The 'Little Big' Scope

[Nightspore]

 

The old joke about the voguish Sky-Watcher Evostar 80ED DS Pro is that it is really a high quality Fraunhofer doublet in a relatively inexpensive optical tube assembly. It originally featured an FPL-53 flint and a Schott crown, although I’m not sure if this is continued in more recent models. The OTA is a tad unusual in the fact that it is around a 102mm tube, but with an 80mm aperture. Which essentially makes it a ‘little big’ scope. The rationality behind this design feature is uncertain. Although my guess would be an economic motive enabling Synta to utilise a modified tube from some of their other 102mm production tube models. Some early ‘Orion’ versions had simpler single speed focusers, there is also supposedly a slightly differently housed Vixen version.

 

 

The Evostar is normally sold as a separate OTA or bundled with a 2” diagonal, a 28mm Synta Kellner (mock Pentax type) and a 50mm ‘straight through’ finder.

 

 

A ‘Jason Statham’ hardcase is supplied with the latter version.

 

 

At f/7.5 there are few (if any) chromatic aberration problems. The Evostar is probably not quite ‘fast’ enough for mostly rich field but is altogether a good all-rounder. Light gathering power of the 80mm (3.1”) aperture is about a 131x increase over the naked eye, it resolves to around 1.45 arc seconds and has a pretty decent limiting magnitude of 11.3. I find maximum magnification can be up to 240x (0.33mm exit pupil) in good conditions. Which is a testament to the quality of its glass. I’ve actually occasionally achieved a good 240x observing Mars and the Moon with my ED80. Although 160x-200x is more usual.

 

 

The stock focuser visual back has a 2” adapter and the focuser mechanism itself has a fine focus ability. It does not rotate. My own stock 80ED was actually delivered on the 23rd of November 2018. Although, due to physical disability, I had to rehearse assembling it indoors until I was comfortable enough to actually use it outside. I usually mounted it on my Sky-Watcher AZ5 which was originally bundled with my SW (Synta) 127mm Maksutov. It balanced well on the AZ5 as it still had the stock focuser in place. At f/7.5 I have discovered I can just about get away with my 32mm 2” GSO reversed Kellner (18.75x) with little obvious edge astigmatism. Something I can’t achieve with my faster StarTravel achromats and my f/5.8 72mm Evostar (we’re talking seagull city lol). The GSO Kellner is a lightweight 2” eyepiece with good transmission. I tended to prefer it to the bundled 28mm ‘Sky-Watcher’ EP, although there is nothing particularly amiss with the 28mm. I believe both are examples of reverse Kellners. Sometime in early 2019 I seriously considered purchasing something like the f/7.5 Takahashi FC-76DS. Although I believe the FC-76 is a modified Petzval doublet (as opposed to a Fraunhofer). For a variety of reasons (I’m talking about you slightly dodgy single speed Takahashi visual backs that look like they were designed in the 1980s!) I went with the similar (and cheaper) Evostar 80ED DS Pro. I actually got first light with my 80mm Evostar on the evening of Sunday the 10th of March 2019. IIRC I only got around 20 minutes before the clouds came to spoil my session. On the 8th of May 2019 I added a custom visual back with a compression ring. 

 

 

It wasn’t until June 2020 that I acquired a second MoonLite focuser specifically for the Evostar (the first was utilised on my ST102 StarTravel). From the start, unlike the almost perfect first unit, there were problems with the new focuser. The wrong (trousers lol) finder shoes were included, and I thought the focusing mechanism was slightly stiff, compared to the MoonLite placed on the ST102. Some stuff seemed ‘bent’ on it (I’m guessing it had been dropped at some point). On the 23rd of June 2020 I got first light with the 80ED fitted with the second MoonLite focuser. Although I had a pretty nice view of Mars, with a fair bit of surface detail, there was always something bugging me about the performance of the second MoonLite. I didn’t really get the 80ED out again until September the 13th 2020, after I’d replaced the ‘BorkLite’ with the aftermarket Long Perng (Sky-Watcher) focuser, originally placed on my ST102. The Long Perng had literally sat on a shelf for months. Interestingly I discovered that the now physically longer modified Evostar OTA still fitted into the ‘Jason Statham’ hardcase!

