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Should I try imaging with my 8" SCT F10, but w a 6.3 reducer?

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#1 Ballyhoo

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Posted 18 April 2019 - 01:45 PM

What will it be like compared to my 85mm triplet? A lot more detail for the faint and fuzzies? the focal reducer puts it down to an F6.3 and my 85mm is F 6.6, so they should be similar, no?



#2 Napp

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Posted 18 April 2019 - 01:47 PM

People do start with sct’s but it is more frustrating and harder than with fast wide refractors.  With refractors you don’t have to deal with mirror issues.  Refractors are just more forgiving. If you have both then see what works for you.


Edited by Napp, 18 April 2019 - 01:50 PM.


#3 pedxing

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Posted 18 April 2019 - 02:00 PM

Yes you should try, why not.

 

Try it with and without the reducer.

 

Experiment and learn.


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#4 mistateo

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Posted 18 April 2019 - 02:09 PM

Things should appear larger and (hopefully) more detailed.  Have you ever even one time taken numerous people's advice to look at targets in Stellarium or other simulators like this one? https://astronomy.to.../field_of_view/


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#5 photoracer18

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Posted 18 April 2019 - 02:44 PM

Well while F6.6 and F6.3 are close when it comes to photographic speed they are way off in image scale. The objects that frame well in one won't be the same ones as the other due to the focal length and resultant magnification differences. All classes of errors will be magnified in the SCT especially guiding, tracking, and polar alignment.

#6 Jon Rista

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Posted 18 April 2019 - 03:00 PM

Definitely try the SCT with a reducer. Which SCT is it, though? Is it a Celestron EdgeHD? If so, you should instead look at the purpose-designed 0.7x reducers for the EdgeHD scope, as it will deliver better results for imaging than the generic 0.63x reducer. 

 

Also, don't be afraid to bin/downsample your results a bit with higher resolution imaging until you are skilled enough to make the most of the longer focal length. You can relax the demands of higher resolution imaging by downsampling, which effectively changes your image scale. As you learn, you can stop downsampling, or remove the reducer, or both, and extract more and more detail from objects the more you learn.


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#7 kathyastro

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Posted 18 April 2019 - 03:05 PM

Image brightness will be about the same for similar exposure times.  But the image scale will be much bigger.


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#8 Eddie_42

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Posted 18 April 2019 - 03:17 PM

Do whatever makes you feel pretty.   But do it with your eyes open to the road ahead.

 

Focal length determines your image field of view (assuming the camera remains constant). Equation is:   Field of view in degress = (width of chip * 57.3) / (focal length of optic)

 

Your 85mm f/6.6 has a FL of 561, where the 8" SCT reduced (0.63) will have 1280mm, Objects will appear roughly twice as large. With those FL and the ASI294 camera, Your Field of view will go from 2 degress to 0.9Degrees.  FYI, imagine without the reducer (@ f/10) will get you a FOV of 0.5deg, but I dont know how flat it will turn out. 

 

With a narrower FOV comes the greater needs for guidance. You are looking down a straw instead of a paper towel roll. Not being a SCT owner, but reading a bunch....SCT generally need OAG for better results. The weight of the mirrors causes flexure as it moves around the axis, and that can cause issues with a guide scope that is piggy backed (it wont flex the same). 

 

Lots of peices and parts, lots of new things to learn if this is a new venture. 

 

Go over to TELESCOPIUS (formerly dso-browser) and pull up M81 or M101. Enter your FL and Chip size (19.1x13mm for the ASI294). Will give you a decent idea of what you can get in the frame.


Edited by Eddie_42, 18 April 2019 - 03:23 PM.

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#9 Rustler46

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Posted 18 April 2019 - 03:31 PM

What will it be like compared to my 85mm triplet? A lot more detail for the faint and fuzzies? the focal reducer puts it down to an F6.3 and my 85mm is F 6.6, so they should be similar, no?

You can capture great photos with either telescope. While they have similar focal ratios, their differing apertures result in greatly differing focal lengths and images scales:

  • 85mm f/6.6 has a focal length of 561mm
  • 8-inch SCT, f/6.3 has a focal length of 1280mm

With such different focal lengths they will have different subjects suitable for imaging. The longer focal length of the SCT makes guiding more challenging.

 

While I have no experience imaging with a small refractor, I've done some with the C-8 with f/6.3 focal reducer. My experience using a consumer digital camera revealed it was operating around f/7.0, due to the spacing between camera sensor and secondary mirror. It is a challenging endeavor, but can be accomplished. Here are some results:

 

  • Telescope - Celestron-8 (Super C-8 Plus, 1985 version), f/7.0 with focal reducer, 1420mm focal length
  • Camera - Sony NEX-7 at ISO 800
  • Mount - Losmandy G-11
  • Guide 'scope - 50mm with Imaging Source mono video camera, 640 X 480
  • Autoguiding with PHD-1
  • Post processing by Chris Rock using PixInsight

 

First is an image of the Needle Galaxy, NGC 4565. Total exposure was 2-1/2 hours (15X ten minute subs) with darks, flats and bias frames.

NGC4565.jpg

 

Next is an image of the Helix Nebula, NGC 7293. Total exposure was 4-1/3 hours (15 & 20 minute subs) with darks, flats and bias frames.

