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Refractors best for reds?

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#1 25585

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Posted 16 February 2019 - 07:19 AM

Are there any refractors that are good for reds particuarly? Red/orange stars - in doubles or singles, Mars, Jupiter details and anything else in the cosmos reddish in colour. 



#2 sg6

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Posted 16 February 2019 - 07:55 AM

Cannot think why one should specifically be.

Would however expect that the AR coatings will have an effect as they can end up biasing the actual wavelengths that get through, at least in percentage terms.

 

Then comes the glass properties, some will absorb one wavelength more then another. And again the color balance alters slightly.

 

Same arguements hold for eyepieces and these could be a greater influence. You will read of people commenting that eyepiece A was "warmer" then eyepiece B. In effect redder. My preference has tended to be for "colder", in that the greens and blues are prominent.

 

Without knowing the glass and I suspect other details it will be a case of peoples opinions. Part of the problem is that just about everything is based on getting the green lines through, what then occurs with red or blue is almost secondary so long as it is reasonable. Look at an astro-camera response, everyone I have seen is designed for Green and the Red and Blue has a lower response.


Edited by sg6, 16 February 2019 - 07:56 AM.


#3 junomike

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Posted 16 February 2019 - 08:06 AM

IME any Achromat using  "Red-shift" to control CA will better show reds.



#4 Hesiod

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Posted 16 February 2019 - 08:31 AM

If you mean which refractors show the least amount of spherical aberration at longer wavelenghts, I suspect that highly-corrected, slow astrographs such as the TOA/TSA designs could be worth a mention (as far as I remember, the TOA design should achieve apochromaticity* even a tad beyond the visual spectrum).

Probably the same could apply to the FOA60, even if could be matter of debate whether its very limited aperture allows it to qualify for the best at something.

Last, I would not discount your own DL: I feel reds in my sample to be really rich and vivid, but it is also a very good performer on Mars and Jupiter, within the limits of its 10cm

 

 

 

 

 

 

*in its original meaning


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#5 Erik Bakker

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Posted 16 February 2019 - 09:13 AM

Good question, often overlooked. This is where my Questar 7 showed me the power of the Maksutov optics over the A-P EDF triplet APO.

 

In doublets, fluorites show the best reds and the FOA60 leading that pack. Most ED doublets struggle a bit here, especially on Mars and red carbon stars. That said, my NIKON ED82 doublet is an exception, showing extraordinary colorful red stars and colorful detail on the planets.

 

The best refractors in this regard are the best triplets, with the TOA being king of that hill.

 

For the ultimate reds, a well coated and superbly figured Newtonian with very small central obstruction at thermal equilibrium in great seeing is perhaps your best bet. And aperture matters too. More aperture means more color. The best refractors add their superb contrast and unsurpassed light concentration in the airy disc to the equation.

 

Some good reading on the subject of red correction can be found on Roger Vine's excellent site here regarding ED vs Fluorite doublets. And here he compares the red correction on Mars in a Maksutov versus a Fluorite doublet.

 

This is also something you can observe during the day, in how vivid the colors (including red) appear in telescopes. And eyepieces or other visual accessories for that matter.


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#6 SeattleScott

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Posted 16 February 2019 - 09:22 AM

Jupiter red spot looks red in a 10” scope. It looks brown in my 6” Mak or 4” Apo. So aperture can matter too.

Scott
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#7 Scott Beith

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Posted 16 February 2019 - 10:08 AM

Jupiter red spot looks red in a 10” scope. It looks brown in my 6” Mak or 4” Apo. So aperture can matter too.

Scott

You beat me to it.  Aperture (due to increased light levels) brings out color saturation to our eyes.  

 

As for refractors that "bring out reds" I would choose the scopes that best reveal the true natural colors of the stars or planets.  As far as I know Tak TOA's are top of the line for color correction.


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#8 naramsin

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Posted 16 February 2019 - 10:48 AM

I was shocked at how much an old TV 17mm plossl (circle R) showed the GRS as deep, saturated red. This was in direct comparison with a Atro-Hutech Ortho 18mm (which itself is no slouch), the Astro-Tech 17mm High-Grade Plossl (which was disappointing), and a few others.

 

The setup: 6" F/8 Newtonian, which shows reds nicely, as expected; and an Orion 3X 4-element barlow.

 

TVs have a reputation for having a warmish tone, and that seems to translate into an advantage for red viewing, at least in my experience....

 

I should add that I enjoy seeing red stars in a super cheap 80mm F11 achro (giants like FZ Persei in the Double Cluster).


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#9 Moondust

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Posted 16 February 2019 - 11:09 AM

I believe the new Stowaway may qualify being optimized for a polychromatic Strehl ratio of 90% for the entire visual spectrum with a peak value of 99.8% at the visual peak. I remember back in late December and early January when Mars was extremely tiny but fairly well positioned in the sky I was able detect the polar cap and make out a subtle darker shading on the red disk with a 4mm ortho and 1.5x barlow. Things can only get better once it's back in opposition.      


