Astro Hutech 25mm Orthoscopic Eyepiece: Some notes and comments

[John]

I’ve had a few chances now to try out the 25mm model from the Astro Hutech orthoscopic range so I thought I’d post a few thoughts on this eyepiece.

I’ve posted in the past on the others in the Astro Hutech ortho range and how they compare with the Baader Genuine and Classic orthos here:

http://stargazerslou...s-orthoscopics/

The 25mm Astro Hutech Orthoscopic shares the same high build quality and finish with the rest of the range. It’s more or less par focal with the 18mm and 6mm but that point is around 8mm further outwards than the rest of the range, which I find slightly odd. Its a little annoying to have to refocus to that degree when viewing an object at high magnifications although I have to do it with other eyepieces I own so I guess you get used to it!

The eye relief is stated by the manufacturer as a somewhat precise 22.22mm, the field stop diameter 17.48mm and the eye lens diameter is 20mm so, for an orthoscopic, it’s quite a generous window to peer into and enough eye relief even for those who wear glasses when observing. The eyepiece retails at £84, which is a fiver more than the others in the range.

The optical design, to give it its full name, is Abbe Orthoscopic. The apparent field of view is 42 degrees.

The lens coatings seem to have the same brownish / lilac hue (depending on the angle you view them at) as the other Astro Hutechs and indeed the now discontinued Baader Genuine Orthoscopics with their “Phantom Multi-Coatings”. The top of the eyepiece is moderately dished and the eye lens is recessed around 3-4mm below the upper rim of the eyepiece. More on this later.

The interior of the eyepiece seems very well blackened throughout and there is evidence of internal micro-ribbing being used around the lens element housings to eliminate, as far as is possible, all internal light scatter.

The 25mm Astro Hutech ortho is a small, lightweight and compact 25mm eyepiece which will put no strain whatsoever on the focuser and balance of even the smallest scope. I used the eyepiece with my 12” F/5.3 dobsonian and my 4” F/6.5 and 4.7” F/7.5 ED refractors and viewed a variety of summer deep sky objects plus the Moon on a couple of occasions.

The field of view presented by the eyepiece seems flat with stars at the centre of the field of view and towards the edges finding sharp focus at the same point. As the Moon drifts towards the field stop it stays round in shape and does not seem to distort as can be seen in some wider field eyepiece designs.

Light throughput seems very good with fainter deep sky objects showing as strong brightness and contrast as the conditions and the scope would allow but please see my notes below regarding eye placement.

The lunar surface was particularly striking on one night with a remarkable range of tones and tints showing across the mare and highland areas paying witness to where ancient lava flows and major impacts have sculpted and painted the surface.

There is little evidence of light scatter around bright stars and the lunar limb as you would hope from a good quality orthoscopic eyepiece. Varying star colours were quite strikingly defined which was especially noticeable in rich tight knit groupings such as the famous double cluster in Perseus.

So all good so far then. There are however a couple of issues with this eyepiece that I found less satisfactory and these do need a mention I feel.

The eyepiece did seem to show some astigmatism in the outer, say, 10% of the field of view when I used it with my F/5.3 and F/6.5 scopes. Not a huge amount but my example did not seem “sharp to the edge” in these focal ratios and I would expect this trend to be consistent in faster scopes too. I re-checked with the 18mm orthos from the Astro Hutech and Baader GO ranges and these did seem to show stars adjacent to the field stop more as points rather than miniature “seagulls” so this seems to be restricted to, or at least much more obvious with, the 25mm focal length.

The second issue which I came across as soon as I put my eye to the eyepiece was that the positioning of the eye lens is such that the eye must find a position around 15-17mm above the top of the eyepiece to see the full field of view and corresponding to the stated eye relief. With no eyecup to help the eye to find the correct placement and to shield unwanted light I found this made observing more of a chore than it ought to have been. Fainter deep sky objects were harder to pick out initially until you created your own light shade with your hand cupped around the eyepiece top and maintaining the correct “hovering” distance above the eye lens took some concentration, which did distract from the viewing pleasure I have to admit. I guess you could fashion a DIY eyecup from soft rubber sheet but it seems a shame to have to resort to this.

With the 18mm orthos from Astro Hutech, the shorter eye relief and deeper cupping / recessing of the eye lens means that this eye positioning issue does not become intrusive for much of the time.

I feel that the 25mm design really needs a longer body to house the optical elements allowing the eye lens to be more deeply recessed and / or a soft rubber eyecup should be provided for this focal length to help find the correct eye position and to shield out that pesky light. The photo below includes the 18mm Astro Hutech and Baader Genuine orthos with the latter particularly having adopted the recessed eye lens in a longer body approach.

