An interesting few hours with a Tak FS128

[F15Rules]

Yesterday was an interesting day.

I've had my Tak FS128 for some 8 or 9 months now. It was always a scope that I admired from afar, but I never dreamed I'd actually own one.  So, having been fortunate enought to find a mint, almost like new 1999 built example, complete with Tak equatorial EM2 mount, (itself a work of engineering art), I count myself very fortunate.

All that said, for various reasons, mainly to do with having moved house just a few weeks before acquiring the scope, and then having a lot of work commitments, and not least, a LOT of mucky weather, I have have very few "proper sessions" so far. In fact, having got the scope out to cool on perhaps 20 occasions so far, I think I've had to put the scope away, unused, on around a third of those, with at least another third being very much shortened by clouds and/or rain.

Frustrating me even more, we have actually had some very clear looking nights before Christmas, in a very cold spell, but work and family commitments meant I just couldn't take advantage of them.

On those few nights I have managed to get out and view some sky, I have still yet, I believe, to get the best out of this wonderful telescope. I have not yet really had a night when I felt I could usefully go above about x190 magnification - compared to the owner's manual, which cheerfully states that you can push the FS range (not just my version), up to x100 per inch of aperture on good nights, and up to x120 on truly excellent nights. As someone who enjoys viewing double stars, it is useful to be able to push the magnification well beyond x200 on tighter pairs, and in the past I have routinely used x300 or even higher on my D&G 5" F15 achromat. Clearly, something is preventing me getting these higher magnifications

So, being puzzled by all of this, I started to try and logically think through what could be the problem -   or problems?? It seemed to me that there were several possibilities, which, individually or collectively, might be holding the scope back:

1. Possible issues with the scope's optics - as a top end premium refractor brand, I would hope not, but every company seems to produce the odd lemon..
2. Local micro-climate - the skies are absolutely, definitely, "better" here in Lincolnshire, than in the industrial midlands where we moved from. However, I think in this context, by "better", I mean darker, so more stars and the Milky Way are visible more often. However, we are in a small village surrounded to the west, north and east by low hills, with the south being flatter, and very flat all the way down to Boston, some 18 miles south of us. The low hills in the other 3 directions peak (sorry) at around 400 feet, with our house being in the dip between the "horseshoe" of higher ground, at about 120 feet elevation above sea level. The difference doesn't sound much, but a climb of 300 feet in less than half a mile means that we are definitely a good deal lower in our village than most of the surrounding farmland. So I am wondering if this topography is causing "rolling" air, with turbulence and unsteadiness to tumble down the slopes of the higher ground, coming to rest down in our village, and so interfere with the seeing locally?
3. The Jetstream. This wretched feature of UK weather has been very active for much of the past 12 months, and the sheer changeability of the weather we have had since we moved, especially since early November, with warm to cold to warm, and breeze to high winds to breeze again (but not many calmer nights, and those that there were were "murky", with lots of mist and smoke to contend with - at least in the early to mid evening when I tend so far to be outside - makes me wonder if this is a nationally noticeable feature. I've certainly seen quite a few SGLers bemoaning the very poor seeing conditions on a regular basis.
4. Something else? Maybe the time of night I am observing? Do I need an eye test??

It seemed to me that the easiest issue to get checked would probably be the scopes' optics: I like to think that I can tell a decent star test when I see one, but I'm no expert at all, and I am sure that my own eyesight has declined noticeably in the past 5 years or so. 

So, I resolved to enrol the help of Mr Es Reid, professional optics expert, engineering wizard and all round very nice chap. Es has checked several lenses for me over the years, including one from a Vixen Pulsar 102mm F13 achromat which I was somewhat underwhelmed with optically, although it looked beautiful: Es was quickly able to conclude that the lens was "only" functioning at c1/4 wave, but would he felt that the lens would work noticeably better at 90mm than the native 102mm, effectively increasing the focal length as well. And he was correct, the scope did in fact perform much better at 90mm. Es identified some issues at the outer diameter of the lens objective, and said that using a 12mm mask would eliminate much of the issue. 

