Takahashi FC-100D Quirk

[Alma]

I’ve had my Tak FC-100 for about 3 years now and I think it’s a fantastic little scope in terms of sharpness, contrast and light throughput.   However it has one annoying little habit and I’m interested to hear whether any other owners have experienced anything similar.   The scope itself b.t.w. is a DC model to which I added a 2.5” Feathertouch, replacing that rather silly (i.m.ho.) FS60 focuser on the DC.

Let me describe what happened about a week ago.   There was a rare break in the clouds and I decided to whip out the Tak for a quick look at Orion.  Now Orion is relatively low on the horizon but not THAT low so that atmospheric issues should be too much of a problem.  The first view was a mess so I changed eyepieces, changed diagonals, checked for dew, etc. but nothing changed that horrible view.  I might add that this isn’t the first time I’ve experienced the same thing with this scope, but it’s rare.  Also, I’ve not experienced this with my other scopes which are f/6 triplets.

Last night I think I finally pinned down the problem - heat plumes!  I dual mounted the Tak alongside my 123mm f/6 triplet and tracked M42 for about 3 hours.  Everything went great, in fact the Tak performed beautifully for a couple of hours giving the bigger LZOS triplet a really good  run for its money.  Then, quite suddenly everything went to pot with the Tak.    It happened precisely when the mount started tracking above my neighbour’s roof.   I suspect his house lacks insulation and there’s a lot of heat escaping through the roof.  So, I kept tracking until M42 completed its roof transit and things appeared to go back to normal.  I say “appeared” because trees partly obscured the view, so difficult to be completely sure.  Throughout this time the triplet maintained its excellent view and didn’t suffer any degradation in the view.  I should add that I also tested an 80/480 triplet separately which similarly remained unaffected.

So, I’m curious.  Is this a facet of the FC100 design?  Is it a particular quirk of fluorite? Is it typical of doublet lenses in general? Has anyone else experienced this problem with the FC-100 in particular or any other scope, or is it just my particular unit that has the issue?   I don’t want anyone to interpret this as Takahashi bashing b.t.w. or to be deterred from buying this model - in fact I think it’s a great performer for its aperture and a very worthwhile investment.

Alan

 

[dweller25]

Heat plumes are bad news for any telescope.

Possible a combi boiler going on and off giving variable results between your scopes ?

[jetstream]

One night I had a quick grab and go with my 90mm SV APO- thought I could avoid plumes from the woodstove by pointing off the roof line a bit. Yeah it was a boiling, blurry mess and would have been in any scope.

[Alma]

1 hour ago, dweller25 said:

Heat plumes are bad news for any telescope.

Possible a combi boiler going on and off giving variable results between your scopes ?

Not really - I had the scopes dual mounted set up on the same mount so I could look from one eyepiece to the other in a second or two.  Also, it seemed to come from the entire length of the roof, not just a single flue.    Think I’ll have a chat with my neighbour about his insulation, or lack thereof.    The main thrust of my question really is why one scope should be so susceptible to heat plumes than others - the difference was very significant.

[Stu]

7 minutes ago, Alma said:

Not really - I had the scopes dual mounted set up on the same mount so I could look from one eyepiece to the other in a second or two.  Also, it seemed to come from the entire length of the roof, not just a single flue.    Think I’ll have a chat with my neighbour about his insulation, or lack thereof.    The main thrust of my question really is why one scope should be so susceptible to heat plumes than others - the difference was very significant.

Sounds intriguing, but I can’t think of any reason why the Tak should affected more than any other scope. I don’t think there is anything inherent in the design that would cause this.

I often choose to observe Orion when it is above either of the gaps between my house and the neighbours, as the seeing is much better then.

Is it a repeatable effect, or have you only seen it this time?

