Planning to buy a first class refractor

[Jose Auger]

I aam planning to buy a first class refractor within 8 to 10 months from now. I have looked at Takahashi TOA 150, CFF Telescopes and TEC. No Astrophysics as I do not have the time nor patience to wait several years for a telescope. I understand that both CFF and TEC are oil spaced. I have had no experience with oil spaced telescopes. I shall appreciate the views of owners or prior owners of these there brands of scopes. Second, what are the pros and cons of the oil spaced (other than providing for the transmission of light)? I understand that the yellowing of the oil is no longer an issue (or is it?).

[Jose Auger]

BTW, it should be 150 or around 150, not less.

[michael.h.f.wilkinson]

The only big APO refractor of that size class I have looked through was Olly Penrice's TEC140, which is outstanding. Nothing wrong with a well-made oil-spaced APO that I can see.

[ollypenrice]

Like Michael I'm impressed by our TEC140.   Visually I can't fault it at all. I bought mine second hand after seeing a couple of others which stay regularly at my place. There's another working robotically here. None of these four has ever given the slightest cause for concern. TEC are a responsive company with Yuri Petrunin responding to questions himself. One thing which particularly impressed me was the TEC's ability to handle Alnitak photographically. The image below shows a 10 minute luminance sub from a monochrome Atik 4000 CCD. On the left the data is linear. On the right it has been given a pure logarithmic stretch (by moving the grey point slider to the left in Ps Levels.) What is truly remarkable is that, despite the stretch, Alnitak has not bloated and is still clearly resolved as a double. I have never seen this from any other refractor and I've used some good good ones.

I don't doubt that you'd be thrilled by a 150 Tak either but, for me, the very reasonable price of the TEC (in the unreasonable world of refractor prices) made it irresistible. Oil spacing is not only optically sound but also brings down costs a little, I'm told.

Olly

[ronin]

The top end refractors over here (sort of in Europe) are the APM ones.

The best are the ones with LZOS lens and it may be that LZOS make scopes - I am not sure if they produce lens and scopes, or just lens.

One other aspect is that LZOS lens are among the best and they are used by others, the obvious one is William Optics and I think TMB do also. So it would be worth looking into the details of WO and TMB scopes as well as APM scopes.

Not sure about oil spaced, and the conditions in Puerto Rico are somewhat different to those we generally meet both here and in most of the US. In design terms I would have thought thtat air spaced offered more design parameters to play with.

One other that appeared and are slowly getting known are Officina Stellare.

[michael.h.f.wilkinson]

Oil spacing gives the same number of degrees of freedom in design as air spacing (actually you have the choice of oil, so an extra degree of freedom is gained). The fact that oil has a refractive index similar to glass means the internal reflections on the glass-oil interface are much smaller than glass-air.

[Steve Ward]

One small thing to consider is that oil-spaced lenses are not suitable for use with the rear mounted Daystar Quark Hydrogen Alpha units for Solar observing/imaging.

[michael.h.f.wilkinson]

One small thing to consider is that oil-spaced lenses are not suitable for use with the rear mounted Daystar Quark Hydrogen Alpha units for Solar observing/imaging.

Unless you use a front-mounted ERF (which is costly).

[Peter Drew]

I seem to recall that oil spacing also eliminates optical figuring defects on the oiled faces, in effect a doublet becomes a singlet with just two external surfaces.    

[michael.h.f.wilkinson]

I would rather say an oil-spaced doublet is optically like a triplet without air spacing (kitted lenses). You only have two air-glass interfaces, with the glass-oil interface acting much like kitted glass-glass interfaces, but without any of the stresses that differential thermal expansion of the different types of glass can produce.

[DaveS]

With oil spacing don't the radii of the two surfaces have to match?

Also with an air-spaced triplet not only do you get the freedom of different radii, but the air space itself can act as an additional optical element.

Not rubbishing oil-spaced triplets by any means, just pointing out there are advantages to both.

BTW debating between a CFF oiled triplet and a APM Triplet.

[michael.h.f.wilkinson]

You may have more freedom with a triplet. I am not sure how thick the oil layer my be.