I contacted Es recently, and he readily agreed to take a look at the Tak objective for me. There was no way I was going to entrust a fluorite objective to Royal Mail's tender care, so I asked if I could take the whole OTA down to him and he kindly agreed. So yesterday I took the scope to Es' home, and we put the scope into his unheated workshop, with a cold air fan blowing also to speed up the acclimatisation of the Tak lens. Es placed the scope in a cradle and aligned it with various mirrors and light point source that he uses. He also put a ronchi grating on the eyepiece end, and showed me how the vertical lines, which should be straight up and down across the field, were in fact slightly curved outwards, more noticeably so the nearer you looked to the edges of the field left and right. Es reassured me that this should normalise once the scope was thermally stabilised, but it did demonstrate the effect that just a move from a warm car interior to an unheated workshop can have - so going from a warmer house to a colder outside must have a bigger effect again. 

We chatted for around 3/4 of an hour over coffee and biscuits (what an interesting guy Es is!), and went back out to the scope. Es chuckled with satisfaction as he looked at the ronchi again and asked me to take a look again. What a difference! The lines were now perfectly symmetrically in parallel, perfectly straight. Es also did an artificial star test with a 4mm ortho, (giving x260) and again asked me to look. A nice sharp central point with diminishing fainter tiny rings around it. Racking in and out, he identified a slight violet shade on one side of focus, and a faint greenish tinge to the other side. There was, at sharp focus, a very faint tinge of violet , but Es said it was very likely caused by his artificial star, which is made from an old brass ballpoint pen tip (!!) and is ready for replacement, being a bit soiled with blue tak residue and other foreign bodies. Es concluded by saying that this was a superb optic, one of the best he has seen...interestingly, Es asked whether I use a mirror or prism diagonal: I said that I normally use a 2" mirror diagonal, although I do have a Baader T2 prism 1.25".  Es remarked that he thought the prism might give slightly better results than the mirror.. I quote from his email from last evening after our meeting.. "Very good to meet a serious visual astronomer and confirm that the Tak is superb in every way but you may yet pull more performance out of it when using a diagonal by switching to a prismatic type. 

All in all a very interesting visit, and I left Es really pleased and not a little relieved that the Tak is as good optically as I had always assumed it would be from all the reviews I had read before committing to buy it.

The next possible issue, the local micro climate.. I still believe that this could be all, or part, of the problem. I think the only way to double check this is a) to take the Tak "up the hill" to another site to observe from there, and b), do a direct side by side comparison with my Vixen ED103s, and I will aim to do this as soon as I have the opportunity to do so.

Thirdly, the Jetstream. I just don't know about this. I need more star time with the Tak, and I'd welcome other's thoughts on how much their local seeing they feel has been adversely affected by this?

Fourthly. I am booking myself in for an eye test!

Dave

Post Script:

Last night the sky remained clear for several hours, with a few patches of high cloud scudding through. Not much breeze, good transparency but not the best I've seen here by any stretch. I could see the Milky Way overhead around Perseus and Cassiopeia, not further down towards the horizon. 

I had let the Tak cool for a good half hour with exposure to the outside temperature. 

I turned to Orion (what else?) and went straight to the Trap. Bingo! - I got E & F readily, both with direct vision. Best view was around the 10-12mm mark on the Pentax zoom ( I am loving this eyepiece!), so between c x85 and x104. The image held nicely up to the 8mm maximum of the zoom (x130), and the 7mm XL gave almost x150.  Once again, though, much higher than that and the seeing just wasn't having it. I just decided after that to enjoy the views, which included:

Praesepe Cluster, Pleiades, Alnilam (Epsilon Orionis, elongated at x148, clear split at x160 (Pentax XL7mm barlowed x1.6), Aldebaran (lovely deep orange hue), and various winter clusters in Auriga and Gemini cruising with the TS Paragon 40mm. The view of the double cluster in Perseus with the LVW 22mm was just stunning, with scattered reddish stars enhancing the view.

Thanks for reading 

[mikeDnight]

Interesting report Dave, with a good outcome! That Jet stream along with the cold weather plays havoc with the view, but come spring, you should hopefully get some breath taking views. 

[Stu]

That is excellent news Dave. I've let that you weren't totally at ease with the scope so it is really reassuring to be certain that the optics are as good as you could hope for.

I suspect seeing is a large part of your issue, and may be handicapping the Tak vs the Vixen a little, as you seem very happy with the 103?

I would be surprised if it was your eyes, again because you seem happy with the Vixen, so get yourself up that hill and find out  

[Alan White]

Dave

Very interesting report and findings.
Most glad the scope is good  and that its possibly cooling and other things, perhaps singly minor, but combined a real spoiler.

I see in your images a Tak Prism diagonal, one item removed from list already then.

The seeing is terrible a lot of the time where I am and this year is far worse than previous years, I think the good? old jet stream is the culprit as you suggest.