 

 

Unlike the ‘BorkLite’ there were no issues with fitting the Long Perng into the 80ED OTA and the Phillips screws were fine, unlike the soft and problematic MoonLite screws, which eventually needed a pair of pliers just to tighten them properly. In fact, the focuser fitted perfectly (like a glove, and then some) into the Evostar OTA, in a manner the MoonLite could only have dreamed of! I’ve never had any real problems with the Long Perng focuser or its screws. It is actually smoother at focusing than either of the MoonLites (inevitably I returned the faulty MoonLite). Although I do keep an eye on the Long Perng’s grub screws holding the rotating part of the focuser onto the main unit (because separation anxiety lol). The MoonLite’s rotation is a better design and as far as I can deduce doesn’t require grub screws holding a flange onto a V-ring. In this respect the MoonLite resembles the focuser on my 102mm Altair Starwave. However, overall the Long Perng is smoother and easier to focus than the MoonLite and I have learned to live with the grub screw separation anxiety.

 

 

The aftermarket LP focuser does alter the balance dynamics towards the rear of the OTA. It isn’t particularly a problem on the Porta II with the TL-130 and I can use a 2” Tele Vue EverBrite and a 27mm Panoptic pretty easily. Another reason I can more easily get the ‘Little Big scope’ outdoors nowadays is that I’ve swapped the ‘Jason Statham’ hardcase for a lightweight soft case. Accessories are also now carried in a small nylon sports bag. Eyepieces, diagonals, et cetera, are selected for their quality, compactness, size and weight. This in combination with a trolley aids the locomotion of all my equipment down my rather long garden to my ‘observatory’ area. I have removed the two safety screws from the dovetail (mostly absent anyway from my other Synta dovetails).

 

 

These apparently ensure that the dove can’t accidentally slide out of the mount jaws, particularly when the scope is aimed at or near the zenith. Setting-up in this way, without the safety screws, makes it easier and safer for me to simply slide the horizontal optical tube dove into only slightly pre-tightened jaws, virtually ensuring it can’t just slide out. The Porta has an additional safety bolt which I tighten with pliers anyway when in use. So it’s unlikely that if the main screw becomes loose the OTA would simply slide out of the dovetail jaws. Also by transporting the OTA in the soft bag I can keep the dovetail in a position that doesn’t entail rotating the tube/dove position after placing it in the dovetail itself. Something I definitely couldn’t do with Jason Statham lol. You appreciate these little things when you have a physical disability and can only effectively use one hand. 

 

 

All these modifications have now made setting-up/taking-down the 80ED much simpler, easier and safer for me. This will ensure more quality ‘Little Big scope’ stargazing time, and less faffing about with equipment! Which can only be good, right? 

 

Eyepieces used with 80ED (some fields are approximate): 

 

Tele Vue

27mm Panoptic (2”): 22x magnification, 3 arc degrees, 3 arc minutes TFOV, 3.6mm exit pupil
15mm DeLite: 40x, 1 arc degree, 33 arc minutes TFOV, 2mm exit pupil
2x Barlow

 

Takahashi

10mm LE: 60x, 50’ TFOV, 1.3mm exit pupil
7.5mm LE: 80x, 37’ 30” TFOV, 1mm exit pupil
6mm orthoscopic: 100x, 26’ TFOV, 0.8mm exit pupil
5mm LE: 120x, 25’ TFOV, 0.6mm exit pupil
4mm orthoscopic: 150x, 17’ 36” TFOV, 0.53mm exit pupil

 

Guan Sheng Optical

SV42 42mm SuperView: 14x, 4° 30” TFOV, 5.6mm exit pupil

 

 

Lowest magnification with used EP’s = 14.2x, highest practically possible = 240x

 

 

Other equipment for this ensemble:

 

