Going from Strength to Strength: the 130mm f/5 Newtonian.

Plotina: queen of grab ‘n’ go ‘scopes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I’ve spoken quite extensively on the considerable virtues of my modified 130mm (5.1- inch) f/5 Newtonian with a 27 per cent obstruction. In contrast to the prognostications of fake theorists and arm-chair amateur astronomers, it has proven to be a fantastic all-round ‘scope, easily trouncing smaller grab ‘n’ go refractors and equivalent sized Maksutovs costing significantly more. It provides very pleasing wide field views at low magnification and excellent performance at high magnifications, especially on a suite of double stars, the Moon and bright planets.

As a double star enthusiast, I have managed to split systems down to one second of arc with this telescope during the winter months but in theory it ought to do a little better. Specifically, the classic Dawes limit for this aperture is given by 4.57/5.1= 0.89″ but is confined to pairs which are reasonably matched in terms of brightness. In this capacity, I was curious to see whether I could resolve the very challenging Lambda Cygni, with a current separation of 0.92″, the components of which differing only by 0.4 stellar magnitudes(5.4 and 5.8).

Taking full advantage of the hot, settled spell (Scorchio lol!) that is currently being enjoyed by residents of the British Isles, I eagerly sought out a system I’ve visited many times before, but mainly with my larger instruments (8″ f/6 and 12″ f/5 Newtonians). I speak of course, of that easily accessible system riding high in my summer sky after midnight during June evenings; Lambda Cygni.

This magnitude 4.5 system is easy to track down, even in twilight, and in preprartion, I have been monitoring an easier system; Pi Aquilae, located much lower down in the east south east at this time. My reasoning was simple; if this lowly system was well presented at high powers in the 130mm instrument, there would be a decent chance that Lambda Cygni would also bear fruit. But that proved to be easier said than done. Over several nights, both this week and last week, I have carefully studied the system but invariably recorded strong elongation. Yet in double star observing, as in so many other arenas of human enquiry, it pays to persevere; and finally in the wee small hours of June 27 2018, I won my prize!

At half past midnight, I noted an exceptionally stable and well resolved Pi Aquilae at powers of 260x and 406x and excitedly turned my telescope on Lambda hoping for a better result. And at 2 minutes past 1am local time, it yielded. Letting the system drift through the field several times over a period of a few minutes I could make out two distinct Airy disks intermittently separated by a delicate sliver of dark sky using a power of 406 diameters!

To put this in context, I have previously just resolved this system using a fine 127mm f/12 achromatic refractor at very high magnifications. And though my recollections show that it was that little bit better at ferreting out the pair, it was always very challenging and only possible under similarly clement conditions. So a most satisfactory result, but not at all violating the rules of classical optics. If something is possible, it will happen; you just have to be there to experience it!

I made a drawing of what I observed at the eyepiece( shown below);

Shumbody stop me!

I’ve been thinking about the eyepieces I wish to bring on our summer vacation and decided to treat myself to an upgrade to my trusty 32mm Plossl. To that end, I gravitated toward the Explore Scientific 24mm 68 degree ocular, which would provide a power of 27x in a 2.5 degree field (the maximum possible for a 1.25″ focuser). So I ordered one up for the princely sum of £142 plus shipping. I had very high expectations about this eyepiece judging by the excellent performance of two other 68 degree Maxvision oculars ((34mm and 40mm also marketed by Explore Scientific) I have already field tested. Both of these provide very highly corrected fields across a very expansive field and provide excellent eye relief for maximum viewing comfort. The eyepiece was ordered on Monday June 25 and it arrived in the early evening of June 27.

The Explore Scientific 24mm 68 degree ocular.

