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Refractors with reducers for use in NV

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#1 GOLGO13

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Posted 22 October 2019 - 12:43 PM

I have a refractor coming (replacing my 81s with a 103s). It has a reducer with it. But I remember folks saying the reducers that come with scopes may not be wanted when using NV. What's the story on reducers for NV use?

 

Thanks.


Edited by GOLGO13, 22 October 2019 - 12:43 PM.


#2 TOMDEY

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Posted 22 October 2019 - 02:10 PM

If you can take the objective lens off of your Gen3 and get the final image to fall directly onto the GaAs photocathode, without vignetting... that is actually a good way to go. Thing is, the 1.25-inch adapter itself and/or the H-alpha filter could restrict the fast cone. Those would be the things to check for --- or just try it! I actually like that direct use, better than the afocal using e.g. the TeleVue eyepieces and adapters. The less glass the better.    Tom



#3 Eddgie

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Posted 23 October 2019 - 12:32 PM

I don't see why a reducer would be any worse than using an eyepiece for afocal and most imaging reducers have broadband coatings that should get you pretty good transmission into near IR.

 

The C mount NV device will fully illuminate the photocathode at speeds up to about f/4 or so when used at prime focus. 

 

Now it will be a matter of personal preference, but I find that even at f/2.8, for visual use, the illumination falloff is not objectionable.  You can though easily see the illumination falloff at f/2.8 in a photo.

 

Since the vast majority of refractors cannot be reduced to speeds faster than about f/4.2 when using imaging reducer/correctors, this should not be an issue, but faster than this and the fully illuminated field starts to shrink, but again, it is very subjective as to where the cutoff for that illumination would be.

 

So, I don't know why you would not want to use a reducer other than the fact that simple reducers usually have poor off axis performance when coupled with faster refractors.  A good imaging reducer with field flattening might not give you the speed of some of the simple reducers out there, but the loss in focal ratio can be a very good compromise for people that like a high quality view out to the edge of the field.   I don't get a fully illuminated field in my 6" f/2.8, but the field is very sharp, and the vignetting is so minor that it does not show in visual use but the camera sees it.   

 

Anyway, there may be subjective reasons for not wanting to use a reducer, but it does work in that it does decrease the focal ratio.  


Edited by Eddgie, 23 October 2019 - 09:01 PM.


#4 bobhen

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Posted 23 October 2019 - 04:59 PM

Depending on the position of the reducer, the amount of reduction, your focuser size, your diagonal, whether you use afocal or prime focus might impact whether you can come to focus using a reducer.

 

My Tak 120 easily comes to focus with the .7 2” reducer used at prime focus because the Tak has a focuser extension that can be removed for a shorter light path.

 

My 102mm F5 refractor would not come to focus with the .7 2” reducer until I swapped out the standard focuser for a GSO 2” focuser that shortened the light path.

 

Bob



#5 GOLGO13

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Posted 23 October 2019 - 05:46 PM

I should mention I have .5 and .7 reducers...but this scope is coming with it's own. So I was wondering if I should try to use the one it comes with or stick with the ones I have since they will be closer to the NV unit.



#6 bobhen

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Posted 24 October 2019 - 07:29 AM

If the scope comes with a reducer, that means the scope was designed with at least some imaging in mind so the scope might have enough in-focus travel to use the reducer it comes with and without any fall-off issues. 

 

I think you are just going to have to try it and use whatever works or is best. It probably won't  take but a few minutes comparing before you'll know. 

 

Bob



#7 Eddgie

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Posted 24 October 2019 - 09:38 AM

So I was wondering if I should try to use the one it comes with or stick with the ones I have since they will be closer to the NV unit.

Why wouldn't you try them all if you already have them???

 

My bet is that the one that comes with the scope will have the best broadband trasmission coatings, and the best off axis performance, but if the faster ones produce a brighter image you might prefer them for dimmer nebula.

 

Heck, you have them and no one is going to call you ugly if you take them for a spin.  



#8 GOLGO13

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Posted 24 October 2019 - 10:18 AM

I'll give it a try. I'm just considering if I should sell it or not. I remember folks telling me those reducers wouldn't work and I needed something closer to the NV unit.




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