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Does a reducer actually decrease FL?

astrophotography beginner refractor
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#1 Astro Babak

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Posted 28 February 2020 - 02:53 PM

Hi all,

 

I'm new to AP and still building my rig. Got a Canon 70D DSLR and HEQ5 PRO mount, so what I'm missing is the scope! I've got my eyes on Evostar 72 APO Refractor telescope, with FL of 420 mm. 

 

My camera is a crop-sensor DSLR, so as far as I understand, I shall multiple the lens' FL by 1.6, which would give me 670 mm. With this focal length, I won't be able to fit M31 in my frame. Will a 0.8x focal reducer actually decrease the FL? 

 

Thanks all,

Babak



#2 Alex McConahay

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Posted 28 February 2020 - 03:02 PM

>>>>>> My camera is a crop-sensor DSLR, so as far as I understand, I shall multiple the lens' FL by 1.6, which would give me 670 mm

 

Crop Factor strikes again......

 

Forget about the crop factor. 

 

THink only of Field of View. The "crop factor" does not change the focal length of your image. It only crops out the outer edge of your image. Instead of looking out a picture window, you are looking out a porthole!!! (OK, that is extreme, but it gets the point across.) At any rate the tree in the distance, whether through the porthole or the picture window is precisely the same size. The only difference in the picture is just how much more of the landscape you see out there. The tree is in the center of both, and precisely the same size. With the porthole, you only see the tree. With the picture window, you also see the surrounding lawn.

 

An 0.8 focal reducer will make your porthole more of a picture window, by a factor of (the inverse of ) 0.8. 

 

I suggest you go to Telescopius.com . Towards the bottom of the home page, you will see a "Telescope Simulator." Do what you need to do to designate your camera and scope combination under "Astrophotography" and ask the website to show you M31. 

 

Take it from there. 

Alex


Edited by Alex McConahay, 28 February 2020 - 06:08 PM.

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#3 OleCuss

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Posted 28 February 2020 - 03:05 PM

Using a focal reducer will make your rig act optically as though it has a shorter focal length.  Of course, the OTA's focal length will not really be changed.

 

Don't worry about a crop sensor in this situation.  Using an astronomical telescope is different from using a DSLR lens.  What you want to be concerned about are things like the size of the image circle generated by the OTA and the diagonal measurement of the sensor and how they match up (to a certain extent the height and width of the sensor as well but mostly we ignore that).

 

Ultimately, if you use the focal reducer you will likely be able to capture M31 in a single frame.  The rule of thumb is that with an APS-C sensor and a focal length of 600mm or less you will likely be able to get M31 in a single frame.


Edited by OleCuss, 28 February 2020 - 03:06 PM.

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#4 Der_Pit

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Posted 28 February 2020 - 03:12 PM

The 70D has a sensor size of 22.5×15mm.  With 420mm that will give you a field of view (FOV) of 3×1.9 degrees.  The focal reducer will indeed make the whole system have an effective focal length of 0.8*420=336mm, and increase the FOV to 3.8×2.4 degrees, this will nicely frame M31.


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#5 StarBurger

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Posted 28 February 2020 - 03:24 PM

+1 for Alex and OleCuss.

I really think we should throw away the old concept of "crop factor". It's antiquated and not applicable to AP.

All you have to work out is the image scale (dependent on fl of your main lens and the reducer or expander used) that falls on your sensor and decide if the size of the object you wish to image will fit on the sensor.  Sensors these days can be any size depending on camera.

It confuses so many people.


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#6 StrStrck

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Posted 28 February 2020 - 03:30 PM

I like to think simple. So remembering as a boy crawling through pipes on a play ground playing hide n seek, the closer I’d crawl to the edge of the pipe (shorter focal length) the wider the view of the play ground.


Edited by StrStrck, 28 February 2020 - 03:34 PM.

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#7 sg6

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Posted 28 February 2020 - 03:59 PM

No, the "focal length" stays about the same.

If it reduced the actual focal length then the image would be somewhere inside the focuser - which wouldn't be any use.

 

What it does is reduce the final image such that it is the size of a "reduced focal length".

 

If a 500mm focal length scope procuded a 10mm image then addig a 0.8 reducer make the image about 8mm but at more or less the same place.

 

Astronomy has a few "errors". We say a barlow makes a 20mm eyepiece a 10mm eyepiece. It doesn't, it makes the image from a 500mm scope the size as if it were from a 1000mm scope and again more or less around the same place. Still somewhere around 500mm.

 

So a reducer or a barlow alters the size of the image, usually at about the same position as the unchanged image - not exactly the same but somewhere near.

 

Forget the rather odd idea of a crop sensor and changing the focal length. A 200mm lens creates the same size image whether the sensor is full size or crop or medium format. What changes is obviously the size of the sensor, and that changes the amount (field) of the image that is detected.


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#8 Iamhondo

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Posted 28 February 2020 - 04:17 PM

 

"...Forget about the crop factor. ..."

 

Amen! The "crop factor" references hearken to an ancient, obsolete reference standard. We might as well list apertures in cubits and use Roman numerals for f-ratios. 

 


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#9 OhmEye

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Posted 28 February 2020 - 04:25 PM

I never did like the tendency of people to use "crop factor" in a focal length discussion. cool.gif The term is only apt in the DSLR world, the sensor image area is "cropped" relative to a "full" sensor. That's all it means, that the "crop" size sensor "crops" the image circle more than than a "full" size sensor, which really only has relevance rooted in the 35mm SLR world. It doesn't have any context in AP, where image circles and sensor sizes vary considerably.

 

The way I think of reducers is that they reduce the image circle diameter. The sensor sees more of the circle and has wider FoV and more light, so behaves similar to a shorter faster scope.


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#10 klaussius

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Posted 28 February 2020 - 04:28 PM

The way I think of reducers is that they reduce the image circle diameter. The sensor sees more of the circle and has wider FoV and more light, so behaves similar to a shorter faster scope.

In the process it changes the light cone to that of a faster scope (F x R, so F5 x 0.5 = F4).

 

Which is relevant when considering filter sizes, bandpass shift, hunting reflections, and other properties that depend on the F ratio.


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#11 Kevin Ross

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Posted 28 February 2020 - 04:29 PM

I happen to like my Stellarvue .175 cubit f/VII scope.


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#12 richorn

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Posted 28 February 2020 - 04:35 PM

>>>>>>

Instead of looking out a picture window, you are looking out a porthole!!! (OK, that is extreme, but it gets the point across.) At any rate the tree in the distance, whether through the porthole or the picture is precisely the same size. The only difference in the picture is just how much more of the landscape you see out there. The tree is in the center of both. With the porthole, you only see the tree. With the picture window, you also see the surrounding lawn.

Alex


As a pro photographer I have long had to try and explain this to others...

That is the best example I have ever seen!!
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#13 Astro Babak

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Posted 28 February 2020 - 05:16 PM

Thank you all so very much. I have been trying to grasp the optics of my imaging train and fully understand it, and I've seen the "crop factor" so many times in different places. Thanks for the clarification. 

 

Alex, Thank you very much for Telescopius.com, it will help me a lot. 

Iamhondo, I'll make sure that I use the roman numerals from now on grin.gif wink.gif

 

Thank you all, I learnt a lot today. 




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