Eyepieces and Lenses

The SkyMax-102 AV set includes two Skywatcher eyepieces with focal lengths 25mm and 10mm, which can be used with the supplied diagonal or else directly attached to the telescope's tube (note that this will yield and upside-down image). Before reading my discussion further below about additional eyepieces and lenses, you should bear in mind the following matters:

• Quality of bundled accessories. The SkyMax-102, the Avant mount and the tripod are well worth the cost you pay for the set. The accessories (eyepieces, diagonal and finder) are included as a free bonus, so that you can use the telescope straight away, but do not expect their quality and performance to match the full capabilities of this telescope. Soon you will find it desirable to invest in additional/better accessories.
• Barlow lens. This is a common “first accessory”: inserted between the eyepiece and the telescope (or diagonal), the Barlow increases the amount of possible magnifications for the beginner's inevitably-limited set of eyepieces.
• Focal Reducer. This is normally required for use with a CMOS Camera (see more details below). It can also be attached to the 25mm eyepiece, doubling its effective focal length for wider-angle low-magnification views.
• Maximum Magnification of the telescope. Due to the SkyMax-102 mirror diameter of 102mm, its “usable magnification” is stated in the specifications as 204x, the result of the traditional twice-the-diameter value. However, we will see in our Resolution/Magnification webpage that this value is significantly over-optimistic.
• Magnification of an Eyepiece. The SkyMax-102 has a focal length of 1300 mm. Accordingly, an eyepiece with a focal length of f mm yields a magnification equal to the ratio 1300/f. So for example the 25mm eyepiece yields a magnification of 1300/25=52x (where the “x” stands for “times the human-eye”).
• Eye Relief. This is the minimum distance you have to locate your eye's cornea from the eyepiece in order to view the full circular field of view (FOV). I have not measured it, and will only qualify eye relief in two classes:
  • poor: when in order to see the full circular field of view ...
    • (a) ... one part at a time, you have to turn your head and eye around,
    • (b) ... all at the same time, you need to touch the eyepiece rim with your eyebrow.
           Inevitably you end up smearing the eyepiece lens!
  • good: when in order to see the full circular field of view
    • (a) you do not need to touch the eyepiece,
    • (b) however, you do need to keep your eye very precisely aligned with the eyepiece.
• Apparent Field of View (AFOV). The apparent angle the eye sees from the eyepiece: values go from 50º (typical of low-cost eyepieces) up to 100º . The larger the AFOV, the heavier and costlier the eyepiece.
• Subjectivity. My impressions below are just that: they are simple observations, linked to my personal use of the telescope and to the particular quality of the units I received.

Let us now discuss different eyepieces and lenses for our telescope. Please note that all the units listed below have sockets of 1.25" (32mm) diameter, required by this telescope and its diagonal. The pictures are all reproduced to scale (same 1.25" cylinder diameter). Please also note the following text convention below:

• The Eyepieces I either have in use or recommend, are commented in this normal type.
• The Eyepieces that eventually I decided not to buy or use are commented in this smaller type and their images are lighter.

32mm 52º Eyepiece. Some websites recommend wide-angle focal lengths, 32mm and even 40mm, yielding a very luminous view for large and faint sky objects. However, of the dozen or so deep-sky objects that you may try to find with this telescope, only three (the M8 Lagoon Nebula, the M31 Andromeda Galaxy and the M45 Pleiades Cluster) are large enough to exceed the field-of-view (FOV) of the bundled 25mm 52º eyepiece (see below). More importantly, at present most 40mm eyepieces have an AFOV of about 43º, thus providing the same sky view (FOV) of a 32mm 52º, which provides a larger eye view and is a recommended low-price eyepiece, luminous, easy to focus, excellent for Milky Way observation and, with the limitations already observed for this telecope, for searching a handful of galaxies and star clusters. (Please note that you may see a minimal fuzzy black ring around the image: this is just telling you that you are barely exceeding the FOV this telescope's optics can provide).

