15 replies to this topic

[CrazyPanda]

At some point I'm going to have an F/3-ish dob. Currently I'm achieving focus with my CZAS binoviewer using a 2x PowerMate, without a Paracorr.

 

In the past I've noted the advantages of coma correction at F/4.5 even very near to the center of the field, so at F/3-ish, I don't think I can get away without coma correction.

 

I'd rather not combine both the Paracorr and a barlow together. The magnification factor starts getting too high, and so does the weight. I'm looking for an all-in-one solution, and the only one I've found is this one by APM:

 

https://www.apm-tele...tric-15x-barlow

 

But the price is fairly steep given it's not a Tele Vue/Baader/Nikon etc. My main concern is overall optical quality. Fixing coma but adding spherical aberration would defeat the purpose, for instance.

 

The backfocus distance appears to be set at 105mm, but the light path through the CZAS is more like 135mm. So to me this means that coma correction won't be optimal, and I'd be worried about other "de-optimizations" sneaking in.

 

Anyone have any experience using this with their binoviewer in a Newt?

[noisejammer]

It may be possible to remove the second barrel and screw the optic directly onto your binoviewers. APM has direct access to the designer (Philipp Keller), so you may want to ask the question directly.

 

I have purchased several optical items from APM including an apo and half a dozen eyepieces. They offer very good stuff.

 

Be aware that even at f/4.5, most of the field will be vignetted by the entrance pupil of your binoviewers.

[wcw]

I can highly recommend the Siebert Optics OCA for binoviewers. My scope is F/4.2 but Harry Siebert claims it works at F/2.9.

The part number is 2OCA1.3x38

The magnification factor is only 1.3x, so you get wider FOV compared to 2x.

You can order it with a T2 thread to attach to a binoviewer if that is what your binoviewer has.

 

https://www.sieberto...Optics-OCA.html

 

Scroll down that page until you find this:

 

B1) 1.3x Large 38mm CA OCA for

Newts & Refractors,  $379.

(2OCA1.3x38)  For F2.9 scopes

on up.

 

 

-Bill

[CrazyPanda]

I can highly recommend the Siebert Optics OCA for binoviewers. My scope is F/4.2 but Harry Siebert claims it works at F/2.9.

The part number is 2OCA1.3x38

The magnification factor is only 1.3x, so you get wider FOV compared to 2x.

You can order it with a T2 thread to attach to a binoviewer if that is what your binoviewer has.

 

https://www.sieberto...Optics-OCA.html

 

Scroll down that page until you find this:

 

B1) 1.3x Large 38mm CA OCA for

Newts & Refractors,  $379.

(2OCA1.3x38)  For F2.9 scopes

on up.

 

 

-Bill

Does it correct for coma though?
 

[wcw]

Yes it does correct coma. Works beautifully on my scope.

[CrazyPanda]

If I search for "coma" on the page, I see no mention of that word. Is there a page somewhere that gives more information on that model?

[wcw]

Search for the part number 2OCA1.3x38

 

Harry is very helpful if you have questions.

[wcw]

Here is a CN thread about it:

https://www.cloudyni...s-and-38mm-oca/

[CrazyPanda]

Here is a CN thread about it:

https://www.cloudyni...s-and-38mm-oca/

Thanks for the link.

 

This post indicates that it doesn't correct for coma. It wasn't used with a Paracorr and there was coma visible at the edges.

 

https://www.cloudyni...-oca/?p=6656867

 

I sent Harry an email to ask him.

[CrazyPanda]

Harry got back to me. Here's his reply about the 2OCA1.3x38:

 

Hi


I will give a flatter field and improve coma a bit as well. But it will not correct all coma in fast telescopes.

 

So it essentially does not correct for coma.

[K.K]

I had APM Coma Correcting Barlow and Baader GPC 1.7x for Newtons.
When I used it with the 20cm F4.5 Newtonian, Baader seemed to perform better.
I think this is because Baader corrects the color fringes that occur in the binoviewer prism, and APM is mainly used for photography, not for binoviewers.
APM also had good coma correction, but when used with F3, color fringes of prism may be noticeable.
Unfortunately, Baader stopped producing GPC in 2022, so now you have no choice but to look for a used one. It seems that Baader is currently considering developing a successor.

[CrazyPanda]

I had APM Coma Correcting Barlow and Baader GPC 1.7x for Newtons.
When I used it with the 20cm F4.5 Newtonian, Baader seemed to perform better.
I think this is because Baader corrects the color fringes that occur in the binoviewer prism, and APM is mainly used for photography, not for binoviewers.
APM also had good coma correction, but when used with F3, color fringes of prism may be noticeable.
Unfortunately, Baader stopped producing GPC in 2022, so now you have no choice but to look for a used one. It seems that Baader is currently considering developing a successor.

The reports I've read about the 1.7x Baader GPC indicate it may produce some spherical aberration which is why I've stayed away from it.

 

My binoviewer is the Zeiss "apo" and it's mirror-based, so I don't have to worry about doing actual glass path correction for prisms. I just need something to help me reach focus while also correcting coma, but without creating a massive multiplier in the process.

Edited by CrazyPanda, 06 July 2024 - 08:22 AM.

[K.K]

The reports I've read about the 1.7x Baader GPC indicate it may produce some spherical aberration which is why I've stayed away from it.

 

My binoviewer is the Zeiss "apo" and it's mirror-based, so I don't have to worry about doing actual glass path correction for prisms. I just need something to help me reach focus while also correcting coma, but without creating a massive multiplier in the process.

APM1.5x might work well, but when I tried it on binoviewer it was about 110mm against F4.5 so I don't know if it will work with your F3+135mm without trying it.

[Fivemileshigh]

I have one, it works well with the MarkV at f/3. Coma correction is very good, comparable to the Paracorr. I have not done extensive testing though.

[CrazyPanda]

I have one, it works well with the MarkV at f/3. Coma correction is very good, comparable to the Paracorr. I have not done extensive testing though.

I'd be curious to know if there is any introduction of spherical aberration or generally how sharp you feel it is on axis.

 

The spot size diagrams of the barlow indicate a fairly large spot compared to the Paracorr which is another of my main hesitations.

[Fivemileshigh]

Hi Sorry for the late reply. As I said, I unfortunately did not get a chance to do extensive testing but my initial impression was very good. I ended up going back to mono on the 20” f/3 because it’s built quite light and the extra weight of the binoviewer plus eyepieces would require a complete redesign and rebuild of the structure. I am working on a f/3 binoscope now, hth.