7 replies to this topic

[whwang]

Hi,

 

There is a blog post from Hutech about BORG's new 0.72x reducer:

https://digiborg.wor...r-redecer-7872/

 

In the post, there is a statement:

 

BORG’s 1st optical design based on F-ratio (All of BORG reducers/flatteners used to be designed for focal length)

 

Does anyone know what this means?  My understanding is that most doublet refractors of similar focal length have similar field curvature, so they can share a common reducer or field flattener.  So it makes sense if a reducer is designed for a certain focal length.  However, this one claims to be designed based on F-ratio.  Does anyone know what is the principle behind this?

 

Cheers,

Wei-Hao

[Denimsky]

Isn't the field curvature different for different focal ratio even with s same focal length?

 

I think this means that the new reducer is optimized for their f5.6 scopes: 90fl and 71fl.

Borg reducers and flatteners have been kind of universal design which work for most of their scopes (and other brand scopes) with different spacing.

This time they went for a dedicated design and they are also working for another dedicated reducer for their f4.5 scopes: 67fl and 55fl.

[whwang]

Hi,

 

Thank you for the reply.  I read from the book "Telescope Optics" that the curvature radius for doublets is usually between 0.3x to 0.35x the focal length.  I wonder how it changes with focal ratio.  Do you have any reference so I can look it up.  I am quite interested about this.

 

Cheers,

Wei-Hao

[Denimsky]

There are much better suited members to answer this but do we see more field curvature on a 10" f/5 scope than a 5" f/10 scope?

That's what I've always thought (the high f ratio scopes are more flat).

[whwang]

Hi,

 

As far as I can tell, focal ratio determines the depth of field.  On a large focal ratio refractor, the image can remain sharp even if the sensor is somewhat away from the focal surface.  So the 10" F5 and 5" F10 scopes can have very similar amount of field curvature, but it will be much less apparent on the 5" F10 one in terms of image sharpness.  The amount of field curvature, and the amount of image blurring caused by the curvature, are two separated things.

 

However, my knowledge is quite limited.  My understanding above may not be complete.  This is why I am curious and looking for answers here.

 

Cheers,

Wei-Hao

[rainycityastro]

Good question. Field curvature should be roughly proportional to focal length. Consequently the same field flattener should work quite well on scopes with similar focal lengths.

 

However different scope designs will have slightly different  monochromatic/Seidel aberrations (coma, astigmatism, field curvature, spherical, distortion). Spherochromatism may also be quite different. An F5 scope will have significantly higher aberrations even if perfectly corrected for longitudinal CA on axis.

 

So in theory a field flattener can be optimized very specifically for the scope in question.  

[whwang]

Thank you!

 

Please allow me to ask more.  I think it is totally reasonable if two scopes of the same focal length have somewhat different behavior on the off-axis aberration and on-axis spherochromatism, depending on the details of the design.  This is of course not simple.  However, the statement from BORG seems to indicate that focal ratio is the most important factor (other than focal length, perhaps) to determine all these, not something else in the design formula (lens spacing in the doublet, first element concave or convex, etc).  Do you agree with this interpretation?  This is still something new to me, and sounds very interesting.  

 

Cheers,

Wei-Hao

[Denimsky]

I found this from Japanese Borg site.

"

It is this product, but in fact there are various "first BORG history". First first one first is that it is "the first of the F value-based design of the correction lens". Correction lens of traditional BORG Although it was the focal distance-based, there was also the case that can not be correct aberrations in bright reducers. This reducer design based on the brightness of F5.6. This, I realized a more high performance Yuku on traditional reducers.

"

 

http://translate.goo...QJoW4KX7LpdW0sQ

 

The google translation is not that good but as Rainycity mentioned, it seemed that they tried to correct different types of aberrations.

 

This reducer is for 90fl and 71fl but the optimal backfocus is different for each scope.

For 90fl, you remove a thin spacer to get the optimal backfocus.

Edited by Denimsky, 21 March 2015 - 11:14 PM.