25mm 52º Eyepiece. The SkyMax-102 AV set includes the SkyWatcher Super MA, a very-low cost eyepiece worth €19 if purchased separately. Nevertheless, it performs very well with this telescope, with a magnification of 52x, AFOV 52º, far exceeding the size of either the Moon or the Sun. I have verified that other users are also happy with this eyepiece. It is also useful for either deep-sky objects or daylight nature observation, and if needed you can widen its angle even more with a Focal Reducer (see below). My only objection to this eyepiece is that it produces a slight vignetting (image borders darker than the centre): virtually invisible to the eye, it becomes clearly visible in photography via a smartphone. As for Moon observation, the 14mm discussed below is certainly a better choice.

26mm 62º Eyepiece. Both Celestron and Explore Scientific produce eyepieces that are significantly better than the Skywatcher 25mm: with 26mm and 62º AFOV you have a 24% increase in FOV degrees. However, at around €100, weighing around 200g and a bit bulky for this telescope, the use of this eyepiece is only justified if you are going to be systematically searching the skies for large deep-sky objects, certainly not the use this telescope is meant for. Otherwise, you should be absolutely fine with the bundled SkyWatcher 25mm. If you really wish to go for a wider angle than the 25mm 52º, a much more economical alternative, with similar FOV, is the above-mentioned 32mm 52º specification.

 

20mm 62º Eyepiece. A very recent Explore Scientific eyepiece, this is a much better proposition than the 26mm as a replacement for the 25mm SkyWatcher eyepiece. Compared with the former (26mm), it weighs half as much and is accordingly much less bulky. Compared with the latter (25mm), it has more magnification but this is compensated by the wider FOV, so you get the same image with a better view, and an increase in overall image quality, with no vignetting. This is my choice for full-moon and detail into deep-sky objects. (Note however that replacing the 25mm SkyWatcher with this 20mm eyepiece is just a “luxury” option.)

14mm 62º Eyepiece. The bundled 25mm eyepiece is too wide even for the full Moon. You may use a Barlow 2X, but you will get significantly better results—when you can afford it—by getting instead a high-grade medium-range eyepiece. Produced by different makers, 14mm with 62º AFOV can be had for less than €100 and allows for a very comfortable observation of the Sun (with filter!) and the full Moon, with much better enlargement than any of the alternatives we have seen above.

10mm 52º Eyepiece. The SkyMax-102 AV carries the SkyWatcher Super MA (€18 if purchased separately),
and the results are not that good: watching Jupiter and Saturn I saw small but noticeable chromatic aberration, less-than-ideal sharpness and poor eye relief. Upgrading from this eyepiece should be your first priority. I tried a well-reviewed but also low cost 8mm eyepiece made by Gosky, and I was not happy with it either. I suggest to wait till you can afford a really good one such as the 9mm listed below.

9mm 62º Eyepiece. With eyepieces of focal lengths 10mm or lower you are trying to extract from the SkyMax-102 all it can deliver, as per a recent review of this telescope. This is an ideal specification for planets viewing. Looking for a high-quality image, good eye relief and a reasonably price—in the region of the €100—we found recently-launched eyepieces by Celestron and Explore Scientific. With a really good 9mm 62º you get a 144x magnification, slightly larger than the bundled 10mm, with improved image realism, no chromatic aberration and good eye relief, the latter in spite of the increased AFOV.

5.5mm 62º Eyepiece. With a 236x magnification (and similar values with other short-focal eyepieces from 4 to 6mm) you are far exceeding the sharpness the SkyMax-102 can deliver, especially with less than optimal sky conditions. Compared with the 9mm eyepiece, with the 5.5mm your sky object will look significantly larger but also visibly less sharp. Also, with a very small field, the sky object will be moving really fast: you will find it really difficult to keep a planet in sight. An eyepiece like this is a luxury for occasional use, such as if you wish to extract the utmost from this telescope for a small object under ideal sky conditions, with a well-calibrated finderscope, or else for planetary photography with a smartphone.

Zoom low-cost Eyepiece. I have found online opposite views about zoom eyepieces. Some advanced amateurs suggest investing a few hundred in a top-quality one and dispense with having to change eyepieces for different sky objects: economical and practical! However, many have noted that even with top zoom eyepieces you cannot expect the quality you have with similar-grade fixed-focal eyepieces. Obviously it also depends on the personal use of the telescope. Looking for an inexpensive zoom eyepiece I bought the Datyson Deluxe Zoom Telescope Eyepiece 8-24mm, which at just €29 has been reviewed as providing a really good value for money. Well, at least the unit I got certainly does not. The issue was not the expected slightly-less-than sharp image at the tele end, but the unacceptable strong chromatic aberration throughout the focal range: only the centre of the image is of any use. A real waste of time and money.

