9 replies to this topic

[Highburymark]

Yesterday was the first time I’d compared a prism binoviewer (Maxbright II) with a mostly mirror BV (Zeiss CZAS) in my TV85 - on Venus. Set up was:
[TV85 - BBHS mirror diagonal - 2.6x GPC - BV - Tak TPL 12.5mm eyepieces].
With the Maxbright, (120x), there was significant CA, more than I’d ever seen in this scope before, purple and yellow, together with a small amount of atmospheric color. I was surprised when I used the CZAS, at around 140-150x with the longer light path, the false color was almost completely cleaned up. Because I do most of my binoviewing with a colourless triplet, I’d never appreciated the difference that choice of BV could make on bright targets when CA is present.

[denis0007dl]

Welcome to the club

[Highburymark]

Thanks Denis - I’ve had the CZAS for years, but had never done this particular test before.
Why a surprise? Well - I’d always presumed that GPCs would provide optimal results with a Baader binoviewer. This time, it was as if the GPC had been specifically designed to work with the Zeiss BV. The difference in false color between the two was very significant indeed.

[denis0007dl]

Yep, direct comparation is best to realise difference.

Actually using all prisms binoviewers turning your APO telescope an ED one, and on some cases, even worse.

Im glad I discovered these mirrors heavens,

thus Im loving to have FAT PRISMS binoviewers in collection 🙃

[Eddgie]

Actually using all prisms binoviewers turning your APO telescope an ED one, and on some cases, even worse.

 

 

 

Unless a GPC is used. 

 

I use a color free Apo and for much lower power observing. I don't use a GPC, and the color error from the binoviewer is simply not visible except on bright stars but I use the scope in this configuration for wider field viewing, which is typically done at lower powers. 

 

For double stars and all solar system observing, I install the GPC, and the view is completely color free (I also use a mirror diagonal).

 

A Barlow will do much the same thing. 

 

The longer light path of a mirror system also means that someone without a binoviewer ready telescope might have to use a Barlow to reach focus anyway, so using a GPC is no disadvantage.

 

Also, I am not aware of any binoviewer sold today that uses mirrors with high quality dialectic coatings, and the transmission loss in these systems is higher than with a good prism binoviewer. 

 

I think a good mirror system is a viable alternative, but you can use a prism system and not change the performance of your Apo. 

 

In all of my writings where the topic comes up, I almost always recommend that a GPC be used for high resolution observing, even in scopes that do not need a GPC to reach focus. If a prism diagonal is being used, I recommend that the GPC be placed in front of the diagonal for high resolution observing. 

Edited by Eddgie, 23 February 2025 - 11:30 AM.

[denis0007dl]

Adding any GPC does NOT completly remove false colours in all prism system binoviewers.

Does GPC helps removing most of false colours there?
Yes!

[betacygni]

I’d be curious if anyone knows (ignoring any special GPC optical prescription) what effective focal ratio would have a complete elimination of any theoretical prism induced aberrations. I’m assuming this is a well understood concept (for those knowledgeable) regarding the effective focal ratio and the prism size as it relates to aberrations. I’ve used both mirror based and prism based binoviewers and haven’t noticed a color difference. That said I’m almost always at f15 or more.

[Highburymark]

This was the reason why I posted.
I have a completely color-free TSA-120 which has never had any issues with either the MBII or the CZAS.
But the combination of TV85, BBHS mirror diagonal, 2.6x GPC, and MBII was not a good one. I should add that the TV85 is a nice example - with single eyepieces, it shows the planets beautifully.
Yet with a mirror binoviewer and 2.6x GPC, the CA almost disappeared.
I’ve checked the MBII subsequently to make sure it’s well collimated.

[Eddgie]

Adding any GPC does NOT completly remove false colours in all prism system binoviewers.

Does GPC helps removing most of false colours there?
Yes!

I have a 130mm FPL-53 Triplet. Without the GPC, there is considerable color error on Sirius with a violet glow around Serious that is easy to see.

 

With the GPC, the star is perfect. 

 

If the scope itself has color error, then that will always be there, and few scopes sold as "Apos" are completely free of color error. An FPL-53 or FCD-100 or other SD glass triplets will almost always be perfectly color free (when used visually). The color error with the binoviewer in faster SD triplet scopes will be easy to see, and the GPC will fully correct it. 

Edited by Eddgie, 25 February 2025 - 08:14 AM.

[Spikey131]

When observing the moon and planets with my AP130 and Maxbright 2 binoviewers, I use the 2.6 Baader GPC behind the Zeiss T2 prism diagonal.  There is no perceptible color error.

 

When observing large deep sky objects, I can remove the GPC and at low power, the star colors are true.

Edited by Spikey131, 02 March 2025 - 07:54 AM.