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3.5mm eyepiece on an APM 70mm ED-APO BT?

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#1 zerodur


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Posted 18 September 2019 - 01:53 AM

Hi all! My first post.

I'm trying to pick a pair of eyepieces for my new APM 70mm ED-APO binoculars (90 degree; 400mm FL or f/5.7). This will be my first binocular telescope (shipped today; Thanks Mike@OSO!). I'm considering the TV Delos 6 or the Pentax XW 7, but am curious to know if a 5mm or even a 3.5mm would be usable in my bino. I'm interested in bright planetary targets - mostly our moon, but being able to readily separate the ring from Saturn would be nice too. A long FL refractor is probably more suitable for this but I don't have one. I live in the city so no DSOs anyway. It's the 90-degree model so no daytime use. Probably.

I've seen a lot of discussion on eyepiece tradeoffs for telescopes, but found little for small BTs. I'd imagine BTs come with their own challenges (limit on magnification by collimation, no tracking mount, weight and bulk of WF eyepieces...). Reading Pinac's review it seems collimation won't be a problem at 80x, but pushing it to 114x with the 3.5mm brings it too close to that oft-cited "50x/inch" limit.

Telescope Calculator also warns me of the small exit pupil (0.61mm with the 3.5mm, 0.88mm with the 5mm), but without any prior experience with BT I don't have a reference to understand what "image may be dimmed" might look like. I wonder if the smaller exit pupil is still usable given that I view with both eyes (I do have slight astigmatism so do wear glasses), it's an APO triplet, and it's mostly bright targets I'm looking at? Not planning to get another scope soon so I don't have to worry about them being too short for scopes with long focal length further down the road.

I also haven't picked a tripod mount yet (trying to decide between a Benro gimbal head with a cheeseplate or something like a Manfrotto MVH502AH - APM's fork mount is too big for the 70mm), as I don't know if the lack of tracking might become an issue at the 114x magnification. My copy of Stellarium won't accept magnification >100x for binoculars so I can't get a sense of how narrow the 3.5mm view will be (should still give a small margin around the moon judging from the numbers?).

The advice for bino beginners is almost always to trade magnification for a wider and brighter view, but I already have a Monarch HG 8x42 for low-power wide-field "sweeping", and the APM already comes with a pair of UFF 18mm (~22x) so I think I'm well covered in the low-mag area. I got hooked on the 60deg aFOV and the bright 5mm+ exit pupil of the Monarch HG, so now I want to see what a 70+ degree aFOV look like (so no DeLite). I can't quite justify the price of Ethos, and I'm guessing it's probably a waste of the 100deg for planetary viewing at a mere 400mm FL (tell me if I'm wrong). Also got spoiled by the long eye relief of the Monarch HG so I'd like to stick to the XW/Delos for their 20mm ER rather than the Nagler.

At this stage in life I can't carry/store/afford a full set of eyepieces, so I want to get it right the first time. I don't know if anything I have said applies to my BT-eyepiece combo, and what other considerations I might have missed. Any advice would be welcomed.



#2 ThomasM


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Posted 22 September 2019 - 06:31 AM

Clearly  the strength of all binoculars and the APMs are no excpetion are wide fields and low magnification. Since you need two eyepieces per focal length a full set can be quite expensive and even more if you make an unthougtful choice. The 70 mm APM is a very nice instrument but certainly not first choice for planets at high magnification. With 3,5 mm the image gets dim, I once watched Jupiter  and the moon with the APM 70 mm and with Siebert eyepieces - I think starsplitter 3.4 mm - and the image was not convincing me. With a Borg 71 FL binocular  it was slightly better, but not everbodies will like it.

Why not considering the Baader Morpheus line, excellent eyepieces, reasonable price, very good for binoviewing. The 6,5 mm works finde with the AMP 70 mm.


best regards




p.s. many different questions in one post, I suggest to start with the 18 mm UFF before making next steps



Edited by ThomasM, 22 September 2019 - 11:05 AM.

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#3 zerodur


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Posted 01 October 2019 - 11:23 PM

It's been "cloudy/rainy nights" since I got the APM 70mm, until last night when the cloud cleared enough for me to try it out on the planets.


