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What’s the refractor sweet spot ?


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Hi All,

I have a Small refractor for wide field AP, just wondering what the next step might be in terms flexibility. An 80, 90 or 100 ?

I know the prices increase considerably with refractors so where is the sweet spot for AP, which should I be looking out for at the bring n buy sales ?

regards

Andy H

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22 minutes ago, Newforestgimp said:

Hi All,

I have a Small refractor for wide field AP, just wondering what the next step might be in terms flexibility. An 80, 90 or 100 ?

I know the prices increase considerably with refractors so where is the sweet spot for AP, which should I be looking out for at the bring n buy sales ?

regards

Andy H

I think a lot depends on the types of target you want to image and the camera you are using. Small refractors with short focal lengths are ideal for large targets, as the targets get smaller such as for smaller galaxies and globs etc you need a longer focal length, and to keep the focal ratio reasonably fast, the aperture inevitably goes up too.

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As refractor objective increases so does the f/ratio required to get decent colour correction so the "sweet spot" might be sub 130mm, larger refactors get unwieldy too requiring very expensive mounts. In my view the cutoff point is around 130mm where even a SW 130PDS or better still a Tac Epsilon 130mm will not only be faster but easier to mount, at 200mm and above it has to be a mirrored scope.

Alan 

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2 minutes ago, Alien 13 said:

As refractor objective increases so does the f/ratio required to get decent colour correction so the "sweet spot" might be sub 130mm, larger refactors get unwieldy too requiring very expensive mounts. In my view the cutoff point is around 130mm where even a SW 130PDS or better still a Tac Epsilon 130mm will not only be faster but easier to mount, at 200mm and above it has to be a mirrored scope.

Alan 

Now this is wisdom on a stick.

Well put, Alan.

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  • Cornelius Varley changed the title to What’s the refractor sweet spot ?

If we consider "capability vs budget" sweet spot for APO refractor, then I'd say this one is in pretty sweet spot:

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html

add in this flattener / reducer:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

You'll be hard pressed to find such aperture, color correction and field flatness at that price point.

 

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12 hours ago, Ags said:

Spotted this today. Totally convinced it is my sweet spot. Next step is to analyze it to death for three years, then buy something else... 🤣

https://www.teleskop-express.de/shop/product_info.php/info/p13519_TS-Optics-ED-Apo-96-mm-f-6-with-2-5-Inchl-RAP-Focuser---ED-Objective-from-Japan.html

Ooh now that looks promising !! 96mm, good bang for buck.

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54 minutes ago, Newforestgimp said:

Ooh now that looks promising !! 96mm, good bang for buck.

Do keep in mind that said scope will be good for visual or EEVA but not for astrophotography.

It uses FCD1 glass and it is larger aperture fast doublet scope. It will have significant color fringing in astrophotography applications if you shoot broad band. For narrowband only it will probably be good choice.

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24 minutes ago, vlaiv said:

Do keep in mind that said scope will be good for visual or EEVA but not for astrophotography.

It uses FCD1 glass and it is larger aperture fast doublet scope. It will have significant color fringing in astrophotography applications if you shoot broad band. For narrowband only it will probably be good choice.

Invaluable detail again Vlaiv, I think to be honest if I go too much beyond a 90 I’ll be into buying another mount too to cope with the payload demands.

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51 minutes ago, vlaiv said:

Do keep in mind that said scope will be good for visual or EEVA but not for astrophotography.

It uses FCD1 glass and it is larger aperture fast doublet scope. It will have significant color fringing in astrophotography applications if you shoot broad band. For narrowband only it will probably be good choice.

Indeed I am concerned only with visual performance, small size and low weight - just 3 kg.

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32 minutes ago, Newforestgimp said:

Invaluable detail again Vlaiv, I think to be honest if I go too much beyond a 90 I’ll be into buying another mount too to cope with the payload demands.

That is true, I was going to suggest this scope:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

I have it and it is really excellent little scope, however, I fear it won't be as much difference to your WO61. Both are wide field scopes AP scopes.

If you want to get into medium resolutions with refractor, you really need to go 100mm+ and that will get you below 2"/px down to say 1.6-1.8"/px range. However, that would require something like Heq5.

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7 hours ago, vlaiv said:

That is true, I was going to suggest this scope:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

I have it and it is really excellent little scope, however, I fear it won't be as much difference to your WO61. Both are wide field scopes AP scopes.

If you want to get into medium resolutions with refractor, you really need to go 100mm+ and that will get you below 2"/px down to say 1.6-1.8"/px range. However, that would require something like Heq5.

My eqm35 has had the Darkframe Ltd Stella drive treatment so sub arc sec guiding should be no issue and I’m assured by Darkframe I can use the full 11kg payload for AP now. Even so I wouldn’t like to run it at its new upper limits for AP.

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  • 1 month later...
9 minutes ago, mauzito said:

Interesting.

I wonder what is your source?

Someone who owns both, he sold the Baader. Mind hair’s breadth between them. He also had specialist application of NV. For visual very close.

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