24mm Explore Scientific 68° Series

[Nightspore]

Almost exactly a year ago I bought the 1.25”, 68° Explore Scientific 24mm eyepiece. This was intended specifically as a rich field eyepiece for my 72ED DS Pro Evostar. It gives a 17.5x magnification for a near 4 arc degrees of field and about a 4mm exit pupil. Which is enough exit pupil to use a narrowband UHC or narrowband line filter for a variety of deep sky objects with a small refractor. Ideally I can get between a 3 ~ 7mm exit pupil on the 72mm doublet depending on conditions. A 4mm exit pupil with a large field can give a satisfying wide view yet still reveal a fair amount of detail. The argon purging, utilised on a great many ES eyepieces, initially attracted me as I’d noticed that my other ES eyepieces handled dewing well. This is only really useful later in the year, but can make a considerable difference to the duration of dew soaked autumnal sessions. 

 

 

Interestingly, I have owned a 24mm Tele Vue Panoptic for many years, but finally curiosity got the better of me and I decided to buy the similar (and cheaper) 24mm ES equivalent. The first thing that I noticed was the near one hundred gram weight difference between the two eyepieces. 

 

 

The ES being by far the heavier at 329g. There are supposedly six lenses in four groups although I don’t know how or if they differ from the Panoptic design. I’m not normally a fan of long eye relief and my 19mm Panoptic at 13mm feels just about perfect. The 24mm ES has a purported 18mm. Although, to me, it doesn’t really feel hugely different to the 24mm Panoptic’s 15mm eye relief. I believe Explore Scientific often overstate the actual relief measurements. I find the 24mm ES quite accommodating and comfortable.

 

 

Both 24mm eyepieces have large field stops for a 1.25” barrel. With the ES fractionally wider at 27.2mm. The other difference that I noticed was the ES has a near 5mm wider eye lens. I make the lens 23mm and for some reason the entire viewing field seems greater and more commodious than the Panoptic. Eye positioning is also quite comfortable with the Explore Scientific eyepiece. There are no blackouts, kidney beaning, or anything that impedes viewing ease. 

 

 

Using a narrowband OIII filter I can usually see the Veil and North America Nebulae more clearly than without a filter. In fact, almost anything that emits the 496nm and 501nm ionised oxygen lines is pretty easily seen in the ES 24mm with a narrowband filter at 17.5x. Most filters thread easily into the barrel. My Astronomik and Celestron OIII filters are fine and go in smoothly, as does an Explore Scientific broadband OIII (as you’d expect). Although an Orion UltraBlock and my old Lumicon OIII won’t thread completely into it. No surprises there then.

 

 

The True Field Of View when combined with the 72ED, which I calculate at 3 arc degrees, 53 arc minutes (almost 7 Full Moons!), is ample for rich field viewing. Oddly, the bigger eye lens seems to intensify or expand the actual field making the overall feel of the eyepiece seem larger than I expected. Well, it does for me anyway. Unless the fact that the entire housing is larger than the Panoptic equivalent contributes to this effect. As a consequence the 24mm ES gets out a fair bit more than the 24mm TV, at least with the 72ED. Admittedly the overall background is a little darker viewing through the Panoptic and it is better edge corrected. The ES has excellent transmission though and is just as sharp virtually across the entire field, and with the same quality colour separation as far as I can tell. The smaller 19mm Panoptic is probably my favourite eyepiece of all time as its ergonomics and performance seem perfect to me. However, I never felt the same about its larger sibling and I really prefer the ergonomics of the 24mm Explore Scientific over the 24mm Panoptic. It just feels good to use.

 

 

I also much prefer it to Baader Hyperion Aspherics converted to 1.25” barrels even though I can get wider exit pupils with them. I like the Aspherics with 2" barrels as they sit lower in a 2" diagonal. At 329g and relatively compact in overall design, unlike the Hyperions, the ES 24mm balances well both in the 1.25" diagonal and on the AZ5 mount. A year later and I'm still pretty impressed with the ES 24mm. The past twelve months it's been out regularly with the 72ED DS Pro.

Edited October 21, 2021 by Nightspore