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Old 29-09-2020, 11:15 AM
Emuhead
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Large scope with reducer vs Native

Hi guys,

Theoretical question:

Ignoring the reduction in image circle size (as right now thats the only difference i can see), would the final image be more or less the same & captured in the same time if you had a large but slow scope combined with a reducer so that after the reduction it matched a small but fast scope's focal length & f ratio.

eg.
Scope 1 is 150mm f/8.25, with 0.66 reducer (ends up at 100mm f/5.5)
Scope 2 is 100mm f/5.5 native scope

Would the image circle difference be the only real difference here?

Last edited by Emuhead; 29-09-2020 at 11:39 AM.
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  #2  
Old 29-09-2020, 01:04 PM
jahnpahwa (JP)
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I'm a bit of a beginner, but I think scope 1 would stay at 150mm. The difference will be a focal length distance if the 150mm is at F5.5, and so is the 100mm (825mm vs 550mm). So your field will be different, smaller on scope one (given that you would be using the same sensor), but speed of exposure should be about/exactly the same.

Image circle here is, as far as I understand, dependent on the reducer and design on scope one, and the design of scope 2, you might be able to find that out by reading the specs of the scopes and reducer. Would you need to flatten scope 2 also? If so, the flattener will have a resulting imaging circle in its specs.

I use stellarium to tackle these sorts of questions, you can input your focal length, reducer magnification, sensor size, and then switch the view between them, see how different objects frame up. This is how I've chosen all of my scopes so far, which is probably sub-optimal, but I've not been able to look through many due to covid-19
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Old 29-09-2020, 02:18 PM
sunslayr (David)
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While the exposure time is the same if you were to crop to just the object the brightness would be different due to the different focal length.

Last edited by sunslayr; 29-09-2020 at 02:56 PM. Reason: Wrong
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Old 30-09-2020, 10:56 AM
Emuhead
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I'm not sure guys, I think f ratio determines everything, once focal length is the same, as it is after using a focal reducer. (Talking astrophotography only, with same camera).

Reducer reduces f ratio, and therefore focal length, and also image circle.

So pretty sure in my example, both scopes would produce exactly the same image (more or less).

Just wondering if a larger aperture scope once reduced gives more contrast etc. Any benefit other than being more versatile (if you ever do use it without a reducer).
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Old 30-09-2020, 11:20 AM
sunslayr (David)
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It your example the scopes have different focal lengths, 150mm aperture at f5 is 750mm while the 100mm at f5 is 500mm thus the different images. If instead the 150 was reduced to f3.33 then the focal length would match the smaller scope and have a brighter image. In terms of contrast, at the same focal length the larger scope would also have an edge, assuming both are the same type of telescope.
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Old 30-09-2020, 12:11 PM
Emuhead
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Yep, youre right, maths was wrong.

Essentially what i mean to ask is, is there any benefit to going a large scope with a reducer vs a smaller scope giving 'roughly' the same focal length and f'ratio. Like large aperture gives more contrast for example (even after reducer added to light path), or pros & cons of reducers vs native.

If for example i was happy to image at roughly 600mm focal length, then would it be recommended to get a native 600mm scope, or a large scope with a reducer for added 'benefits'.
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Old 30-09-2020, 12:26 PM
sunslayr (David)
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Same answer really, 'no replacement for displacement'. If your equipment can handle it and it has the focal length you want bigger is better. It would take a lot of glass to affect the image enough to offset a larger aperture.
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Old 30-09-2020, 12:31 PM
Emuhead
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Thanks, will keep adage that in mind.
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