As soon as it was getting dark, I fielded the 130mm f/5 Newtonian once again in order to test the new eyepiece out. Centring Vega in the field, I focused the image and to my chagrin, I noted pretty harsh field curvature and coma in the outer part of the field. When Vega was sharply focused at the edge of the field, the stars in the periphery of the field were quite badly out of focus. This was patently not the result I expected for such a pricey eyepiece. What is more, the eye relief was pretty poor too. I had to bring my eyeglasses right up to the field lens, with the ruuber eyecup pushed down, to try to take in the entire field but it was barely possible and far from comfortable. “As tight as a knat’s chuff” is an expression that immediately sprang to mind lol!

When I compared it to my 32mm, the eye relief was far more comfortable (being about 0.73 times the focal length in millimetres, so ~ 23mm) and though the magnification was slightly lower (20x) the Plossl proved significantly better corrected at the edge of the field!

Was I disappointed? You bet I was! I don’t know if I received a lemon or not but on the morning of June 28 2018, I phoned the dealer and explained that the eyepiece did not perform as advertised and that the 32mm Plossl I tested alongside it delivered better performance. Thankfully, they gave me the go ahead to send the eyepiece back so they could test the unit out. I expect a full refund in due course.

What a palaver!

Anyway, while thinking about my next move on the low power eyepiece front, I intend to bring my trusty 32mm Plossl on my vacation, together with my old Mark III 8-24mm Baader Hyperion zoom. This will provide all the medium power viewing I will likely do, and the 2.25x Baader Barlow will enable me to increase the power to 183x; good enough to study quite a few double stars under dark, Hibernian skies.

Travelling light ( from left to right); my 32mm SkyWatcher Plossl, 8-24mm Baader Hyperion zoom and dedicated 2.25x shorty Barlow.

 

After several more nights of observation with the 130mm f/5 Newtonian, I managed yet another sighting of the companion to Lambda Cygni. Specifically, on June 30 at 23:20 UT, using the same power of 406x, the stars were seen cleanly separated on and off during several minutes of observations. With the fine weather continuing for UK observers, I would warmly encourage others with instruments of 5 inches and over to have a go at this system at high powers. It’s very accessible with an 8″ f/6 Newtonian (confirmed once more around local midnight on July 1/2)

The Explore Scientific eyepiece arrived safely back at the dealers this afternoon (July 2 2018). I am now considering the Celestron 25mm X-Cel LX, which is purported to work well in f/5 optical systems and with a 60 degree AFOV should give nice, expansive views, well over 2 degrees in extent.

 

July 4th 2018: A Very Happy Independence Day to all my viewers in the Colonies!

We leave for Ireland on Monday next, July 9. I’m very excited about sampling the skies of my youth with my 130mm f/5. Of course, I’ve had other telescopes over there, back when my folks were still in the land of the living; a 90mm f/10 achromat (which is still at my sister’s home), and when the ETX 90 was all the rage in the late 1990s, I astounded my late father with its go-to capability and almost magical ability to centre and track down the planet Jupiter in the field of view one chilly Christmas Eve.

Plotina being readied for another night of sky gazing.

 

Bringing a Newtonian is a big change for me. It was always a small refractor or Mak that made it, but this time ’round, I can think of no better telescope to enjoy my vacation with. My wife got a bit of shock when I told her I’d be lugging the 130 in its case, but she has since come round to the idea of having me put all my astro junk in one neat place lol..

I’m relatvely new to Newtonians you see. I pretty much overlooked them, owing to the rise of refractor mania and catadioptrc telescopes in the last few decades. I did have one in my youth however; my second telescope, a Tasco 114mm (4.5-inch), f = 900mm or some such that came in a big yellow box decorated with fabulous photos of planets and deep sky objects. Back then though, I knew next to nothing about the rigours of fine collimation, or how to precisely align its equatorial mount(non-motorised). I bought it second hand from a jeweller that lived near me. It cost me £100; an enormous sum of money for a young teenager in the early 1980s. I eventually sold it on to raise some funds for University a few years later.

