positions for several EPs from many different makers with a Pentax Xw20 as the '0' point. The range was quite large. Nagler31T5, 22T4, Ethos21 and 13? Needed the most back focus. Whereas the TV55 was way out the other way.

[janoskiss]

It's usually not possible to figure out from the usual published specs of an EP, unfortunately. I can't give any better advice than what Matt already said.

[Stonius]

Thanks, I'll check out that thread.

Seems amazing they don't publish this info. It's entirely possible you could but an eyepiece and find it doesn't work, through no fault of your own, or the seller's.

Does this happen often?

[Kunama]

Some light reading for anyone interested in this topic..... Enjoy!!!

Thanks to Brandon (bdawg6381 on CN)

http://www.cloudynights.com/index.php?app=core&module=attach&section=attach&attach_id=282845

also much kudos and thanks to Mr Tamiji Homma for his permission to post this here:

Quoted from Tammy's great post on CN

Here is what I measured with Vixen ED80SF and Baader 2" MaxBright diagonal against several eyepiece focus distance when I focused to a target about 1 kilometer away, to focus to infinity, less than about 1mm inward distance required. The first column is distance between end of OTA and diagonal. The second column is relative distance, negative value shows need inward focus distance, positive value shows outward focus distance.

14.47 -16.94 Hyperion 31 1.25" mode
19.38 -12.03 LE 50
22.65 -8.76 Ethos 17
24.00 -7.41 Nagler 22T4
24.37 -7.04 Nagler 22T4
25.93 -5.48 Ethos 8 1.25" mode
26.09 -5.32 TMB SM 7
26.17 -5.24 Hyperion 24
28.62 -2.79 LE 30
29.57 -1.84 Paragon 40
29.58 -1.83 LVW 42
30.24 -1.17 Denk 14
30.37 -1.04 XW 40
30.88 -0.53 NAV-SW 17.5
31.02 -0.39 Paragon 30
31.09 -0.32 AII-16
31.14 -0.27 Hyperion 31
31.25 -0.16 AII-6
31.32 -0.09 LE 12.5
31.38 -0.03 XO 5
31.41 0.00 AII-4
31.41 0.00 XW 20
31.46 0.05 XW 30
31.53 0.12 AII-10
31.59 0.18 LE 7.5
31.61 0.20 NAV-SW 7
31.61 0.20 XW 30
31.61 0.20 XW 5
31.62 0.21 XW 10
31.65 0.24 XW 3.5
31.89 0.48 LE 5
33.52 2.11 Denk 21
34.09 2.68 UWAN 16
34.52 3.11 UWAN 28
35.75 4.34 TV PL 32
36.70 5.29 Panoptic 24
36.92 5.51 Panoptic 19
37.53 6.12 TV PL 15
37.72 6.31 TV PL 11
40.04 8.63 Zeiss 25
47.64 16.23 Hyperion 24 2" mode
51.15 19.74 Ethos 8 2" mode

note) unit is in millimeter.

PS: I used very low (less than 1mm) profile 1.25" -> 2" adapter to measure 1.25" format eyepiece.

Tammy

[Stonius]

Thanks Matt;

That's useful info to have. The ones in my collection that bottomed out on the focusser were a couple of GSO/Bintel superviews. It doesn't bother me because I have since bought Pentax XWs, but it suprises me since they are a commonly recommended beginner's EP, and if they require a lot of backfocus it could be a problem.

Interestingly I have two GSO 30mm - one rebadged as Bintel and an original GSO - both have different backfocuses which would imply a difference in design even though they're supposedly the same EP.

I'll do some measurements myself when I get the chance and post some results to add to the data.

Cheers

Markus

[janoskiss]

With cheap EPs you can expect quirks like this. The photographic versions of the Superviews should be more versatile. But perhaps your scope is just a stretch out of or near the limit of its factory specs. That said, if a GSO EP did not work in a GSO scope I'd take it back.

But first note that you have some room to move with the three primary mirror collimation screws. One of them is redundant (for collimation), so you can move the mirror up and down the tube a few mm by tightening/loosening the screws. That often solved problems of EPs not quite reaching focus for me.