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Guide scope vs lens

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#1 AllStarez

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Posted 06 August 2020 - 08:24 AM

Hello there,

 

I see a lot of people using guide scope as an alternative to camera lens with their NV device, and I  was wondering if there is any big difference into viewing through a guide/finder scope versus a lens (with kind of the same magnification) ? Which one will give the better image quality ? If there’s any.

 

For example a 50mm guide scope (let’s say  about 160mm focal f/3.2) compared to a 160mm camera lens (with a similar f/) ?



#2 GOLGO13

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Posted 06 August 2020 - 09:43 AM

I'm interested in this also. I can say my 50mm guide scope does have some issues. Stray light gets in the system very easily. I'm not sure where to buy a dew shield for it. I tried to make one, but at this small size it has proved a bit tough. I'll keep trying.

 

But the aperture is pretty small and I'm guessing a larger camera lens would do better. 

 

While NV helps improve smaller aperture, I find aperture still makes a decent difference.



#3 Mazerski

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Posted 06 August 2020 - 10:07 AM

I have 50mm CCD Finder (@ f/3.8) and Nikon 200mm (@ f/4) and I think both do well, provide sharp image to edge of view. If there is a difference it has to be subtle as I can’t tell. I have LP to deal with and maybe in dark sky any differences would show.

 

As for stray light / dew shield... The Finder I have has permanent shield that protrudes 2 inches above lens. If I needed more, I would make a shield out of Reflectix and slide over Finder but so far, I haven’t noticed any stray light.



#4 Eddgie

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Posted 06 August 2020 - 10:31 AM

I use both types and both have their advantages and disadvantages.

 

Let's compare two leneses, a Nikkor ED 180mm f/2.8 and a 60mm CCD Guide scope, that I am guessing is about 220mm in focal length (maybe a bit more) and I am guessing around f/4.1 (about 250mm). 

 

The Nikkor is an ED scope does not have to be used with long pass filters and maintains much of its sharpness, while the guider has a cemented doublet, and benefits from long pass filters. Under light polluted skies there is no advantage here for the ED Nikkor, but under dark skies, the Guider needs to be filtered, while the Nikkor does not have to be.

 

The Nikkor is very sharp off axis. It is a lens that was designed to provide excellent illumination and corner of frame performance over a 35mm full frame.   The cemented and unflattened  Guider is slighly comatic (if it were air spaced, this could be corrected) and has some field curvature, but it is not terrible, just not as sharp as the Nikkor.

 

The Nikkor, being f/2.8, is a bit brighter on nebula. Not a lot, but enough that you can see it is a faster system.

 

Now for the advantages of the Guide scope.   First it is lighter.  The Nikkor weigh 32oz vs 25 oz or the guider. 

 

The ergonomics of the guider are superior. SLR lenses have a focuser ring and it is easy to move that ring and while hand holding, on this particular Nikkor lens, the focus is done by a large rubber knurled ring, and it is easy to disturb that ring while panning around.  The guider by comparison has a microfocusing ring at the rear so that the entire tube of the guider can be used as a hand hold or a place to mount rings and nothing disturbs the focuser because it is very near the front of the NVD.  Also, the micro-focuser is better for getting exact focus.  

 

The Guider also provides a bit more magnification than you could probably get in a hand held SLR lens that would be light enough to hold.  The 50mm is much lighter and closer to the Nikkor in focal length. 

 

The guider, when used at prime focus, puts the filter on the front of the 1.25" nose and it is quicker and easier to change the filter and depending on the guider, it might even be possible to mount a filter wheel or filter drawer, though I have not done this to mine. 

 

The guider costs a lot less than a good SLR lens.

 

Now the important thing is that it is not an "Either/Or" proposition. I have two SLR lenses that I use a lot and I have the guider that I use some.  One of my most used SLR lenses is a Vivitar SMS f/4.5 70mm to 210mm zoom that cost me $30, including shipping.  It is lighter than either of these lenses (19 oz) and while not great for nebula, it is fine for brighter nebula but being a zoom, it is a really nice general use scope for working the dark rift and being able to locate features then zoom in on them.  The Vivitar Series 1 is f/3.5 and is a very respected lens, but it is considerably heavier than the Nikkor 180.  Still, you get a zoom and more speed, but the Nikkor is already pobably as much as I want to hand hold. 

 

The SLR lenses though are typically better optically and faster, but heavier, more expensive, and a bit more complicated to use due to filter changing. 

 

I use them all and find that all have their strengths. 


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#5 GOLGO13

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Posted 06 August 2020 - 10:33 AM

I have 50mm CCD Finder (@ f/3.8) and Nikon 200mm (@ f/4) and I think both do well, provide sharp image to edge of view. If there is a difference it has to be subtle as I can’t tell. I have LP to deal with and maybe in dark sky any differences would show.

As for stray light / dew shield... The Finder I have has permanent shield that protrudes 2 inches above lens. If I needed more, I would make a shield out of Reflectix and slide over Finder but so far, I haven’t noticed any stray light.


Interesting. Mine is the Orion version and it lets in stray light very easy. I will try again making a few shield.

#6 Mazerski

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Posted 06 August 2020 - 11:01 AM

GO,

 

I have (I assume) a typical Chinese Version finder with Helical Focuser (I sold the rings and baseplate). Have used Reflectix for telescope finder at times on very humid nights and it works so this should work with your finder & stray light.

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#7 Eddgie

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Posted 06 August 2020 - 11:09 AM

I have no issues with stray light either.  In my 60mm, the lenses are recessed a good 50mm from the front of the dew shield.  I would think I would have to get a light source quite close to the optical axis line to get light into the tube.

 

Is it possibly some issue with the the finish of the inside of the dew shield?   Maybe wrap some black felt in there??



#8 AllStarez

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Posted 07 August 2020 - 03:28 AM

Thanks for all the helpful posts, i get it more clearly now. I really though there were much difference visually, with better results using a guide/finder scope, but it is not that easy.



#9 PEterW

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Posted 07 August 2020 - 12:07 PM

No straylight.... you lucky then!! I have a foot long flocked lined black plastic tube I take out to block any stray photons... I do have some nasty local lights and it makes a noticable difference. I need to make a smaller version for the lower power optics I use. I mainly use a focal (and length) reduced f4 80mm jumbo finder, possibly working at f2.5. Seems to frame the larger nebulae quite well.

Peter


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