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When to use a focal reducer?

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#1 petro62

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Posted 12 May 2021 - 09:10 PM

So I have a nexstar 6se in a bottle 8-9 location. I use a canon 70d to photograph. The other weekend I took a lot of pictures of ngc 5236. I got ok results but it was such a small part of a bigger picture and didn’t have great detail. I am trying to determine if I should have taken off my f6.3 focal reducer so I was more zoomed in on the NGC. Just trying to determine when I should use the reducer and when not to. Any help?

#2 StarAlert

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Posted 12 May 2021 - 09:31 PM

You’d normally use a focal reducer to increase the field of view. So… if you want a larger image scale, you should remove the reducer. Your target will be bigger as a result. 


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#3 petro62

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Posted 12 May 2021 - 09:34 PM

You’d normally use a focal reducer to increase the field of view. So… if you want a larger image scale, you should remove the reducer. Your target will be bigger as a result.


Thanks. Is there a app or website to help you calculate. I guess I just do t know when to use and when to not. Kind of late to figure it out after taking a 1000 photos.

#4 StarAlert

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Posted 12 May 2021 - 09:43 PM

Thanks. Is there a app or website to help you calculate. I guess I just do t know when to use and when to not. Kind of late to figure it out after taking a 1000 photos.

https://astronomy.to.../field_of_view/

https://telescopius....scope-simulator
 


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#5 Mr. Pepap

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Posted 12 May 2021 - 09:45 PM

Is there a app or website to help you calculate

Telescopius.com should give you a good idea of what to expect with a given setup. Hover over "toolbox," then click "telescope simulator." You put in all your information, then select some targets and adjust to find out what's good and what isn't. Focal reducers are especially useful for large focal length telescopes because they can enable your scope to capture larger objects. You will notice that the Andromeda Galaxy will look huge with your setup with no focal reducer, but put on a 6.3 reducer and it's a bit better.

 

Be warned though: there's only so much you can do with a focal reducer. The more you reduce, the greater the image quality hits. You won't be able to capture the entirety of M31 or the Veil Nebula without specialized equipment, so try sticking with smaller DSOs to start.



#6 StarAlert

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Posted 12 May 2021 - 09:46 PM

With some practice, you’ll start to figure out how big of a FOV you need to frame your target. 



#7 idclimber

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Posted 12 May 2021 - 10:58 PM

Imaging with a SCT at f/10 is not ideal. There are a couple issues/problems.  The first is the longer exposures that are going to be necessary with it at the native focal length. This is over one stop so a little more than double the time. 

 

The second issue is the f/6.3 Celestron reducer is also a field flattener for imaging. Your images will look better with less visual defects in the corners and edges of the frame. 

 

Depending on the quality/steadiness of your sky and the actual pixel size of the camera SCTs can start to push the recommended pixel scale. This is real problem with large SCTs like my 12" LX200 and less of a concern with a 6" version. 



#8 ChiTownXring

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Posted 12 May 2021 - 11:12 PM

You can setup Stellarium (free) and quickly add or remove your f/6.3 reducer and see how the framing looks for a particular object..



#9 petro62

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Posted 12 May 2021 - 11:58 PM

Thank you all for the great replies. I started using telescope is and so far that is working well. We will see how the photos turn out tonight.


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