With the older "volcano top" orthoscopics the longer focal lengths would have had this same issue of course but I guess we have got rather "soft" now with features such as adjustable eye cups

The above issues did detract from my enjoyment of the Astro Hutech 25mm orthoscopic somewhat and I have to admit and, rather to my surprise, I found the Celestron X-Cel LX 25mm (also pictured for scale) that I currently have, with it’s twist up soft rubber eye cup, provided a more enjoyable viewing experience overall, especially on deep sky objects, despite its more complex optical design. Not a result that I would have predicted but there you go.

As ever many thanks are due to First Light Optics for the loan of the eyepiece, along with the others in the range

Its great to have the opportunity to try these items out, to explore their quirks, characteristics and foibles, and to report on them here

[majapala]

Thanks again John, for an informative and interesting report.

[nightfisher]

Good write up John, the 25mm ortho is a suprising EP, i have the circle T V/T and find it a lovely EP to use at f10, very sharp and pretty good on smaller DSO`s

[Pig]

Excellent informative review John, I am sure this will be of much use to SGL members

[mdstuart]

I do find the x cel lx eyepiece I have 12mm to be very good on deep sky objects as well.

the eyepiece I have is fairly sharp to the edge even at f5.

the only issue is the cap is too small for it.

mark

[jnc71106]

thanks for the review John. Does the BGO suffer the same eye placement issue ?

[John]

thanks for the review John. Does the BGO suffer the same eye placement issue ?

Not really - the longest focal length Baader GO is 18mm which has 15mm of eye relief and the eye lens is recessed within a cupped eyepiece top. Similarly it's not really much of an issue with the 18mm Astro Hutech.

It took me a bit by surprise with the 25mm to be honest.

[Damo636]

As we have come to expect now John, another excellent informative review :-)

[Naemeth]

The comments about eye placement and especially astigmatism do put me off this eyepiece. Do you think it's anything in the design that makes the eyepiece more astigmatic with longer focal lengths?

[John]

The comments about eye placement and especially astigmatism do put me off this eyepiece. Do you think it's anything in the design that makes the eyepiece more astigmatic with longer focal lengths?

I'm not an optical expert but I don't think the abbe orthoscopic design places edge correction as high on the list as on axis sharpness which is their forte. Nevertheless, I've not noticed much or any in other orthos I've used however most of my ortho observing is done with short focal lengths so I'm not a long ortho aficionado really

I could live with the edge correction of the 25mm really I think but the eye placement was a bigger issue for me.

[alan potts]

Very nice report John, I might just add one of these to my collection next year when I come across, as you I believe said it is a good yard stick to have a optically excellent eyepiece around this focal length. I may well sniff out one of the 32mm monsters as as a test for the 31mm Nagler.

Alan.

[Andrew*]

It's a shame they did not just send it with one of these: http://www.telescopehouse.com/acatalog/Orion_Winged_Rubber_Eyeguard_1_25__.html

I think they're great and bought a set for my Zeiss orthos. Makes them even nicer to use.

Ortho design seems to be stuck in the past a little. Most new eyepieces on the market have twist up eye cups etc. Why not fit one to 18mm+ orthos? Why not use tapered barrels? Or rubber grips?

John, did you find any differences in the quality of the view with the Celestron LX, such as contrast, sharpness or scatter differences?

Great report by the way.

[John]

....John, did you find any differences in the quality of the view with the Celestron LX, such as contrast, sharpness or scatter differences?

Great report by the way.

Thanks Andrew. To be honest I did not see much difference between the 25mm ortho and the X-Cel LXW 25mm in overall quality of view terms although I did not set out to do a detailed head to head between the two. The Celestron eyepiece does show some astigmatism too in the outer 10% or so but, as it's field of view is that much wider than the orthos, the sharp area exceeds that of the ortho. The ease of eye positioning of the X-Cel LX due to it's twist up soft rubber eye cup and the blocking of extraneous light from around the eye lens that the feature gives gave it the edge over the ortho as a deep sky viewing tool I reckon.

[nodsasgoodasawink]

jOHN

Good review. I think I read somewhere that the orthos being sold by Glen Moulton of Lyra are the same as the Astro Hutech. Just google Lrya and go for eyepieces, they are £79 each. Seemingly they are all rebadged from a Japanese designed/manufactured ortho.

[John]

jOHN

Good review. I think I read somewhere that the orthos being sold by Glen Moulton of Lyra are the same as the Astro Hutech. Just google Lrya and go for eyepieces, they are £79 each. Seemingly they are all rebadged from a Japanese designed/manufactured ortho.

Thanks. I agree that the Fujiyama HD orthos that Lyra Optic are selling seem very, very similar to the Astro Hutech's and probably perform the same as well. Some of the Astro Hutechs have a longer body format than the Fujiyama's so reach focus a bit further in than the others.