Focuser travel requirements test results

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Focuser travel requirements test results

Post by starfinder »

I've this weekend tested the amounts of focus travel required in a refractor, between eyepieces, diagonals, binoviewer etc.

I used a William Optics 80mm 10th anniversary Zenithstar doublet refractor, which is binoviewer friendly. Daytime target was used, a building about 5km away.

Common eyepieces used were Plossls, Televue Delos, Naglar, Delite, Pentax XW, Explore Scientific 68° and 100°, and Opticstar 70°.

>>> Conclusions:
1. General eyepieces focus travel range:
With the same diagonal used (a 2" WO mirror diagonal), the focus travel range of eyepieces is 1.3cm between all 1.25" and 2" common eyepieces tested (excluding orthoscopic eyepieces, which require on average 0.9cm more inward travel than other eyepieces).

2. 1.25" diagonal vs 2" diagonal (both mirror):
- difference is 1.3cm additional backfocus required for 1.25" diagonal, vs 2".
- or 1.3cm additional inward travel for 2" diagonal, vs 1.25"

3. Barlows:
- 2x Ultima Barlow requires 1.0cm more inward focus travel.
- 3x Antares apo barlow requires 0.3cm (3mm) more outward focus travel

4a) Difference between 2" & 1.25" eyepieces:
-- 2" eyepieces: range between them is 0.8cm.
-- 1.25" eyepieces: range between them is 0.6cm (excluding orthos).
-- 2" eyepieces generally do not particularly require more or less travel than 1.25" eyepieces (assuming both types used with a 2" mirror diagonal), with any differences due to brand rather than size.

4b) Difference between brands:
- Televue 1.25" (Plossl, Delite, Delos) and 2" (Naglar) eyepieces are nearly parfocal: 0.2cm difference between them only.
- ES 2" eyepieces require average 0.7cm more outward travel than Televue.
-- Pentax XWs require 0.6cm more inward travel than Televue Delos.
- Orthoscopic eyepieces tested require about 0.9cm more inward travel than Televue eyepieces.

5. Difference without spectacles (myopia): 0.05cm (half mm) more outward travel, i.e. v slight.

6. Difference with 1.25" light polution filter (Lumicon UHC 1.25"): 0.05cm (half mm) more outward travel, i.e. v slight.

7. WO binoviewer without barlow requires 10.2cm more inward focuser travel. With stock 1.6x barlow the binoviewer requires 4.6cm more in-focus travel (than without Binoviewer).

8. Prism diagonal (generic 1.25"): required 2.8cm more backfocus than mirror diagonal

9. Nearer object (building vs building, 4km difference) required 0.3cm more back focus than further object. Conversely, further objects require more inward travel than nearer objects.
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Re: Focuser travel requirements test results

Post by Airconvent »

Wow Gavin, this looks like an academic report :)
Thanks for sharing. Thankfully all my current eyepieces can focus within its normal travel [smilie=angel.gif]
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