Still, the 130mm is far superior in many ways to that old Tasco; greater aperture, light gathering power and sharper optics owing to its nicely figured parabolic mirror. No fooling around with complex mounts either; just stick it on the Vixen Porta II and I’m off to the races! Eyepieces have improved vastly as well; the Tasco came with two cheap Huygenians and a junk Barlow lens.

Mars’ fiery red mien graces my horizon after midnight and Jupiter is quite a bit past opposition, but I hope to get a better view of both worlds as they will rise a few degrees higher in the sky than they do here in rural central Scotland. And having the chance to explore the vast skyscape within the confines of the Summer Triangle (marked by Vega, Deneb and Altair) from a truly dark sky will be an enjoyable experience. That said, last night I lingered a while on a reasonable view of the Ring Nebula in Lyra with my 34mm widefield and 8-inch reflector in summer twilight. How much better will the views be under true July darkness?

I decided to pull the trigger on a Celestron X-Cel 25mm LX eyepiece, which should arrive here by Friday. Fingers crossed for reasonable performance in the f/5 optical system!

The sky is darkening more now as the days and weeks have flown by the Solstice. The stars of Delphinus are beginning to show in the deep twilight and so a chance to visit another summer favourite; the delightful colour contrast pair, Gamma Delphini.

After midnight on July 5 2018, I began searching for two systems; Gamma Delphini and Mu Cygni. The latter proved much more challenging to track down, even with the Celestial Swan now having gained a considerable altitude. Gamma is widely spaced and presents with an aureal primary and lemon-white secondary. Mu is much more challenging though, especially at this time of year; the secondary is situated right up next to the primary but the great light gathering power and resolution of this grab ‘n’ go telesope on steroids made light work of it at 260x but the view was even more compelling at 318x. It’s such a delicate system to study telescopically; like budding yeast seen through a powerful microscope. I made a couple of drawings of what I saw (shown below):

Two gems of the summer twilight: Gamma Delphini & the challenging Mu Cygni, as seen through the 5.1″ f/5 Newtonian in the wee small hours of July 5 2018.

 

July 6 2018

The Celestron 25mm X-Cel LX 60 degree eyepiece.

 

Well, the new 25mm Celestron X-Cel LX eyepiece arrived late this evening. It was packaged well and a quick inspection revealed no internal dust in the optical train. It is very light, considerably less so than the 24mm Explore Scientific (ES) 68 ocular I sent back to the dealer and that’s a bonus, given the low mass of the optical tube. Like the Baader zoom, this eyepiece has a twist-up eyecup which allows the user to adjust the distance between the large eye lens and the eye. Testing it on the telescope in the bright evening sunshine revealed some very good things; the image of a distant rooftop was very sharp. Constrast was excellent. It showed a very small amount of field curvature and/or distortion at the edge of the field but best of all the eye relief was just right; that is, I was able to comfortably view the entire field with my eyeglasses on, in sharp contradistinction to the ES 68. Things were indeed looking good.

The Celestron 25mm X-Cel eyepiece has a large eye lens and twist-up eye cup for optimal viewing pleasure.

Around local midnight, I was able to test the new eyepiece on the stars. I am happy to report that it produced very sharp images of Vega and its hinterland, with good contrast and, to my relief, off axis performance was much better than I had experienced with the 24mm ES 68. Nor did I detect any internal reflections. There was a liitle distortion at the edge of the field but it was more than acceptable, certainly a notch up from the 32mm Plossl I have used for so long with the instrument. And like my daylight experiences, the eye relief was more than adequate when used with my eye glasses(which corrects for the natural astigmatism in my viewing eye).  This improved perfomance may at least in part be attributed to the smaller AFOV of this eyepiece (60 degrees as opposed to 68 degrees with the 24mm ES) but maybe also to its design differences. Nevertheless, I am more than content with its optical performance as a low-power, wide-field scanning ocular, delivering a power of 26x in a true field of 2.3 degrees. And in consideration of the fact that it set me back just half the price of the 24mm ES 68 (£70), I think it represents a really good bargain.

