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F6.3 FR/corrector or not?

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#1 psuaero

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Posted 19 September 2019 - 11:22 AM

I'm just getting started in the astrophotography. I recently bought the F6.3 focal reducer/field flattener for my NexStar 6SE (6" SCT) with Az-El mount. So, now for the questions.

  1. Should I use it all the time when taking images? I've gotten decent results on Jupiter/Saturn without the FR/corrector already.
  2. Should I only use it only for the DSO?
  3. What about (optically) smaller planets like Mercury, Venus, Mars, Uranus, Neptune?

My thoughts are to use it for full disk of the moon and sun. Leave it off for closeups of craters/sun spots. Put it on for all deep sky photography. Not sure about the smaller planets. I like the idea of more resolution/magnification so I'm leaning toward NO FR/corrector.

 

Thanks for any advice,

John

 



#2 dcollier

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Posted 19 September 2019 - 11:39 AM

It would be used to widen the FOV and/or shorten the exposure time,  mostly for bigger deep sky stuff.  FR would not be used for Planets.   You could likely get a full disk of the Sun or Moon with it dependent on your cameras sensor size. 

 

             -Dave 



#3 TrustyChords

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Posted 19 September 2019 - 11:42 AM

 

  1. Should I use it all the time when taking images? I've gotten decent results on Jupiter/Saturn without the FR/corrector already.

You probably will want to take it off for Jupiter/Saturn or other planets especially if you are using a barlow.

 

 

2. Should I only use it only for the DSO?

You can use it for DSO, you can also use it if you want a larger field of view for the moon full face .

 

 

 

3. What about (optically) smaller planets like Mercury, Venus, Mars, Uranus, Neptune?

Probably without, but ymmv.



#4 Hesiod

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Posted 19 September 2019 - 11:55 AM

For planets, you should shot at the best focal for your camera to attain a sampling around 0.2-0.3 arcsec to exploit all the telescope's resolving power (e.g. 2x Barlow with a 3.75 micron pitch camera).

The same applies to the Moon but may opt to undersample if want to take a picture as a whole (through a mosaic); if want a single shot may try with the focal reducer, but it is not guaranteed that will get the result.

Even for deep sky objects you should use it to increase the field of view if the object is too large, or want a specific framing.

The biggest advantage is the speed gain, which is hard to let pass: but if want to shot a very small target, may opt to use the telescope at its native focal (it goes without saying that the whole process will be much harder)



#5 biomedchad

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Posted 19 September 2019 - 12:06 PM

Also since I’ve recently been down this path. The 6.3 reducer really needs a smallish sensor to play nice with. My asi224 with a 6mm sensor is fine at 6.3 but a large format like my asi294 at 23mm must be scaled down in order to combat the coma and effects produced. I scale the 294 sensor size down in order to make them meet up optically as close as possible. This is for dso imaging btw

#6 Stelios

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Posted 19 September 2019 - 12:22 PM

You want to remove the reducer and use a barlow (2 or 3x) for *all* planets. 

 

For the moon you should use it, because of the flatter field.

 

And you practically *must* use it for DSO's (or you'll need to significantly crop--which might be OK for some *really* small galaxies and planetary nebulae). Note also that imaging without it will take more than twice as long for DSO's for comparable results--the ratio is by the square of the F/ratio, so compare 6.3^2 (~40) to 10^2 (100).



#7 barbarosa

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Posted 19 September 2019 - 12:27 PM

Also since I’ve recently been down this path. The 6.3 reducer really needs a smallish sensor to play nice with. My asi224 with a 6mm sensor is fine at 6.3 but a large format like my asi294 at 23mm must be scaled down in order to combat the coma and effects produced. I scale the 294 sensor size down in order to make them meet up optically as close as possible. This is for dso imaging btw

In several places on the web including Celestron (at some point), the image circle of a C-8 and up with the f/6.3 reducer is given at 24 or 25 mm. For example here is what Agena Astro says about reducers. It is worth a read because it covers aspects that we sometimes overlook.

 

4.1 Field of View and Image Circle
For imagers, the main purpose of a focal reducer is to increase the brightness of the image at the focal plane. But while the image gets brighter, the size of the image circle gets proportionately smaller. For example, an 8" SCT without a focal reducer has an illuminated field of 38mm at 50% fall-off. Add a 0.63x reducer, and the brightness of extended objects increases by (1/0.63)2 = 2.5. But the diameter of the image circle decreases by a factor of 0.63 to about 24mm. Such an image circle is still large enough to encompass the relatively large sensor of many deep-sky astronomy cameras. If a stronger level of focal reduction is used, say 0.5x, then the image circle may be too small to fill the sensor of larger cameras.

 

How about for visual observers? Using this same example of an 8" SCT and a 0.63x reducer, a visual observer can also enjoy brighter images and a wider field of view. But the smaller image circle means there is a limit to the field stop of an eyepiece that can achieve an unvignetted image. An image of about 24mm across, approximately, allows an observer to use a 1.25" eyepiece with a maximal field stop. That includes, for example, a 1.25" eyepiece with an apparent field of view of 68° and a focal length of 24mm (eg. a Tele Vue Panoptic), or a Plossl eyepiece with an apparent field of view of 50° and a focal length of 32mm. With this telescope and this focal reducer, it does not help to move to a 2" eyepiece and a 2" diagonal as the visual view will be akin to looking through a porthole within the larger apparent field of view of the eyepiece.

 

Another factor to consider: focal reducers also increase the angle at which light approaches the focal plane. This may be a problem if the focuser tube or the diagonal (for visual observing) is too narrow to accept light at this larger angle. As a result, the smaller tube may cut into the light cone and effectively reduce the working aperture of the telescope.

 

Visual observers are more likely to overlook a degree a vignetting that would be instantly obvious in an image, but imagers can use flats and or correct in post.



#8 psuaero

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Posted 19 September 2019 - 02:21 PM

Thanks everyone. All those statements align with my general understanding. Some nice clear and calm nights forecast for this area the next few nights. Hopefully I'll make the best use of time and have something to show for it.




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