I've spent about 2-3 weeks trying to solve a very annoying issue I've had with certain 2" eyepieces not coming into focus using a very short (as In short backfocus distance) 100mm f/6 Orion Astroview Achro and with a TSFLAT2 (I can also confirm the TSFLAT2 does add 6-8mm of backfocus). I've read every thread in existance about this scope and everyone seemed to have had issues from way back than in 2005....The answer was always to cut the scope...But I'm sure many don't know how or simply don't want to do it, I wouldn't do it... I gave in and ordered a 2" William Optics Durabright diagonal because many including Jon Isaacs have measured it to have an optical length of 105mm compared to my 2" Baader Clicklock which has 112mm.
I definitely noticed the difference when I received it yesterday by measuring the distance of the focuser but still many eyepieces like the 25mm ES 100 and TS Optics 100 XWA would not come into focus. I almost gave up until I lucked out and found this thread...Post #39 saved the day....https://www.cloudyni...infocus please
I've been walking around wearing glasses for a while now and I have a bad case of myopia or short-sightedness but I don't like observing with them, I find all my eyepieces show about half the field, so I put my contacts on and everything worked lol. With about 2-4mm of infocus or backfocus to spare. So yea, maybe this post helps someone who gets this scope or another with short backfocus in the future, just get a WO diagonal and put on your glasses or contacts....if you need those.
I'll leave all this info here I wrote down while going down this rabbit hole. Eyepieces that have their field stops way up the flange or shoulder are the most annoying ones by far. The 17mm Nikon HW would still not come into focus because the long barrel does not slide all the way into diagonal....the 14mm version almost did. The eyepieces that need a lot of outfocus are way easier to come into focus....I'm new to all these refractor issues, therefore If I've made any errors in the terminologies you can correct me..
Above the flange (Intfocus)
40mm Pentax XW 0.5mm
40mm WO 72 6.5mm
32mm Celestron Ultima-LX in 2" 8mm
31mm TV Nagler 14mm
30mm 2" Vixen Does not come into focus
30mm Pentax XW 0.5mm
28mm William Optics UWAN 3.5mm
28mm Astro-Tech UWA about 1 mm above the flange
26mm 2” Agena SWA 1 mm above flange
26mm(25.1) Meade MWA 2mm
25mm ES 100 10.2mm
20mm APM XWA 100° 6mm
20mm ES 100 8.9mm
20mm Meade XWA 8mm
20mm Sky Rover XWA 9.1mm
20mm TS-Optics XWA 10mm
17mm Celestron Ultima-LX in 2" 10mm
13mm TV 100° Ethos 7mm
13mm Apm XWA 6mm
13mm Sky Rover XWA 5.8mm
13mm Celestron Ultima-LX in 2" 3mm
12.5mm Nikon HW 1.5mm
10mm TV 100° Ethos 7mm
9mm Sky Rover 2.8mm?
8mm 100° Ethos 18mm
6mm TV 100° Ethos 18mm
5mm Meade XWA 2mm
5mm Celestron Ultima-LX in 2" 10mm
3.7mm TV Ethos 8mm
Below the flange (Outfoucs)
56mm Meade Super Plossl 25mm
50mm Takahashi LE 11.5mm
48mm Brandon 10mm?
42mm Vixen LVW 1.5mm
41mm TV Panoptic 5.8mm
40mm ES 68 1mm
40mm Maxvision 68 0.5mm
40mm TMB Paragon 1mm?
35mm TV Panopic 4.2mm
34mm Explore Scientific 68° 4mm
32mm Q70 4.2mm?
31.5mm Antares 90 3-4mm?
31mm Celestron Luminos 0mm
30mm APM UFF-Sky Rover 6mm
30mm Optik BW 14mm
30mm TMB Paragon 0mm
30mm Explore Scientific 82 3mm?
30mm Maxvision ES 82 0.5mm?
30mm UFF 6mm
27mm Panoptic ?
26mm T5 Nagler 82 0mm
23mm Celestron Axiom LX 1.2mm
22mm T4 Nagler 82 7mm
22mm TV Panoptic 2" 6.2mm
21mm TV 100° Ethos 10mm
17.5mm Baader Morpheus in 2" 21mm?
17mm Nikon HW 16.5mm
17mm TV 100° Ethos 10mm
17mm ES 92 0mm
17mm T4 Nagler 0mm
14mm Baader Morpheus in 2" 19.7mm
14mm Meade XWA 4mm
12mm ES 92 3mm
12.5mm Baader Morpheus in 2" 19.7mm
12mm T4 Nagler 18.5mm
9mm APM XWA 3.6mm?
9mm Baader Morpheus in 2" 19.8mm
9mm ES 120° 2mm
9mm Meade XWA 5mm
8.1-17.9mm Leica ASPH Zoom 16.5mm
8mm Celestron Ultima-LX in 2" 0mm
7mm APM XWA 3.5mm
6.5mm Baader Morpheus in 2" 20.3mm
5mm APM XWA 4mm?
4.5mm Baader Morpheus in 2" 20.3mm
3.5mm APM XWA 3mm
Edited by Procyon, 01 May 2025 - 04:07 PM.
