The Siebert ETX 2 inch Eyepiece Adapter: A Wide Angle 2 inch Eyepiece in an ETX?

Click to Enlarge The adapter and 36 mm eyepiece in place – it’s much lighter than it looks!

A few months ago I was looking around for a reasonably decent low power, wide field/finder eyepiece for my newly completed “ultra-light” F4.9 16-inch dob .  One night, a fellow member of my astronomy society loaned me a Siebert 36 mm Observatory series eyepiece with a 70 degree apparent field of view to try and I was impressed by its performance.  It may not have been quite as corrected for a short focal length as a 35 mm Panoptic (though much better than many other budget wide fields) but gave good, sharp images for the majority of the field, had minimal field curvature,  and was very comfortable to use - with 20 mm eye relief, forgiving eye placement and no “kidney-beaning”.  What I really liked, however, was that the 36 mm Siebert was light enough that I did not need to re-balance my scope when changing to smaller eyepieces.  There are other good reviews on the Siebert Observatory series eyepieces on Cloudy Nights and elsewhere, so I won’t go into too much detail here, other than to say I think they are good value for the price.

When looking at the Siebert Optics web site , I was surprised to see that Harry Siebert had designed an adapter so that this 2” series of eyepieces could be fitted to the 1&1/4 inch back of an ETX scope!  Sounded hard to swallow, I know, so I searched in vain to find any reviews, only finding some threads on the Cloudy Nights forum below - containing some heated discussion about whether or not it could work in theory: 

CN thread 1

CN thread 2

As I was going to purchase the eyepiece anyway, I decided to buy the adapter as well and let others know how it performed on my ETX 125, which I now use for my “ultra-portable” and “quick-look-at-home” scope.  The cost of the 36mm Siebert was $199 USD, with a 40mm model also being available that is also compatible with the adapter with the same apparent field of view, eye-relief, weight and price.  There are two types of ETX adapter – one with a built-in helical focuser to eliminate the mirror shift inherent in some of these scopes ($165 USD), and the one I purchased which was a simple adapter with no focuser ($65 USD).  I also received a $20 discount for buying both items at once. 

The adapter appears to be made out of machined aluminium, which obviously helps with the weight and has an “industrial” finish.  The inside of the adapter is in essentially three sections:

• The lowest part, which fits over the outside of the tube where the 1&1/4” eyepieces normally are inserted.  This means that the light path is less constricted than an adapter that fitted inside the tube.
• A conical section that moves outwards towards the 2” inside diameter section.  This part has ridges machined in it, which I guess acts to help baffle internal reflections, and is very dark flat black.
• The upper section, which holds the 2” eyepiece or supplied 1&1/4” adapter.  The lower part of this is also flat black so that 1&1/4” eyepieces aren’t viewing polished aluminum either.
  

Click to Enlarge View of the inside of the adapter

The adapter came without instructions but was fairly easy to fit to the back of the ETX.  First, the set screw that normally fastens the eyepieces in the ETX is removed.  Then an o-ring is placed over the outside of the eyepiece holder of the scope, with the adapter going over the top of the o-ring.  This provides the stability for the lower part of the adapter, with three set screws fastening the upper part and also allowing exact centering on the optical axis.  The adapter is a little clumsy to take on and off quickly so it is more convenient to leave it on permanently and use the provided1&1/4” reducer with smaller eyepieces.  The setup is very snug and stable, with no slop once it is in place.

Click to Enlarge Rear view, showing how the adapter & 36mm eyepiece fits onto the ETX.

Once fitted, any eyepiece will protrude about 2 inches higher than without the adapter.   This might annoy some, but I actually found that this made it easier and more comfortable to look into and also facilitated right eyed viewing (for those who so prefer) because it was well above the finderscope.  The finderscope consequently needs to be rotated to the side (which many have done anyway) and is easier to use with the right eye.   Most users would not want to add too much weight to the back of an ETX because of balance and the possible strain it may place on the drive.  The adapter is very light weighing in at 100 grams and the 36 mm Seibert (275grams) is only slightly heavier than my Meade 24.5 mm Super Wide Angle (210 g).  In fact, the scope is still nose heavy even with the adapter and the 36 mm eyepiece in place, except when pointed straight up.

Now, according to my limited knowledge of optics,  this Maksutov-Cassegrain telescope does not allow a wide enough passage of light to fully illuminate the field of a 70 degree AFOV 36 mm eyepiece without some vignetting of that light.  However, as anyone that has built Newtonians knows, a secondary size can be selected such that only 70% illumination at the edges of the field of view exists, with the viewer usually unable to perceive the decline in brightness from the centre to the edge (though this is different for photographic applications).  The question with this setup is how much vignetting of the light is going to occur and what will the observer see in the eyepiece?  Time for a look:

I first set the telescope up in the daytime looking across the valley from my home.  As with all wide field eyepieces, the less dilated pupil of my eye meant that the eyepiece was more prone to blackout and required careful placement to take in the whole field of view.  It was no more difficult to use than my Meade series 4000 40 mm plossl or 13 mm nagler however.  Looking carefully into the eyepiece, I could see a small crescent-shaped obstruction in the view right at the edge of the lower field stop but it was small - I estimate no more than a few percent of the total field of view.  This was not objectionable to me as, besides its small size, it blended in to the curve of the adjacent field stop.  The rest of the view showed no discernable dimming towards the edges to me at all and was basically identical in quality to my Meade 24.5 mm Super Wide Angle, but with a wider true field of view and lower magnification (53X).