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#10 25585

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Posted 16 February 2019 - 11:50 AM

Thank you everyone! I shall try out what I have of course, but now a TSA120 is more appealing than before. A TOA 130 would need a new mount, so barring a lottery win, the 120 is it.

 

Betelgeuse and Aldebaran (alpha Tauri), are great in the DL and Equinox100, 200 EDs. I will try for FZ Persei and others. Jupiter was good last summer but missed the spot somehow. What do I need for Barnard's Star and red dwarves + R, N, C in Pocket Sky Atlas. There seem to some reds in the Pleiades. Not seen many in globulars yet. I want reds to show up more. 


Edited by 25585, 16 February 2019 - 11:51 AM.

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#11 Hesiod

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Posted 16 February 2019 - 12:03 PM

In your boots I would rather look for a fine reflector or more frequent trips to really dark sites instead.

If reds are not bright enough in your DL, have the suspect that even adding 2cm will not bring much advantages.


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#12 mikeDnight

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Posted 16 February 2019 - 02:50 PM

Some scopes can be warm while others cool and this may give a different view of a planet or star. Some years ago one review on eyepieces,  revealed that TV plossl's brought out reds on Jupiter while RKE's gave a better, cleaner view of Saturn. So for planetary observing, eyepiece type can play an important part in revealing detail. Perhaps the eyepiece type/make would be worth experimenting with!

Personally I've found that larger apertures tend to reveal star colour more easily than smaller apertures, though a sharp refractor can produce some nice colour contrasts. 


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#13 gnowellsct

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Posted 16 February 2019 - 10:26 PM

If red carbon stars are what you want you're going to get more of them as you increase aperture.  And the ones that are already comfortably within reach of smaller scopes will take on greater color saturation.    Mars shows salmon color and various hues in all my scopes regardless of aperture from 81 to 356 mm.  Color saturation on Jupiter is certainly *easier* to get to know and view the available palette, it becomes easier to pick up at least hints in smaller apertures. 

 

There is tremendous variability in detection of color hues from one person to another, especially men, so on line discussions of color are pretty problematic.  

 

Go ahead and take this color test.  Take it several times till you get it figured out, don't count the first two times that you do it, so you get a hang for the procedure.  If you have color deficiencies in the eye you're likely never to be able to make zero mistakes, except as a statistical fluke.  My kid can't do it, unfortunately, which confirmed color tests he's had since kindergarten.  Neither can/could some other people in my life but I won't go into details.  Suffice it to say that when you're freaking out over the color display on Jupiter not everyone, even experienced observers, is going to be able to share fully in the experience.

 

Greg N  


Edited by gnowellsct, 16 February 2019 - 10:27 PM.

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#14 barbie

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Posted 16 February 2019 - 11:46 PM

I see reds very vividly in all of my refractors, no filters or anything else needed to enhance the color either.



#15 DeanD

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Posted 17 February 2019 - 12:59 AM

Thank you everyone! I shall try out what I have of course, but now a TSA120 is more appealing than before. A TOA 130 would need a new mount, so barring a lottery win, the 120 is it.

 

Betelgeuse and Aldebaran (alpha Tauri), are great in the DL and Equinox100, 200 EDs. I will try for FZ Persei and others. Jupiter was good last summer but missed the spot somehow. What do I need for Barnard's Star and red dwarves + R, N, C in Pocket Sky Atlas. There seem to some reds in the Pleiades. Not seen many in globulars yet. I want reds to show up more. 

Mars, Betelguese and red stars in general look very nice in my TSA102, so I assume the TSA120 would be similar. I think it would be hard to go wrong with one!

 

The design strehl ratios for the TSA102 are in excess of 0.94 for wavelengths as long as 677nm (with a max of 0.999 at 550nm). See: http://www.takahashi...-102.optics.php

 

All the best,

 

Dean


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#16 Redbetter

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Posted 17 February 2019 - 04:13 AM

For the stated purpose I would be wary of achros, as the red is on one end of the spectral spread, while the blue is on the other.   The red secondary spectrum aberration is only a fraction of the blue/violet as I recall, but it is there to be seen.  On the other hand...a red filter for Mars works well enough for long ratio small aperture refractors.  The problem with achros is that the unfiltered actual color is more distorted because of the secondary spectrum.   


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#17 Starlease

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Posted 17 February 2019 - 09:51 PM

If you want to easily see red colors everywhere you need to get the BBHS mirror diagonal. My w.o. 5 inch apo triple could easily see Saturn's moon Titan as reddish. Never saw colors on that moon with that small a scope.  And then there are all the reddish stars that just look bland in most scopes like all those red giants in the Double Cluster.



#18 francesco italy

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Posted 18 February 2019 - 01:58 AM

Are there any refractors that are good for reds particuarly? Red/orange stars - in doubles or singles, Mars, Jupiter details and anything else in the cosmos reddish in colour. 

 

Yes, of course. CdE corrected refractors, like Zeiss AS. They were made with Mars in mind.




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