Technology has come a really long way since the days of my youth.

A happy camper am I.

 

At 00:45 local time on Saturday July 7, I turned the telescope toward Cassiopeia, now low in the northern sky and washed out quite a bit by the presence of twilight. I used my 6 x 30mm finder on the 130 to track down the creamy white magnitude +4.6 star, Iota Cassiopeaie. Ordinarily, I wouldn’t go near such a system at this time of year owing to how bright the sky is, but the persistant good weather here inspired me to give it a go. From memory, this star is annexed to a triangular configuration of three fainter stars so it was easy to identify. This is a famous triple system and its delicate cast under good conditions never fails to impress. I was pleasantly surprised with how well it presented at 260x, with all three members showing up clearly and distinctly. I made a quick sketch of what I recorded in the small Newtonian telescope, and is shown below. This is well worth a try at high northern latitudes in small telescopes with easy access to the northern horizon.

 

Iota Cassiopeiae as seen at 23:45 UT on the night of July 7th 2018 using the 130mm F/5 Newtonian at 260x. Note that the date should read July 7 and not July 10. Mea culpa.

 

I spent the afternoon of July 7 2018 deliberating about which eyepieces to bring and which to leave behind. The new 25mm Celestron is definitely coming on the trip. Further daylight tests showed that it Barlows real well with the 2.25x Baader shorty Barlow giving a nice medium power of 59x in a one degree true field. I have had second thoughts about the Baader zoom though, as it’s a bit on the heavy side and even with the same Barlow would only yield 183x, grand for most systems but since the telescope can very comfortably accommodate 50x per inch of aperture, I wished to coax that little bit more power out of the instrument. Racking my brains, I pulled out an older eyepiece which I had boxed away under my bed; a Meade Series 5000 5.5mm Ultra-Wide Angle (82 degree field). As I’ve said in a previous blog, I’m not overly enamoured by 82+ degree AFOV oculars, preferring 60 to 70 degree units in my field work, but upon testing it out in the 130mm f/5 Newtonian on a bright sunny day, I was quite impressed with what it delivered. A high-end eyepiece like this has very good edge-of-field correction and yields a power of 118x in a field of 0.7 degrees. Coupled to the 2.25x Barlow I can squeeze a high power of 266x out of it; perfect for the most challenging doube stars should they present themselves!

So, in the end, two eyepieces, both significantly lighter than the Baader zoom, will be coming on the trip with me, together with a single, 2.25x shorty Barlow. That combination will tick all the boxes!

Final choice (left to right): the Celestron 25mm X-Cel LX, the Meade 5.5mm Ultra Wide Angle and the 2.25x Baader short Barlow lens.

 

This is a good place to wrap up this blog. Take care and see you all when we get back in a couple of weeks.

 

All the best,

 

Neil.

 

 

 

De Fideli.

12 thoughts on “Going from Strength to Strength: the 130mm f/5 Newtonian.

  1. Hello Neil

    Congratulations for the impressive achievement of resolvind Lambda Cygni with the 130mm reflector!

    I was able to do it only with my 200mm F/6.16 Dobsonian which I call ”Toleascope” to honour my friend Alin Tolea who gave this telescope to me.

    However , I remember once I was on the brink of resolving Lambda Cygni with the 125mm F/6.9 Dobsonian using 288x /Radian 6mm+Barlow 2x.
    What I was seeing then was a small , roundish agglomeration or condensation inside the first diffraction ring of Lambda Cyg.
    But as you said , I should be more perseverant so I will try again in the future.
    Of course ,after finishing the overall refurbishment I’m currently doing to my beloved 125mm F.6.9 reflector which I call with affection ”Uggly Duckling”.