The micro-climate comment you make has resonance with myself.
I live at the top of a hill, but have a large area of open ground around me and it dips to my property, a bowl in which the climate varies to 300 metres away at times!

As Stu says, agree that your eyes are unlikely off as the Vixen pleases you so much.

I look forward to your updates as you remove the minor issues, one by one.
 

[Ben the Ignorant]

In order to be sure of what type of diagonal you have to use, send an email to Takahashi. A prism may give the last bit of correction the scope needs at high power, but the light path inside a 2" or 1.25" diagonal is not the same, and the glass type matters , too. BK-7 is the most common but maybe Tak designers chose BaK-4 or something else.

A friend owns a 130mm Vixen apo doublet with nearly the same specs as your Tak. Per Vixen's technical diagram, it requires a 2" mirror for low-power, wide-field viewing. However, high-power views call for a 1.25" prism. We tested that on real stars, and indeed, at 213x the defocus star test showed less spurious green and violet thanks to the prism. It was the old prism from my Celestron 5 that got a mirror instead.

And since that prism is not a chinese one, but precisely a Vixen-made prism, no wonder it improves a Vixen refractor. I believe Vixen didn't make 2" prisms when they issued the 130SS, anyway.

[fireballxl5]

That must be a great relieve Dave, I'm sure that you've got a great scope!

I got hold of a similarly used and pristine Tak TOA-150B last March. As I'm mainly an imager I very quickly realised just how much time the scope needed to thermally equalise with the outdoor temps. I've not felt the need to have it tested as when the scope has cooled and conditions are fair or better, it gives stunning views 

[dweller25]

Dave, an interesting writeup - and one that MIRRORS my experience with my 1998 FS128.

ES cleaned and checked my scope some years ago - his advice to me was to never sell it - which I am happy to adhere to

However, I often notice that other visual astronomers who live close to me see more planetary detail and I put this down to 3 main reasons :-

1. My southern view is straight over a large town (Bury) and then Manchester, both with a lot of lighting, cars and central heating systems.

2. My eyes are not getting any younger - and I know from comparing drawings of Mars with my wifes that I have some astigmatism.

3. The ever present Jetstream.

When I took the FS to France for a couple of weeks vacation the views of Jupiter were in a different league to those I see at home

So, I will have to be patient for the right seeing conditions and will choose my location carefully when I retire and move to Devon

[YKSE]

Dave, A very interesting story and enjoyable as usual

Let's keep our fingers crossed for more clear and steady nights.

[Chris]

What a read! I'm really pleased the scope is as every bit as good as you hoped, and I look forward to your side by side comparison with the Vixen. What an interesting comparison that will be!

[F15Rules]

1 hour ago, Alan White said:

Dave

Very interesting report and findings.
Most glad the scope is good  and that its possibly cooling and other things, perhaps singly minor, but combined a real spoiler.

I see in your images a Tak Prism diagonal, one item removed from list already then.

Thanks Alan.

It's encouraging that other members are seeing so much disruption due (very likely) to the Jetstream.

Regarding the Tak diagonal, I changed that for a Baader T2 prism...only because the Baader came with a lockable helical fine focusing eyepiece holder. I feel more confident with the Baader holding some quite big, heavy eyepieces (Pentax zoom, XL 7 & 10.5), as compared to the (very good) Tak prism with its' lightweight compression retaining ring (if I was using plossls or orthos I'd have no problem using a Tak prism again).

Optically, I couldn't see a difference.

[F15Rules]

2 hours ago, Stu said:

I suspect seeing is a large part of your issue, and may be handicapping the Tak vs the Vixen a little, as you seem very happy with the 103

Thanks Stu, I hope that's it. I haven't used the latest 103s much yet, but first light looked nice a few months back. 

It's looking clear outside at the moment, and the Vixen is on the Tak mount cooling as we speak?..

[F15Rules]

1 hour ago, fireballxl5 said:

That must be a great relieve Dave, I'm sure that you've got a great scope!

I got hold of a similarly used and pristine Tak TOA-150B last March. As I'm mainly an imager I very quickly realised just how much time the scope needed to thermally equalise with the outdoor temps. I've not felt the need to have it tested as when the scope has cooled and conditions are fair or better, it gives stunning views 

Thanks FBXL5?. That 150 of yours sounds like a beast! Can you share any photos? What do you mount it on?

Dave

[F15Rules]

3 hours ago, dweller25 said:

Dave, an interesting writeup - and one that MIRRORS my experience with my 1998 FS128.