At night, I spent a good deal of  time panning slowly through the star clouds of the Milky Way, open clusters and  globulars around Scorpius and Sagittarius, to see if faint stars dimmed as they moved to the edges of the field of view.  Only the very faintest stars on the edge of visibility disappeared when moved right to the edge of the field of view. In fact, the astigmatism at the edges of the field of view that I saw in my 16” F4.9 dob virtually disappeared at F15 and so gave an even  wider (though of course dimmer) useful view.  The blackout that occurred during the daytime was not a problem at night but for some reason I did seem to need to place my eye a little more carefully than when using this eyepiece in my F4.9 dob.  Generally the ETX did not seem to have any major problems with the extra weight in goto or tracking - but sometimes at zenith the goto placed the object a little below the field of view, presumably because this is at the point which the scope became a little back heavy.  Likewise, the tiny crescent-shaped obstruction that I saw during the day was even less noticeable during nighttime viewing, although when I looked carefully the field of view appeared slightly elliptical.  As stated above, I also found that the increased height of eyepieces allowed more comfortable viewing.  When placing my other eyepieces in the 1&1/4” adapter provided I was able to easily reach focus with all them (Meade 40, 26 & 15 mm plossls; 24.5 mm SWA; 13 mm T6 Nagler) as well as with a Meade Barlow.  I did not notice any increase of internal reflections with these eyepieces as a result of using the 2” adapter on brighter objects like Antares or Jupiter.

The widest true field of view I was previously able to get from my 1&1/4” eyepieces in this scope was about 0.8 of a degree which is possibly the biggest drawback of the ETX’s optical design.  With the 36mm Siebert in place I was able to get 1.3 degrees, which is more than double the area of the sky (a 40mm would give even more – over 1.4 degrees).  I am now better able to frame wide angle views of many brighter extended nebulae such as the Lagoon, Eta Carinae and Orion, as well as moderately large open clusters such as M7, M6, M25, M41 & NGC 2516.  My only slight quibble with the adapter is that, with a UHC filter screwed into the 36 mm eyepiece it would not go quite all the way in and protruded a couple of millimeters out.  With the set screws fastened it was still very stable, however, and 1&1/4” filters were no problem.

Now I know what you’re thinking – can you whack any wide field 2” eyepiece into the adapter and get fantastic, unobstructed views in the little scope?  Well…probably not.  You see, according to Harry it’s also the design of the Observatory series that avoids the apparent vignetting, due to the unusually large size of the elements in the eyepiece:

"In the early stages of developing the adapter a number of eyepiece combinations were tested. I noticed that even 1.25" eyepieces in the 32mm range were completely dark, roughly at the last 10% of field before the field stop. This became even worse with a 40mm losing almost 20%. This is a byproduct not simply of the scope’s ability to illuminate an eyepiece without vignetting, but is a fault of the eyepiece design. When optimizing 2" eyepieces for the ETX series it was noticed that the size of the elements, particularly the upper element dictated how much of the available field would be illuminated. So what this means is that what is achieved with the adapter with the Observatory 36mm and 40mm very well may not be duplicated with certain other eyepieces, particularly if the upper element on those eyepieces are under 40mm.”

Harry Siebert, from Weanser’s Mighty ETX site.

Harry goes on to say that in the ETX adapter the 36mm Observatory eyepiece gives about 95% of its field of view (which corresponds to what I saw), whilst the 40mm eyepiece - with its 50mm  elements - gives 100%.

I did not test any other 2” eyepieces with the adapter for the above reason and also because I did not want to tax my ETX’s drive with heavier eyepieces (e.g. the 35mm Panoptic is 725grams compared to the 36 mm Siebert’s 275 grams), so I will leave that for others to try if they wish - my opinion is that buyers should view the adapter and the Observatory series eyepiece as two halves of the same unit. I also did not see how the combination fared in the other, smaller ETX models but can’t think of a reason why it would not also work in a similar fashion  with these and other small Maks.  If in doubt, I would contact Harry Siebert and ask him – at the very worst you can always ask for a refund if it does not work in your scope.

In summary, I found – somewhat to my surprise - that this product works very well.  It more than doubles the true field of view possible with a 1&1/4” eyepiece, and allows me to more satisfactorily view many more of the objects that one often wishes to see with a small scope.

There is certainly no thick, dark “donut” shape around the edges of the field of view as I initially expected.  Vignetting must be occurring to some extent but, apart from the small obstruction on the edge mentioned, seems to fall well below the threshold that most viewers would notice, let alone find objectionable.   Each person must make up their own mind as to whether it’s worth spending the cash ($250 USD plus) to get these views, but in my case the extra cost of purchasing the adapter to go with the 36 mm eyepiece was trivial and I am very pleased with the result.  The 40 mm eyepiece would have had too large an exit pupil in my F4.9 dob so was not an option for me, but would probably be a better choice for those who wanted to get a maximum true field of view at a higher focal ratio.

I have no association with Harry Siebert other than as a customer.

 Article by Tim Nott (tnott). Discuss this article on the IceInSpace Forum.