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APM 100 and 120mm. Semi apo compared to SD apo series

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#1 dozerdan

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Posted 15 January 2022 - 06:11 AM

Looking at APM binoculars.  90 degree.

Considering both 100mm and 120mm.

I am asking what difference in perform is seen between the Semi APO and the SD APO.

Within the Semi APO is there a gain in moving up from the 100mm to the 120mm, both being the Semi apo.

Or is it best to go with the 100mm SD APO.

Truly asking comparison of Semi apo to the more costly SDapo in the 100mm series.

 

 



#2 astrokeith

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Posted 15 January 2022 - 06:33 AM

Its a good question.

I dont have the APM's but many years ago I went through the same decision on Miyauchi 100mm binos, a choice between ED and Fluorite APO with a big price jump.

 

Fortunately at the time the vendor sent me both and I had a couple of clear nights to compare. The APOs easily won.

 

Twenty years later, the decision would be very different. My eyes have deteriorated and I know they are the limiting factor. Sometimes I look through my scope or the Miyauchi's and get upset about how much I have deteriorated - its so obvious now.

 

So your question needs to be considered along with your eyesight (and your expectations)


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#3 drt3d

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Posted 15 January 2022 - 08:26 AM

Do you like chromatic aberration? I don’t, so the answer is clear for me.

#4 ArsMachina

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Posted 15 January 2022 - 08:38 AM

Semi Apo is another name for Achromat.

So the Semi Apo is the lowest quality, the ED is better and SD is the best.

Which one to get depends on your needs and expectations.

 

Jochen


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#5 Nightfly

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Posted 15 January 2022 - 02:45 PM

I have the 100mm Semi-Apo which is an achromat.  CA is well controlled,  for an achromat. It does well for low to medium powers.  A great sweeper of the Milky Way.   I've been happy with it for such use.  I'd expect a bit more CA from the 120.  Neither would be good for planetary.  I do get good contrasty lunar views with just a tinge of color really.

 

For lunar and planetary, and the absolute best deep sky views,  the Apo no question.  I've got deep dark skies, but small pockets, so the semi-apo provided very well for my needs.  I'm glad that option is out there.    


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#6 Beg

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Posted 15 January 2022 - 03:27 PM

I have both an SA and an ED 100mm BT. Both are excellent for sweeping the star fields. Both have excellent contrast and general sharpness. The main difference between the two is CA can become present with daytime views, and Lunar and brighter stars and planetary. If that is important to you. Other than that light and dark Nebula and clusters with both are very good and a lot of fun.


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#7 ECP M42

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Posted 15 January 2022 - 04:31 PM

I have the 100mm Semi-Apo which is an achromat.

I'd expect a bit more CA from the 120.  

For what reason? 

 

 

I do get good contrasty lunar views with just a tinge of color really.

But isn't it for this reason (avoid seeing the color fringes) that SD glass is preferable?

 

Sorry if I point this out (not to turn the knife into the wound), but the OP asks for exactly an estimate of the visual differences of the color fringes between the two models Semi-APO (marketing term) and SD glass. smirk.gif  



#8 Rich V.

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Posted 15 January 2022 - 04:47 PM

I agree with Nightfly's statement; both BTs are f/5.5 so I'd expect the 120mm achromat to show more CA than the 100mm. CA increases with aperture for any given focal ratio; that's why focal ratio is increased with larger lenses to maintain the same level of chromatic correction.

 

Of course, "ED" or "SD" glass will dramatically lower the CA over an achromat in either size.  That's the point. 

 

Rich


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