Made a leap of faith and got a pair of 5XW. At 80x the view was definitely much dimmer than what I'm used to in my 8x42, but it was still pretty bright for Jupiter and Saturn. Jupiter was a warm-white disk with 1~2 belts. I have no experience with better instrument, but I'm guessing this is due to a lack of contrast rather than sharpness or resolution. Don't know how that can be improved though (bigger aperture? different magnification? filter? better observation site?).


Last time I saw Saturn not as a point of light was when I was playing with a "department store" reflector when I was a small child. So even though binos are not the best tool for planetary viewing I'm still pretty happy to be able to see Saturn's ring again, and for the first time see the belts on Jupiter.


I don't know how much I could improve on the "wide field low mag" end. The Delos only goes up to 17.3mm; the 20XW is too close to the UFF 18mm. Most other wider options are 2" EPs, and I need to do more reading on what tradeoffs/limits are going wide-angle. I feel it's hard to beat the comfort and ease-of-use of my 8x42 though.


A bit off topic: I was a bit surprised by the size of the XW - after IPD adjustment there is maybe a 1mm gap between the pair of EP (I measured the OD of the 5XW to be 61mm, so I guess my IPD is 62mm). If the OD was any bigger I'd have to return/sell the EPs. Now I'm not sure I could use the other big EPs like the Delos / Ethos. Weight turned out not to be an issue on the fluid video head I got so I decided not to get a gimbal / fork mount.

#4 Allan Wade

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Posted 02 October 2019 - 06:16 AM

Astronomy equipment is like any other tools you use to get your jobs done. The APM binoscopes are great at lower power, widefield, deep sky observing. From my dark site my APM 120’s and 82’s are so enjoyable for that type of astronomy. I never look at the planets with mine, as I have other scopes that are more suitable for that observing.


By using such short focal length eyepieces for high power and small exit pupil you are pushing the bino way beyond what they are intended for and the image quality will suffer. The moon might be a better target because of its large size, you will be able to back the power off and stay above the 1mm exit pupil which will be a pleasing image.


I’m a big fan of the Delos and Ethos but find them not suited to binoscopes for my eyes. My IPD is 65mm and the ergonomics are uncomfortable for me. I use the Docters, and the Morpheus which are great bino eyepieces. I would recommend you look at those. The 9 Morpheus will give you 44x and a 1.6mm exit pupil.

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#5 garret


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Posted 02 October 2019 - 01:42 PM


By using such short focal length eyepieces for high power and small exit pupil you are pushing the bino way beyond what they are intended for and the image quality will suffer.


The 70mm APM binocular telescope is great for low power and very wide field with 1.1/4" eyepieces  (3.3 degree field with the 17.5mm Morpheus!) 

I recommend 9mm, 12/ 12.5mm, 17.5/ 18mm eyepieces for your APM...Morpheus, Delite, Delos... (if possible try before you buy)

#6 zerodur


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Posted 02 October 2019 - 02:27 PM

The binos came with a set of UFF 18mm and I'm pretty happy with their optical quality, so I got the low-end 22x covered. Indeed I love the wide angle view (tfov 8.3°!) of my 8x42, but I don't think 70mm is a big enough step up in terms of aperture to justify another set of eyepieces in similar magnification (say the APM UFF 24mm which yields 16.7x. Would I see significantly more stars when sweeping the milky way?).


It's true I don't have anything in the mid-magnification with 9~13mm, but I also don't know what kind of targets I could use them for. The 18mm seems to frame M31 perfectly (according to calculation; last I checked I could only see its core from my roof), and that's the largest target I know that I want to see in whole (unless there's similar easy targets for backyard observation?). I'm definitely interested in wide-field sweeping of the milky way and globular clusters especially, but I feel any shorter EP from 18mm would just reduce my tfov while granting not much extra power in seeing fainter stars.

Edited by zerodur, 02 October 2019 - 05:56 PM.

#7 nicoledoula



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Posted 02 October 2019 - 06:27 PM

I'd start with a pair of 3.2mm ED/Paradigms for $120 And if they aren't good enough (they will be) spend $400-$600 on a better pair




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Posted 03 October 2019 - 07:46 AM

You're not wrong with the ED Paradigms 3.2 mm, in my APM 82 I split well double double in lyra.
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#9 zerodur


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Posted 05 October 2019 - 07:09 PM

Could not split the double double (into four) with the 5mm last night. I probably need a bigger scope than a 70mm f5.7. But then splitting doubles was not my goal with this instrument, so I will probably get something in the mid mag like garret and Allan suggested instead.

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