[DaveS]

From what little I've been able to gather, in the case of the CFF triplets it's only a capillary film, so the radii would have to match pretty closely, much like a cemented triplet.

[mikeDnight]

Just to add to your dilemma. A number of years ago I decided to buy a TOA 130 after rather foolishly selling my incredible FS128. I phoned the supplier, who I know well and who I trust, and he said the TOAs are not as good visually as the FS128. He advised I looked for a second hand FS128.

The Takahashi Super Apochromat are a different ball game, perfect in every way, except they only go up to 120mm.

Given your initial choice of candidates I would unhesitatingly say go for the TEC. They have been producing triplet apos for a long time now and I've never heard of any problem with the oil leaking, drying out or discolouring. Many of these telescope myths are perpetuated by manufacturers and fans of other designs. Astro physics have used oil spaced lenses for years and there's no problem at all with them.

Mike

[Davey-T]

I was looking into this recently and decided on the APM 150 as first choice with TEC140 a close second, just need some money now

The APM is available unlike some advertised scopes.

Dave

http://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AG1200TLZAPO/SubProducts/AG1200TLZAPO-0002

[Peter Drew]

Without equal radii you would have a job keeping the oil in. 

[Stu]

Is this for imaging or visual use?

[Jose Auger]

WOW!!! Many recommendations, but it seems that TEC has the advantage. The telescope will be used for both, visual and imaging.

[Jose Auger]

BTW, no one recommends CFF. Why is that?

[DaveS]

They'er a small company with a limited output. They're also not cheap (Though not as expensive as eg APM) so I guess no one on here has one to be able to recommend.

[ronin]

Problem I have with oil spaced lens is I have an old 5" seperated doublet, the seperation is about 6mm so no way could it be oil filled. :eek:  Even Hypoid 90 would not manage that. The White Lightening Crystal Clear Grease might manage.

When you have one of those you sort of discard all thoughts of oil spacing. :grin:

I have assumed that if oil spaced then R2=R3 if not exactly then very closely as the oil is not there to sort of just fill in the gap, it seems to be almost an alternative to cementing and I presume that the surface tension that is utilised for this, and that means a thin film generally and so very similar if not identical radaii. Will say that just about all air spaced lens seem to be seperated by what are fractions of a millimeter so likely not much real difference. :rolleyes:

It does aid the irregularities created by grinding the lens also, although I would hope these were very minimal and should not really impact.

Main concern would be the temperature and humidity in Puerto Rico. I assume higher for both and not sure how the oil would take the repeated cycles.

[DaveS]

Oiling does appear to be an alternative to cementing which has its drawbacks for large lenses. Incidentally, from what I gather, the CFF objectives have at least one hand figured aspheric surface. Possibly to compensate for the reduced degrees of freedom without the air-spacing.

[Jose Auger]

Temperature does no vary much in Puerto Rico. Well, these days we are hitting 90+ Fahrenheit due to a mass of warm air and Sahara dust (yes it gets up to here). However, temperatures oscillate between 70 degrees Fahrenheit and 80 degrees. In the mountains, it can get down to 50 degrees, but its not an ideal place for telescopes as it rains too much. So coastal areas (specially south, which is very dry) are much better.

[timwetherell]

Oil has the advantage over cement that it has some flexibility so it can handle the slightly different expansion coefficients of the various glasses, enabling far larger lenses to be made than cement will allow. Cement can also contract slightly on setting which could potentially deform a big lens. It's true that air spacing does allow the designer to introduce a small difference in the adjacent radii but you only need enough degrees of freedom to achieve better colour correction that the diffraction limit of the scope. I guess it's why generally people stop at 3 or 4 elements in an apo rather than having six or nine - if you can get the result you want without more then they're not needed. I don't think that either oil space or air space are inherently superior or inferior, in my experience a well made lens of either design will perform superbly. Most of the high end manufacturers offer excellent after sales support too, so whatever you go for, I'm sure you'll be well looked after I can certainly recommend TEC in that respect!

[dweller25]

Here are a few to consider.....

Takahashi TOA150

APM/Lunt 152ED ** visual only

APM 152-1200T

TEC 160FL