[mikeDnight]

Hi Alan,

  First off, its almost certainly nothing to do with your scope, either mechanically or optically, and almost certainly due to some form of atmospheric disturbance.  A similar thing has happened to me a couple of times. The first was back in 2003 just after I'd bought a brand new FS128.  I'd taken my scope excitedly round to my friends house where we had first light with a view of Jupiter. It was absolute perfection, and the Galilean satellites were perfect tiny discs each subtly different in colour and size. Then I swung the scope round to look at Vega in the east. The star image was terrible!  It wasn't boiling or colourful, but just a stationary blur. I was distraught, as I'd paid £3,850,00 for this scope and I was far from rich. I arranged for the scope to go back to the vendor but the currier didn't turn up. So I decided to take the FS to my local astronomy club to get some moral support. That was a mistake!  As i walked up the hilll towards the big observatory, my so called friends were all hysterical with laughter and rolling round on the grass desperately trying to grab their breath.  My friend Gain tried calmly to reassure me saying "Michael - Takahashi don't make bad scopes"!  At the same time my other friend Paul had tears streaming down his face and was almost wetting himself. Then Peter Drew took the scope off me, mounted it on a GP, and aimed it at Vega. After a few seconds Peter stepped back from the telescope and said "That's one of the best star images I've seen".   I looked and saw a perfect image of Vega. Then I tried to explain that it wasn't like that two days earlier, but they just looked at me as though I was nuts. The FS never gave a bad image after that first view of Vega.  

 The second time this happened was in April 2017.  The night was one of exceptionally good seeing and the first quarter Moon was high in the sky.  My friends Paul and Rodger had called round to observe with me. I was using my FC100DC and Paul brought his FC100DL. Both scopes performed perfectly and we all observed together for several hours. Rodger was amazed at how well colour corrected the DC was.  As Jupiter rose in the east we aimed both scopes at the planet. Both scopes were pretty much on an equal footing, until suddenly the DC gave the same stationary blur that I'd seen on Jupiter all those years earlier. Detail on Jupiter's disc all but disappeared in the DC, which was mounted on my GP in my observatory. Paul's DL was around twenty feet further down the garden and gave a spectacular view of the festoons, garlands and white ovals. At first I wondered If the lens had dewed over, but it was as clear as crystal. Looking at the situation the following day, I noted the DC had been looking directly over the apex of a bungalow a few yards away. The DL however looked past the house. The atmosphere on both occasions was steady, so any mild heat plume would rise like a steady column. I can only imagine such a steady mild heat plume to be the only reason possible for creating that steady blur. The FS and DC were capable of extreme high power, which they wouldn't have been if the optics weren't top class.  May be in your case, one scope was looking through a stable heat column while the other wasn't?  

My friend Gain posing with my FS128 mounted on the Astronomy Centre's GP mount, spring 2003.

below, my glorious FC100DC on its GP mount in my observatory, spring 2017. 

[dweller25]

It must be a UFO......

Unidentified Fumes Overhead 🙂

Edited February 20, 2021 by dweller25

[mikeDnight]

We're the scopes mounted piggy back on an equatorial or on an altazimuth?  And could the Tak have been closer to the tree tops? Trees can cause problems at times, even if they aren't actually covering the object being viewed. My western horizon has a high tree line that disturbs the view of an object as it nears the tree tops.  Also, could there be air vents in your neighbours roof that may allow heat to roll up the roof? 

[Alma]

36 minutes ago, Stu said:

Sounds intriguing, but I can’t think of any reason why the Tak should affected more than any other scope. I don’t think there is anything inherent in the design that would cause this.

I often choose to observe Orion when it is above either of the gaps between my house and the neighbours, as the seeing is much better then.

Is it a repeatable effect, or have you only seen it this time?

Hi Stu.  It’s repeatable, but only on rare occasions and thinking about it only during the Winter months.   I don’t recall the same effect with the FC-76 (the one I bought from you) but then I didn’t use it that much.  It just seems odd that the 80mm and 123mm triplets seem hardly effected at all whereas the Tak went into meltdown.

Before I turned to M42 b.t.w. I took a look at the Moon to make sure all 3 scopes had cooled properly and the view through the Tak Fc-100 was outstanding - almost as much detail as the superb LZOS 123/738.  

