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Hello 

I have a rookie question, here goes, does a focal reduce reduce the focal length  of your scope or increase FOV ? Also how do you determine the size you need 

clear sky's 

Ken

Edited by 10-7
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It does both. But the latter is hindered by the physical characteristics of the optical path and you may get vignetting. 

Most scopes have a dedicated focal reducer which is matched to that scope.

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I would say neither. The image still comes out the end of the scope so the focal length isn't say halved. The final image is reduced in size to match that size as if the focal length were reduced. However it still comes out the end of the same length scope tube.

You can consider that is the actual focal length was halved then the image would be inside the scope tube. It would also be before the reducer had been able to do it's job.

The field of view will usually be increased but by exactly how much I am unsure as I would expect that it depends on the field of view of the scope initially. If the scope had a maximum of 5 degrees, dependant of aperture and focal length, then adding a reducer will not enable say a 7 degree field to be produced.

Easy way to think about it is to incorporate the phrase "as if" into it all.

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I'd agree with jambouk that it won't necessarily increase the field of view because there may be other limitations of the optics that prevent doing so in any useful manner.

It must reduce the effective focal length of the entire optical train when compared to not having a reducer however, because of the way focal length is defined.  If it didn't then it wouldn't be acting as a reducer.

James

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It reduced the focal length of the telescope as seen by the eyepiece or camera, but not the physical focal length of the telescope.  It does this by compressing the image circle into a smaller footprint.  That's why they're also referred to as telecompressors.

@10-7You never mentioned if you intended to use it with eyepieces or a camera or even with what type of scope.  The focal reducer/correctors for SCTs work pretty well with 1.25" eyepieces.  Refractor focal reducers are mainly intended for astrophotography.  The cheap, generic 0.5x focal reducers don't work well at all with most eyepieces and telescopes.  They are mainly intended for very small imaging chips.

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  • 2 weeks later...
On 04/10/2020 at 20:02, michael8554 said:

You can enter your equipment details in a FOV calculator.

Then change your focal length to FL x O.XX, where 0.XX is the FR ratio, and that will give you the reduced FOV.

https://astronomy.tools/calculators/field_of_view/

 

Still on topic as a rookie looking at the need for a focal reducer.

But I can't match my real views with the FOV calc.

astronomy_tools_fov.png.82ea94242c52da964b42a2e0dacce16e.png

 

In the image above, the Sun is shown as fully visible on my SE8, but I came to SGL to see how a focal reducer would help me get to that point :o  i.e. I have only part of the Sun in view as there is too much zoom.

I have two EP, the Celestron supplied 25mm and a Hyperflex 7.2mm-21.5mm Eyepiece (hence what I setup in the FOV Calc)

Missing from the equipment entry list is the  90 degree adaptor but that is 1:1 and only flips the image.

What mistake am I making?

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it might be down to the way your scope focuses. An SCT focuses by moving the primary mirror. This in turn changes the focal length of the scope. The focal length in the specification (and used in the FOV calculator) is just a nominal value. Depending upon which diagonal and eyepiece (and any other component) you attach, you will need to move the mirror by differing amounts in order to bring the image to focus (as well as how far away the target is). That will change the actual focal length away from the value used in the FOV calculator, and your actual view will be different to the calculator view.

 

Tim

 

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Sorry the question was unclear.

I found this post while looking for help with how a Focal Reducer could help my observations.

The FOV calc shows my two eyepieces should have full views of the Sun.

In real life the Sun is too large viewed by either.

I asked if I was making a mistake in my FOV calc setup.

astrotim suggests I am not making a mistake but that I should allow for how my focuser is adjusted/positioned - and that I had not done.

Edited by kevenh
Confused Username with ranking and clarity
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There was some PICNIC

In today's Sun viewing my kit 25mm  Plössl eyepiece could show a full sun disk (sunspot 2776 seen ✅ )

So now I am where I wanted to be Wednesday: How to get my 760D DSLR able to see a full disk sun?  I'll get my hands dirty with a little bit of my own research...

Thanks for the feedback my earlier posts gained👍

 

edit: OK, a f/6.3 focal reducer looks like the extra bit of kit needed: -

917880935_astronomy_tools_fovDSLR.png.6acc1eacba013872e8df17c04bd1d775.png

Edited by kevenh
Short time between related updates
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I'm with @AstroTim on this one

SCT has changing focal length with respect to focus position - "further out" you focus (on a regular telescope that would be further out, but in case of SCT - closer two mirrors are) focal length is increased.

It is not uncommon for F/10 SCT to operate at F/12 or more. This is particularly pronounced when using 2" accessories (longer FL) and using focal reducers.

Focal reducers shift focal point towards the scope - this means that one needs to push it "further out" to reach focus in standard configuration (diagonal + eyepiece) - again extending focal length of the scope.

Take a look here:

 

Edited by vlaiv
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49 minutes ago, vlaiv said:

I'm with @AstroTim on this one

SCT has changing focal length with respect to focus position - "further out" you focus (on a regular telescope that would be further out, but in case of SCT - closer two mirrors are) focal length is increased.

It is not uncommon for F/10 SCT to operate at F/12 or more. This is particularly pronounced when using 2" accessories (longer FL) and using focal reducers.

Focal reducers shift focal point towards the scope - this means that one needs to push it "further out" to reach focus in standard configuration (diagonal + eyepiece) - again extending focal length of the scope.

I agree with this as I found out recently. My binoviewers add quite a bit to the focal length to my 127 mak, can't remember the exact detail but think it took it from F12 to F13.8 with the added focal train distance. 

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2 hours ago, bomberbaz said:

I agree with this as I found out recently. My binoviewers add quite a bit to the focal length to my 127 mak, can't remember the exact detail but think it took it from F12 to F13.8 with the added focal train distance. 

However, it's far less of an increase than using a 2x Barlow nose piece to reach focus.

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6 hours ago, bomberbaz said:

That becomes a 4x barlow in my mak with binoviewers

I use a Meade 140 nose piece screwed onto the front of my binoviewers, and I get right about 3x with the combination.  The 140 Barlow is 2.4x by itself.  I think being a medium length Barlow helps limit the power growth when used with the longer binoviewer path.

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