The Zoom Eyepiece

The “stock” eyepieces that came with my 8″ 200P Dobsonian were great for me to get started. A new owner should definitely get used to them before considering any “upgrade”.

For me, the stock 25mm produced good results but the 10mm was dim and I was disappointed with the clarity of what I was seeing.

I did consider going down the conventional path of getting a few, mid range (like BST Starguiders) or better eyepieces in the 4.5, 6, 8, 12, 15, 20mm range but in the end, I took a different path and chose a zoom eyepiece.

A Zoom eyepiece sounds like the perfect solution: a wide range of focal lengths available in a single ocular, to be changed as simply as twisting the barrel. Zoom eyepieces are of course a compromise. Budget zooms tend to have narrower fields of view at the long-focal-length end, they can be heavy, their many glass elements result in internal reflections and loss of contrast and brightness, and of course, they’re more expensive than a single mid-range eyepiece.

“With my 200P I have a 50X to 338X magnification range”

There are several advantages to a zoom. There is no need to swap out eyepieces in order to find the ideal focal length of the object being observed. You will have everything from 24 to 8mm with the twist of a hand. For a non-tracked telescope convenience and speed are useful features. At the short-focal-length end, they actually tend to have longer eye relief and wider fields of view than Plossls, Kellners, and Orthoscopics. With the addition of a low power, wide-angle eyepiece and a Barlow having just three oculars can be very convenient. With the 200P I have a 50X to 338X magnification range and no wasted glass.

With all optics, you get what you pay for. If you buy a very cheap zoom eyepiece, don’t expect it to perform anything like as well as a single eyepiece. I did consider a good budget zoom eyepiece like the Celestron 8-24mm or the Svbony SV 135. At twice the cost, the Baader Hyperion IV Zoom and Barlow combo is not a budget option but considering this is an all-in-one option you may be saving money in the long run. With bright, sharp, crisp images, a “widefield” AFOV ~70 @ 8mm, 2” mode with camera threads, Barlow friendly, comfortable eye relief with no blackouts or beaning it gets great reviews. 

I cannot praise the Baader Hyperion IV Zoom and Barlow combination highly enough. It has made a massive difference to my observing in terms of what I can see and the convenience of my observing. The only other oculars I have are a pair of 2″ StellaLyra/Revelation/ GSO wide angle. These are ideal for targeting and observing wide-field objects like the Pleiades and Andromeda nebula.
With the 2″ supplied adapter fitted, I now no longer use 1.25″ eyepieces. 

Recommendation

If you can afford the Baader Mk IV and matching Barlow it’s a great combination with a 2” 30mm wide angle eyepiece. However given the rave reviews on the astro forums, the Svbony SV135 combined with a SV215 planetary zoom would be an excellent alternative at a very good price.

Fitting the Barlow

Note that with the Barlow and supplied 2” sleeve, in order to achieve focus, the eyepiece needs to be withdrawn about 5mm.

Without the Barlow the focus tube sits a long way in the OTA.  I have purchased an additional shorter 2” extender which I use as follows:

No Barlow: Short 35mm 2” extender 

Barlow: Original 47mm 2” extender and short 35mm 2” extender.

You can fit the Barlow directly to the eyepiece shaft (Barlow turns as zoom changes) or to the 1.25” sleeve (Barlow does not rotate as zoom changes). I use the latter method with both sleeves fitted.

Specifications

Baader Hyperion IV Zoom with 1200mm focal length reflector, 203mm diameter mirror, focal ratio 5.91. Maximum recommended magnification x406
EP Focal Length (with 2.25x Barlow)	Magnification	Apparent Field of View	Eye Relief	Field Stop	True Field of View	Exit Pupil
8mm (3.6mm)	150x (338x)	68°	19mm	9.5mm	0.45°	1.5mm
12mm (5.3mm)	100x (225x)	63°	18.2mm	13.2mm	0.63°	2mm
16mm (7.1mm)	75x (169x)	58°	17.4mm	16.2mm	0.77°	3mm
20mm (8.9mm)	60x (135x)	53°	16.7mm	18.5mm	0.88°	3mm
24mm (10.7mm)	50x (113x)	48°	16mm	20.1mm	0.96°	4mm
Revelation GSO 2″ Wide Angle
30mm	40x	68°	22mm	37mm	1.7°	5mm
42mm	29x	58°	30mm	43mm	2°	7mm

Summary

It may seem strange to spend half a much on an eyepiece as I did on my telescope but I am so glad I did just that.
Having said all this, I totally get why a few good fixed eyepieces could be the best option for many. There are plenty of recommendations on r/telescopes, Star Gazers Lounge and Cloudy Nights.

Focal Length of Eyepiece – The distance between the lens and point of focus (focal plane). 

Magnification – How much bigger an object will appear (power).

Apparent Field of View – How much of the sky the eyepiece can see before it is fitted to the telescope

True Field of View – How much of the sky the eyepiece can see when in the telescope 

Field Stop – The aperture (opening) that limits the field of view of the eyepiece.

Exit Pupil – The size of the image in the eyepiece. It should match the size of your pupil at night. For a 30-year old this is typically  7mm, reducing by 1mm per decade, so for a 60-year old, it will be 4mm. All figures are averages

Eye Relief – How far your eye should be from the eyepiece to see all the field of view