Tonal variations in refractors.

[GlenM]

I was out last night during a couple of hours of excellent observing. I was out with my Borg 77EDII and my Sharpstar 61 EDPH. I was using my Morpheus 17.5mm. What I did notice was that the Borg certainly showed a warmer image than the Sharpstar. The striking difference was when viewing Albireo. I know the Borg uses FPL 51 and the Sharpstar used FPL53, but the Sharpstar is certainly a faster scope. I noticed similar variations when I used to sell 4" F/11 Achros, these always had a cooler tone although I was using Orthos at the time. I know eyepieces show variations, but I think scopes do also.

Does any one else see tonal changes with their objective lenses?

 

Glen.

 

 

[Peter Drew]

I think the main thing that drives tonal differences is the type of coatings used.  For an exaggerated example, try the "ruby" coated binoculars.   😀

[F15Rules]

Hi Glen,

Interesting thought.. I just have two scopes at the moment, an F8.1 Tak FS128 Fluorite apo and a vintage 80mm F15 achromat. I find the "tonal" appearance of both to be remarkably similar (insofar as I can judge them from memory, I've not done a side by side comparison).

I would describe the image tone as being very "pure" or "neutral" in both scopes..I seem to see more variation in eyepiece tones, which can sometimes be very noticeable.

As an example of this, using a good ortho as a benchmark, I find Tele Vue plossl to be slightly "warm" whereas I find a Morpheus  to be "cooler" or more neutral, which I prefer.

This isn't a scientific judgement, just my opinion based on what I see☺.

Dave

[mikeDnight]

I've never looked through a Borg 77EDll or a 61 Sharpstar EDPH, but I've certainly noticed tonal differences in the numerous refractors I've used over the years. 

Glass types and focal length do play at least some part it seems. As an example, a 150mm F8 SW achromat will show Saturn to be yellow, where as a Vixen FL 102 will show it as a almost white/yellow. I suppose in the case of a short achromat such as the SW, not all colours are in play equally at the point of focus, but in an ED or apochromat they are, and produce a truer, cooler view. Looking through a 150 F8 achro alongside a 125mm uncoated F11 achromat some years ago, the F11 gave a far purer, better defined view of Jupiter.

With regard to ED's and apo's, I found my TV NP101 was warmer and softer than my Tak FS128. To my eye the FS128 F8.1 gave an ice cool piercingly sharp view. My SW Equinox 120 was similarly very Tak like giving a cool contrasty view. In contrast, my Tak FC100DC F7.4 has a slightly warmer tone but with an unusual vibrance that I've not seen in other warm toned scopes, and is also piercingly sharp and free of visible CA. I don't use Naglers or for that matter anything Televue, as I'm not a fan of their warm tones. Instead I prefer the cooler Pentax XW's or Baader Morpheus. In a side by side comparison with a Vixen FL102 F9 (cool), the FC100DC (slightly warmer), both gave a stunning highly defined view of jupiter showing equal amounts of fine detail in the form of festoons, white ovals, barges and garlands, all visible with ease. Again in a side by side comparison, both the Tak FC100DC F7.4 and its longer sibling the FC100DL F9, gave equally sharp contrasty views, with the F9 being cooler. I suspect the two scopes use the same eco front crown element to match its fluorite rear element, though it did appear there was a subtle difference in the colour of optical coatings. Ultimately, I suppose if the optics are well figured and performance is high end, the rest just boils down to preference.

 

[GlenM]

Thanks guys. It's always interesting to hear peoples views on the subject.

 

Clear skies,

Glen.

 

[Alkaid]

Both TAL’s I owned showed a warm tone. One was the ‘RS’ and the other was a TAL-1 newt, though the tone cast on that would have to be down to the eyepieces.  I quite liked the tones of those scopes tbh.

[whipdry]

I think you'll find your eyes/brain adjusts to colour variations quite quickly comparable to the white balance of a digital camera.

It's easy to see in daylight, turn on a warm white light bulb and stare at it with one eye closed or covered for 30sec then look away towards a plain coloured wall while blinking from one eye to the other... over a short time both eyes will return to seeing the same tones. 

Peter 

[paulastro]

As others have said, different refractors do indeed have different colour casts.  I've noticed this over many years, some are 'warm' (too warm to my eyes) whilst at the other end of the spectrum others are 'cooler', the best examples being Tak fluorite in my view.

Of equal effect on the image is the colour casts of  eyepieces.  To evaluate the colour casts of optical systems you have to use a colour neutral (true colour rendition) eyepiece or the tests are virtually useless.  Some (and that includes expensive ones) eyepieces have shocking colour casts.  I did some extensive eyepiece tests of my own some years ago with eyepieces I owned at the time or could borrow.  Bearing in mind that my tests took place before many of the comparatively recent eyepieces were available, Pentax XWs came out as true and neutral as you could wish for - vibrant and honest to life. I also recall that Hyperions were  on a par with them if I remember correctly.  The test scope was a top quality Pentax 80mm spotting scope.

I would say the tests must be done in daylight and not very early in the morning or late in the evening when other factors come into play. If you have good selection of different eyepiece types, compare them on your scope in the daytime and depending on what you have you may be shocked and surprised the differences there are, and not always in the way you expect.

Edited December 1, 2019 by paulastro