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what is the advantages of this configuration ?

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#1 delorehal

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Posted 15 July 2020 - 01:25 AM

Here :

https://explorescien...iant-2221347844

 

OTA is shorter but requires a big flat mirror instead of a diagonal



#2 Tapio

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Posted 15 July 2020 - 01:33 AM

Really don't know what the benefit is but at f/4.5 it will sure show serious CA.


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#3 sg6

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Posted 15 July 2020 - 02:18 AM

Looks like they have just added a fairly large diagonal in the tube to direct the light to the focuser. Would therefore I guess make the OTA longer as a normal refractor is OTA+Diagonal whereas this is in effect just OTA with a diagonal inbuilt.

 

Probably more robust. Disadvantage would be you cannot alter the diagonal, likely cannot use 2" eyepieces as the required position of the eyepiece changes with physical size. OTA I guess has to be used on an AltAz - you cannot reposition/rotate the focuser.

 

One advantage is the user cannot do a lot to it.

 

Really looks like someone has rather simply moved the diagonal up inside the OTA, then glued a focuser on the top surface as you still have Lens-tube-diagonal-focuser. Exactly as in a more common refractor arrangement.



#4 futuneral

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Posted 15 July 2020 - 02:20 AM

As the description says - it's an "ideal compromise"

I would guess the goals of the design were: 1) refractor 2) not less than 102mm 3) portable 4) dirt cheap

So they figured out the objective lens part of the equation, but that probably required a massive 3" focuser which are expensive. So they cheated by putting a mirror there. Plus a mirror is probably also cheaper than a diagonal.


Edited by futuneral, 15 July 2020 - 02:21 AM.

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#5 Rutilus

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Posted 15 July 2020 - 04:23 AM

Reminds me of something from the book (all about telescopes) by Sam Brown.



#6 PETER DREW

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Posted 15 July 2020 - 04:31 AM

I recently built a 6" F5 binoscope using a similar principle.  I had to do this as the number of prisms necessary for 90 degree viewing angle and IPD adjustment meant that even a 2" diagonal would vignet the light cone, I had to use 3" minor axis diagonal mirrors.  I can't see the same advantage with the Bresser offering apart from shortening the tube.



#7 ButterFly

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Posted 15 July 2020 - 05:14 AM

You can't forget the diagonal at home?


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#8 SeattleScott

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Posted 15 July 2020 - 11:27 AM

I suspect the goal was keeping the tube short enough to fit the backpack.

Scott

#9 Jon Isaacs

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Posted 15 July 2020 - 11:48 AM

As the description says - it's an "ideal compromise"

I would guess the goals of the design were: 1) refractor 2) not less than 102mm 3) portable 4) dirt cheap

So they figured out the objective lens part of the equation, but that probably required a massive 3" focuser which are expensive. So they cheated by putting a mirror there. Plus a mirror is probably also cheaper than a diagonal.

 

I think you nailed it. 

 

At F4.5, vignetting concerns will require a large diameter drawtube. This eliminates the drawtube before the diagonal.  It's cheap.. 

 

Jon


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#10 Hesiod

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Posted 15 July 2020 - 11:49 AM

It avoid the buyer to waste money by upgrading the diagonal...


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#11 howardcano

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Posted 15 July 2020 - 12:30 PM

I think this arrangement is a good idea, as long as it's used with mounting rings so the OTA can be rotated to a comfortable position.  As Jon says, it gives better field illumination at low powers than is usually possible with the focuser and diagonal of a "normal" refractor.

 

My sample of this particular scope didn't impress me optically.  Even when stopped down to 70mm aperture, the image was soft.


Edited by howardcano, 15 July 2020 - 12:32 PM.


#12 junomike

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Posted 16 July 2020 - 06:22 PM

Along with the advantages, no one noticed (or pointed out) the huge disadvantage (IMO)......1.25" Focuser.

IMO these OTA's are best at wide field DSO's which will require a 2" eyepiece (to get the widest views).

 

For that reason I went with the Bresser Cousin which requires a Diagonal but has 2" capability.


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#13 stevew

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Posted 16 July 2020 - 09:34 PM

Here :

https://explorescien...iant-2221347844

 

OTA is shorter but requires a big flat mirror instead of a diagonal

If your looking for a short focus achromat this would be a much better scope in the same price range. https://www.telescop...yCategoryId=331



#14 Steve Cox

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Posted 16 July 2020 - 10:29 PM

You can't forget the diagonal at home?

No, but you could still forget and leave your eyepieces at home.




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