ES cleaned and checked my scope some years ago - his advice to me was to never sell it - which I am happy to adhere to

However, I often notice that other visual astronomers who live close to me see more planetary detail and I put this down to 3 main reasons :-

1. My southern view is straight over a large town (Bury) and then Manchester, both with a lot of lighting, cars and central heating systems.

2. My eyes are not getting any younger - and I know from comparing drawings of Mars with my wifes that I have some astigmatism.

3. The ever present Jetstream.

When I took the FS to France for a couple of weeks vacation the views of Jupiter were in a different league to those I see at home

So, I will have to be patient for the right seeing conditions and will choose my location carefully when I retire and move to Devon

Hi David,

It sounds as though your experience is very similar to mine. Interesting that a trip overseas to good skies transformed the views...perhaps we should all emigrate to France to squat at Olly Penrice's place??

It does make me very envious of those Cloudy Nights members living in places like Arizona, where they can virtually be sure that a given night will be clear!?

Dave

[F15Rules]

1 hour ago, YKSE said:

Dave, A very interesting story and enjoyable as usual

Let's keep our fingers crossed for more clear and steady nights.

Thanks Yong?.. it's clear outside here at the moment, o guess where I'm going in 10 minutes???

Dave

1 hour ago, Lockie said:

What a read! I'm really pleased the scope is as every bit as good as you hoped, and I look forward to your side by side comparison with the Vixen. What an interesting comparison that will be!

Cheers Chris, the Vixen is cooling outside..it won't be literally side by side, but I had a good almost 2 hours with the Tak last night, around half of that at least on M42 area, so I think I'll be able to make a good mental comparison..

Dave

[fireballxl5]

48 minutes ago, F15Rules said:

Thanks FBXL5?. That 150 of yours sounds like a beast! Can you share any photos? What do you mount it on?

Dave

Yes, it's defo a serious lump of glass and I love it! It's carried by an MI-250 mount but I don't have any decent photos of the setup - I must sort some. I have this unpacking photo...

[F15Rules]

Wow! That's like a missile - with the mount (MI-250) sounding like a launcher. When was it made?

Thanks for sharing?.

Dave

[25585]

Great read, you must be both relieved and pleased.

A couple of things... Mr Reid's home made artificial star! Is that the one he tests Esprits bought from FLO with?!

High end refractors needing a glass prism diagonal to perfect an image at high magnifications?! Is that more for shorter focal lengths than longer? I hope Bak-4 is used, Bk-7 are what is used in older and cheaper binoculars. Disturbing that something so lowly should be required in addition to (presumably) state-of-the-art lenses!

[F15Rules]

The artificial star.. it's interesting.. apparently the properties of a used biro type pen ball (cleaned of course)  make them ideal for creating a nice round point source. He's expecting a supply anyway now from a local charity group who collect unwanted ball point pens and send them to poorer countries overseas: every so often they get a dried out one they can't use, so Es takes them!

Seriously, though, Es knows his stuff and has a number of tools he uses to check the properties and effectiveness of lenses. I believe he used to figure lens himself in a former life. I'd happily trust his judgement on any lens I owned☺.

Regarding the prism Vs mirror. I don't think it's a case of a high end frac needing a prism to perfect the image. The objective places the image at it's point of focus, (its' "perfection" or otherwise being dependent on the quality of the lens' glass/figuring/polish) and the diagonal/prism/eyepiece combination then presents the image to the observer - an image that is only as good as the eyepiece end of the optical train. 

Es' view was that a good prism scatters less light than most mirror diagonals. It's sometimes said that below around F7 or so a prism can show a little more CA than a mirror, whilst reversing that effect on slower scopes. I tend to agree, from my experience. I think Es' point was that the real enemy of fine detail resolution is light scattering, more so than a bit of violet halo. Again, I'd agree. My D&G F15 showed a smidge of CA, but with a Baader prism diagonal showed minimal scattering, and on good nights the scope really showed fine detail.

BK7 is cheaper than BK4, but not necessarily inferior. Ask Steve Tonkin, our SGL binocular expert. He would rather have a well made BK-7 binocular with decent prism cages than a cheaply put together pair with BK-4 prisms that are not properly secured in their housing. 

I've read somewhere that Tak 1.25" diagonal prisms use BK-7 glass. If that's true, it's proof to me that BK-7 prisms can put up excellent images!