[jock1958]

Hi @Alma I very much doubt you have a problem with your Tak.

I was out last week with my FS-60Q during a rare cloudless night, conditions were steady so decided to give Sirius a quick peek and Wow!! it looked excellent at x200.....this could be it and I'm in with a chance of seeing the Pup with my FC-100DL 😀 

I quickly set up the mini Taks big brother only to be hugely disappointed with the views, I persevered for well over 45mins but things didn't improve so put it down to the dratted central heating plumes, cool down time of my Tak or atmospheric conditions changing 🤔 and of course Sirius being too low for my location ☹️

[Deadlake]

18 minutes ago, mikeDnight said:

We're the scopes mounted piggy back on an equatorial or on an altazimuth?  And could the Tak have been closer to the tree tops? Trees can cause problems at times, even if they aren't actually covering the object being viewed. My western horizon has a high tree line that disturbs the view of an object as it nears the tree tops.  Also, could there be air vents in your neighbours roof that may allow heat to roll up the roof? 

It's interesting you mention that Mike. Last night I was looking forward to the clear skies, however when I looked towards the Moon it was shimmering. In the end I had to drop down to a 6 mm eyepiece. I put this down to maybe moon light being reflected off low cloud on the horizon when maybe it was because a tree was to the right of the moon, which was going quite low on the horizon. 

[Alma]

13 minutes ago, mikeDnight said:

We're the scopes mounted piggy back on an equatorial or on an altazimuth?  And could the Tak have been closer to the tree tops? Trees can cause problems at times, even if they aren't actually covering the object being viewed. My western horizon has a high tree line that disturbs the view of an object as it nears the tree tops.  Also, could there be air vents in your neighbours roof that may allow heat to roll up the roof? 

Thanks Mike.  They were mounted side by side alt-az on a Rowan AZ100.   I’m pretty sure it was the roof and not the trees as I observed the same thing for about 45 minutes to an hour as Orion made its transit over the house.  The eastern side of the house was well away from the trees.   Your earlier description of a “stationary blur” describes it well.  If I kept my eyeball perfectly still over the eyepiece there was a reasonably pin sharp view, but the merest movement sent the image into a mass of scintillating blurs.   I’ll have a good look at my neighbour’s roof but it looks just like your standard, tiled roof with nothing out of the ordinary.    I think it’s a case of poor maintenance and extremely poor insulation.   

[Alma]

18 minutes ago, jock1958 said:

Hi @Alma I very much doubt you have a problem with your Tak.

I was out last week with my FS-60Q during a rare cloudless night, conditions were steady so decided to give Sirius a quick peek and Wow!! it looked excellent at x200.....this could be it and I'm in with a chance of seeing the Pup with my FC-100DL 😀 

I quickly set up the mini Taks big brother only to be hugely disappointed with the views, I persevered for well over 45mins but things didn't improve so put it down to the dratted central heating plumes, cool down time of my Tak or atmospheric conditions changing 🤔 and of course Sirius being too low for my location ☹️

Certainly.  I’m loathe to describe it as a “problem” as such, more of a quirk as otherwise I think it’s a brilliant scope in all respects.  Mike’s experience with his FC-100DC describes perfectly what I saw.   It may well be you were experiencing the same thing with your shot at the Pup.

[mikeDnight]

1 hour ago, Alma said:

 If I kept my eyeball perfectly still over the eyepiece there was a reasonably pin sharp view, but the merest movement sent the image into a mass of scintillating blurs.   

Can I ask what eyepiece you were using?  I'm just wondering if the eyepiece had a smallish eye lens, its possible that some moisture or oil from an eyelash could have transfered onto the lens?  It's strange that moving your eye over the eyepiece changed the view.

[Alma]

Actually I used several Televues alternately in all 3 scopes ... Panoptic 19 and 24mm, Delos 10mm, Ethos  8, 13 and 21.  Same result whichever I’m afraid and I also alternated between 2 diagonals ... Baader 2” mirror and 1.25” mirror and prism.