2” Tele Vue Everbrite diagonal
1.25” Tele Vue Enhanced Aluminum diagonal 
Baader 1.25” ‘Zeiss’ Amici prism diagonal (only for lunar observing)
Baader 2”-1.25” adapter
Aomekie LED two colour reflex sight
10cm Omegon flexible vinyl dew tube extension
Vixen Porta II mount with Vixen TL-130 tripod
Custom alt/az knobs
2” Astronomik UHC-E filter

 

Rationale about the choice of equipment:

 

I’m a great fan of Panoptics in general and I’m also firmly in the ‘anything greater than 68° is unnecessary’ camp. As any FOV wider than 65° requires moving your actual eyeball I don’t see the need for a greater field than around 70°. Large 80°or more fields tend to have odd eye relief IME (I’m talking about you Nagler T6’s). I’m of the opinion that the size of the field stop itself is more important. I just don’t see the advantage of a huge field with a limited field stop size. The DeLites need no introduction to those that have used them. The pancake flat 62° field with almost supernatural colour separation and equally impressive contrast is virtually unparalleled. My first DeLite was the 18.2mm. I also have the 3mm and 4mm which I often use with my 102mm Altair Starwave. 

 

 

The finest albedo surface images I’ve ever visually seen on Mars were in 2020 with my 3mm DeLite at 237x and the Starwave. I could actually see the same surface features with the 80ED (240x). Although there was obviously more definition with the 4” scope. My only grievance with the DeLite series is their adjustable eyecup mechanism; obviously not aimed at disabled people! The exceptional 44° Takahashi Abbe orthoscopics also need no introduction. 

 

 

 

The extra 2° is merely to aid target acquisition and will still exhibit a tiny bit of lateral astigmatism. On-axis observation cannot be beaten in my experience. The Takahashi LE series EP’s have steadily evolved over time. Some earlier LE’s apparently had ghosting issues. Mine are fine. I like the LE field stops. The 10mm 50° LE compared to the 9mm 82° Nagler T6 is interesting. I can see all 50° of the LE quite comfortably. Unless I push my eye right into the 9mm Tele Vue and really swivel my eyeball I can only just see the field stop. So, even though the Takahashi FOV is 32° narrower, the LE wins on pure usability ergonomics. The 42mm GSO SuperView (64° AFOV) is easier to use than a 55mm TV Plossl as it doesn’t require an extension tube to focus. Furthermore, the SuperView is comparatively lightweight (368.5g) and the 30mm eye relief is fairly comfortable for me. It enables some nebulae to be observed with a UHC filter.

 

 

Diagonal choices were mainly dictated by design and compatibility. The 2” Everbrite plays nice with the 2” visual back and is well designed and manufactured. I tend to use the 1.25” ‘Enhanced Aluminum’ because I’ve owned a couple for many years and find them robust and practical. Plus there is a distinct possibility they don’t display any real light scatter with bright planetary targets, compared to many dielectric mirrors. I honestly can’t tell any difference between the 'aluminum' (aluminium) and the dielectric ‘1/10th wave’ regardless of all the Tele Vue advertising spiel. 

 

 

The monocoque design is particularly useful as the nose won’t unthread if I rotate the diagonal in the Baader adapter rather than rotating the entire focuser. Which is easier for me. I do tend to rotate the entire focuser, but only with the 2” accessories. I originally considered Baader and Takahashi diagonals, but the TV’s won! I prefer a mirror at f/7.5 in case of in-focus issues. The 1.25” Takahashi prism diagonal is excellent, well priced, and one of my favourite diagonals, but I find some shortcomings with the housing quality and the twist lock mechanism. 

 

 

The often maligned Astronomik Ultra High Contrast Economy (UHC-E) filter has excellent transmission and also passes some of the Swan bands. The filter passes between 465nm to 530nm and above 645nm, which includes OIII, H-beta & H-alpha. Making the UHC-E useful for comet observation. Many dismiss these sorts of filters as not being narrowband enough. However, used visually, and in combination with apertures of 80mm or below they can be quite effective. Exit pupil is a big factor, of course, and needs to be at least 4mm, preferably wider, with the f/7.5 80mm Evostar.

 

 

All I need now is less clouds!