    Best regards, Mircea

  2. Hello Mircea,

    Good to hear from you and hope all is well. Enjoying some real summer weather here for once; a grand settled spell.
    I was deligted to see this phenomenon for the first time; it really pushes the ‘scope to the very limits. The small brightness differential really helps with this one though, but it still requires ideal seeing conditions which we have just now. There’s a huge high pressure system sitting over Scotland at the moment.
    I was intending to do another observation last night but alas got sidetracked with a new eyepiece I was testing. The whole episode proved rather frustrating though, as I shall reveal later in this blog.
    Still, my little “ugly duckling” gives some impressive views! And remember, beauty is only skin deep!

    With best wishes,

    Neil.

  3. well done Neil, I can just split Lambda Cygni on a good night
    with my 150mm F11 rellector.

    Pat.

    • Hello Pat,

      Great to hear from you! Hope you’re keeping cool over there in Northern Ireland! Some weather we’re getting at the minute!
      Today, Scotland enjoyed its hottest June day(32C) in 120 years!
      I’m chuffed to bits with how far this little Newtonian has taken me!

      And it’s great to hear that you’ve managed it with the 6″ f/11 OO Newt. Envious of that high f ratio instrument!

      With best wishes,

      Neil.

  4. Hello Neil

    I’m very glad you are a happy camper and you enjoy your well deserved
    vacation !
    Thank you for the review of the Celestron 25mm X-Cel eyepiece.
    I have a WWII 30mm Erfle eyepiece which belonged to a 10x80mm Flak binocular.
    It have a 70 or 72 degree aparent field but I was not happy with the edge performance of it on my 200mm F/6.16 Dobsonian.
    I simply added a smaller field stop which limit the aparent field to 60 degree.
    I like much more the ocular after this modification.
    Looking further to read more about your adventures with Newtonians.

    All the best ,Mircea

  5. Good evening Mircea,

    Thanks for the thumbs up in regard to the 25mm Celestron eyepiece. My first impressions of it have certainly been favourable.

    Re: the field stop fitting: that was most ingenious of you! I admire folk who can tinker with their equipment the way you have.

    We’re all looking forward to the trip. It will good for my boys to meet up with their relatives after all these years. Hoping the weather stays fair!

    God bless you!

    Neil.

  6. Hello Neil,
    Thank you for an excellent article. I was keen on getting a reflector due to all the advantages you highlight but found the views slightly uncomfortable (The full story is here- https://www.ashfordastro.org.uk/from-the-garden-to-the-stars-by-martyn-fuller). I haven’t done enough research to pin down why this is but it may be a contrast issue. Would you say that your recoated 130mm is an equivalent contrast to a 100mm achromat refractor?

    Regards,
    Martyn

  7. Hello Martyn,

    Many thanks for your message and very glad you enjoyed the blog. I’m on vacation at the minute so am out of the loop most of the time.

    The 130mm f/5 gives images roughly equivalent to a good 4 inch refractor on Moon and planets but is much closer to a 5 inch refractor on pretty much everything else, especially deep sky objects and double stars.

    Regards,

    Neil.

    Ps. Enjoyed your blog!

  8. Thanks Neil,
    I shall look out for a 130mm reflector to experiment with. One last question, would the F5 focal length be ok with my BST eyepieces or would I need better ones?

    I’m glad you liked my beginners booklet. That and my Messier booklet were inspired by helpful people such as yourself giving so much to help us novices.

  9. Good morning Martyn,

    I have heard many good things about the BST line of eyepieces. I actually had a set for testing a few years ago but they never made it to review in Astronomy Now.

    From memory, my own recollection of these eyepieces is that they were were generally very good, especially at the longer focal lengths. Some time later, I purchased a 3.2mm BST for my own personal use. When I tested it out in a variety of telescopes at home, it yielded quite bad, off-axis flaring on bright stars and the Moon, which caused me to promptly return it to the seller, after which time, I received a full refund.

    So, my opinion on these is mixed. If you are happy with them, then more power to you!

    All the best,

    Neil.

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