 

Barlow 2X lens. Barlow lenses come mostly in 2x or 3x, but a 3X will convert the 25mm into a 8.3mm with less sharpness, and this is pointless. As for using an affordable Barlow 2X with the bundled SkyMax-102 eyepieces:

• 25mm eyepiece: a Barlow 2x converts it to 12.5mm, but with some degradation in sharpness.
• 10mm: a Barlow 2x converts it to 5mm, attempting significantly more magnification (260x) than this telescope yields, and with less quality and luminosity than a 5.5mm eyepiece (236x). The sharpness is abysmal in low-luminosity conditions, such as when watching deep-space objects.
• With caps removed, you swap eyepieces in about 7 seconds. Removing an eyepiece, inserting the Barlow and re-inserting the eyepiece takes about 11 seconds. Furthermore, the insertion of the Barlow introduces a significant change in focusing: you have to refocus the telescope and then chase the object again with the finder. If instead of a Barlow you have a set of eyepieces by the same brand and series, you swap them in a breeze with minimal need for refocusing and re-centering of the sky object.

Needless to say, the cost of an affordable Barlow is about €45, while the cost of two reasonably good eyepieces is about €190. Accordingly, the amateur on a tight budget may find the Barlow an useful tool. In the long term however, in my opinion and (limited) experience, I strongly prefer adding to the bundled 25mm the 14mm and 9mm eyepieces as suggested above.

Focal Reducer 0.5X. As we saw above, this can be taken as either halving the main optics focal length from 1300mm to 650mm, or equivalently doubling any eyepiece focal length. Its main use is with a CMOS camera (see our Astrophotography webpage) bringing its field of view (normally equivalent to a 5mm eyepiece) up to 10mm. It can also be used with the 25mm eyepiece (in case you did not get a 32mm eyepiece) to get a wider angle that should in theory be equivalent to 50mm but in practice behaves like a 36mm eyepiece.

A TABLE OF FOCAL LENGTHS AND MAGNIFICATIONS

This table lists all the eyepieces recommended above.
We can see that an ample range of magnifications is well covered.

EYEPIECE AND LENS	MAGNIFICATION
25 mm + Focal Ext. 0.5x (real 0.7x)	36 x
25 mm	52 x
20 mm	65 x
14 mm	93 x
9 mm	144 x
5.5 mm	236 x

 

AVOID EXCESSIVELY HEAVY EYEPIECES AND BARLOW, AND DO NOT TIGHTEN SCREWS TOO MUCH

Quality correlates somehow with price and weight (“There is more stuff inside!”), as shown in the following table of four 1.25" eyepieces of similar short focal length that I tested in July 2018.

EYEPIECE 1.25"	PRICE (Europe approx.)	WEIGHT
SkyWatcher Super MA 10mm	€  18	23 g
Gosky Plössl 8mm	€  22	63 g
Explore Scientific 62º Ar 9mm	€  89	107 g
Vixen LVW Lanthanum 8mm	€290	429 g

Enormous weights are also found among some top-quality Barlow lenses. If you handle any of the above you will soon realise that the 107g of the 9mm Explore Scientific is almost the maximum we would load our small SkyMax-102 telescope and lightweight diagonal with, especially if later for astrophotography you will add hundreds of additional grams with a smartphone and its adapter. So be careful to read the specs before buying any eyepiece or Barlow.

BE CAREFUL WITH SCREWS AND THREADS

Finally, please note that the small screws that tighten in place the eyepieces and Barlows you insert (either directly into the telescope or into a diagonal or else a Barlow) have threads a few millimetres long and are made of a hard metal, but are mostly inserted into a threaded 2mm-long hole in a cylinder of a softer metal. Therefore, when you insert diagonals, Barlows and eyepieces, be careful to tighten the screws only the very minimum needed for them not to fall or vibrate. Otherwise you will find that the threaded hole gets worn in no time and you have to insert inside it a copper wire for a not-too-long-lasting fix. Eventually I decided it was worth having a screw-less diagonal (see the Telescope Accessories webpage).