Dave

[fireballxl5]

52 minutes ago, F15Rules said:

Wow! That's like a missile - with the mount (MI-250) sounding like a launcher. When was it made?

I purchased the mount directly from Larry Myers at Mountain Instruments back in 2006. He stopped making them in around 2010. The website is archived here: http://www.mi250.com/

I've upgraded it to Gemini 2 and it's great, carrying the 150B with a Tak FSQ85 piggy-backed. Currently using this combo as a dual-rig set-up.

PS the 150B is approx 6 years old now.

[F15Rules]

Well, I went outside with the Vixen ED103s. It's really cold outside here, and the scope objective had fogged up while cooling down!

So out came the hairdryer on a low power, low heat, to gently clear the fogging. Of course, having done that, I had to wait for about 10-15mins for the images to settle down. After 30 minutes I was sure that there were no image artifacts that could be attributed to the heating/defogging of the lens.

And the result so far? Same as for the Tak! On a clear looking night, quite transparent, I couldn't keep a steady image beyond about x120. The Trap in M42 was nicely defined up to around x85, but the Trap was quite small at that power, and I only suspected the E star a couple of times, and no sign of F.

I came in after 50 minutes to warm up and get a drink. I brought the scope in as well, as it was dripping wet on the tube outside.. I'll pop out again soon, bit I'm now pretty sure that it's the sky conditions at work, just not sure if it's the Jetstream, or local conditions, or both. 

At least I'm now sure it's not the scopes' fault, either of them?.

Dave

[F15Rules]

5 minutes ago, fireballxl5 said:

I purchased the mount directly from Larry Myers at Mountain Instruments back in 2006. He stopped making them in around 2010. The website is archived here: http://www.mi250.com/

I've upgraded it to Gemini 2 and it's great, carrying the 150B with a Tak FSQ85 piggy-backed. Currently using this combo as a dual-rig set-up.

PS the 150B is approx 6 years old now.

That's a beautiful looking mount. It can clearly take some weight!

Do you post any of your images here on SGL?

Dave

[fireballxl5]

44 minutes ago, F15Rules said:

Do you post any of your images here on SGL?

I tend to post mainly on FB groups to be honest. I've put a few on SGL though. A couple I've taken with the 150B since last April. I'm currently working on others taken with the dual config.

 

Jupiter 20170415.bmp

[dweller25]

1 hour ago, 25585 said:

Great read, you must be both relieved and pleased.

A couple of things... Mr Reid's home made artificial star! Is that the one he tests Esprits bought from FLO with?!

High end refractors needing a glass prism diagonal to perfect an image at high magnifications?! Is that more for shorter focal lengths than longer? I hope Bak-4 is used, Bk-7 are what is used in older and cheaper binoculars. Disturbing that something so lowly should be required in addition to (presumably) state-of-the-art lenses!

I have done numerous tests with miror diagonals and prisms in my Tak and can confirm that a Zeiss PRISM gives the best views.

[Ben the Ignorant]

A Zeiss APQ 100mm f/6.4 triplet is improved by a glasspath (prism diagonal). Its chromatic spread is reduced from 175 microns to 52 microns:

http://r2.astro-foren.com/index.php/de/9-beitraege/01-aeltere-berichte-auf-rohr-aiax-de-alles-ueber-apos/524-a037a-zeiss-apq-96998-100-640-mit-glasweg-verwenden

Proving you can't generalize, the 130mm f/7.7 APQ Zeiss triplet loses performance with a prism, its chromatic spread stretches from 33 microns to 145 microns:

http://astro-foren.de/index.php?thread/9049-apq-ohne-glasweg-ein-guter-apo/

Lastly, the american-made Traveler triplet from Astro-Physics prefers prisms, too; the colors from red to blue are spread over a 45 microns length when they cross the prism, but they spread over a 93 microns length when they don't benefit from the extra glass:

http://astro-foren.de/index.php?thread/15464-mit-oder-ohne-glasweg-was-wollte-der-designer/

Too many combinations of glass, focal ratio and number of lenses are around to draw broad conclusions. Ask your telescope maker, they know what kind of diagonal their scope is designed for.

[F15Rules]

1 hour ago, fireballxl5 said:

I tend to post mainly on FB groups to be honest. I've put a few on SGL though. A couple I've taken with the 150B since last April. I'm currently working on others taken with the dual config.

 

Jupiter 20170415.bmp

Lovely pictures, just stunning. Imaging isn't my thing, but I can really appreciate the effort and time that goes into such pictures. You must be delighted with your Tak?.

Dave