NGC 7293 Whew.jpg

 

Some have held the opinion that a Schmidt-Cassegrain telescope is suitable only for small deep sky objects. Yet the Helix Nebula is almost 1/2 degree in angular extent. Certainly the 85mm refractor will handle subjects of much larger size. You just need to match the size of your subject with the focal length you will be using. Use SkySafari or Stellarium to see how your focal length will frame potential subjects. So results can be had with the C-8. It might be better to get some experience with a shorter focal length before attempting with the longer focal length. As mentioned guiding at 1400mm focal length is difficult. Using the focal reducer brings the focal length down from over 2000mm. So that's a real plus for imaging with a SCT. But with attention to all factors involved, you can be successful. Give it a try!

 

Clear Skies,

Russ


Edited by Rustler46, 18 April 2019 - 05:28 PM.

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#10 Ballyhoo

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Posted 18 April 2019 - 03:31 PM

Do whatever makes you feel pretty.   But do it with your eyes open to the road ahead.

 

Focal length determines your image field of view (assuming the camera remains constant). Equation is:   Field of view in degress = (width of chip * 57.3) / (focal length of optic)

 

Your 85mm f/6.6 has a FL of 561, where the 8" SCT reduced (0.63) will have 1280mm, Objects will appear roughly twice as large. With those FL and the ASI294 camera, Your Field of view will go from 2 degress to 0.9Degrees.  FYI, imagine without the reducer (@ f/10) will get you a FOV of 0.5deg, but I dont know how flat it will turn out. 

 

With a narrower FOV comes the greater needs for guidance. You are looking down a straw instead of a paper towel roll. Not being a SCT owner, but reading a bunch....SCT generally need OAG for better results. The weight of the mirrors causes flexure as it moves around the axis, and that can cause issues with a guide scope that is piggy backed (it wont flex the same). 

 

Lots of peices and parts, lots of new things to learn if this is a new venture. 

 

Go over to TELESCOPIUS (formerly dso-browser) and pull up M81 or M101. Enter your FL and Chip size (19.1x13mm for the ASI294). Will give you a decent idea of what you can get in the frame.

I Think  I might be jumping of the proverbial cliff but at the same time I do want to justify possession of my various toys.  For example my pride ad joy homemade 16" dob just collects dust. and why is that? because when  opportunity for astronomy presents itself I am now much more interested /motivated or whatever you wanna call it, to grab my imaging rig. 

 

I do not know if I am like others here, I have acquired a lot more astro-toys than I actually need.



#11 Gary Z

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Posted 18 April 2019 - 03:39 PM

I've seen what an IEQ45 Pro and a 8 inch SCT can achieve for imaging.  It can be pulled off, especially if you have a pier or at least a setup where the mount isn't moved around.  But yeah, it can be pulled off.  As others have indicated, don't be surprised by what results you get good or bad.  Truthfully, I'm taking the safe route until I know my used Ioptron mount can guide properly.  I'm hoping to take it out tomorrow night. 

 

Have fun!!!

 

Gary



#12 Eddie_42

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Posted 18 April 2019 - 03:47 PM

 

I do not know if I am like others here, I have acquired a lot more astro-toys than I actually need.

I have the opposite problem. I took my first steps into the hobby last winter (just a few months).  I am trying to pick and choos what I grow into.  I just use my 80mm f/6 and a DSLR for now. Guiding that scope is likely my next step.  Long term, Id like to jump to a long FL scope, the 8" SCT strikes me a decent option, so I can do some visual with the kids too.  Not a lot of "wow thats cool" moments visually with the 80mm.

 

I built a 6" dob nearly 15 years ago that I keep in a closet. The mount is just too clumsy to locate and track an object. Made it out of OSB board and rough cut shop tools. It is not good. Maybe someday that will get fixed



#13 dakinemaui

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Posted 19 April 2019 - 11:30 AM

For sure, try it since you have it. It will certainly be educational. The smaller field of view with the C8 is an obvious consideration for the composition of your images. As others have indicated, FOV simulators will be very useful to gauge these effects.

 

As for your main question, it is doubtful that you will see "a lot more detail for the faint and fuzzies". In fact, it's far more likely the detail will be the same, because that resolution will probably be limited by seeing. In other words, the larger scope just puts more pixels across the same blurry pattern. You would need seeing better than about 1.3 asec FWHM before you start to see a difference. Thus, your particular conditions will play a critical role.

 

The similarity in focal ratio means the exposure times and the signal to noise ratio (SNR) will be comparable at the "native" image scale of each scope (assuming the same camera). However, the larger scope does give you the option of binning those pixels. A 2x2 binning would give a similar SNR in half the time without losing detail in "average" seeing (which I assume is about 2 asec). Or you can have a higher SNR (about 1.4x) for the same integration time. (SNR goes as the sqrt of time once photon noise is the dominant noise source.) The binning will also hide errors in guiding that are visible at the larger magnification.

 

I will hedge my above statement about the detail being the same in average seeing conditions. IF you elect to go with the higher SNR, it's possible that some faint details will no longer be buried in the noise. I mean, that's why we image for hours on end -- to increase the SNR over what our eyes see. However, I wouldn't say it would result in "a lot" more detail with SNR only 1.4x higher.



#14 Stephanh

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Posted 19 April 2019 - 01:30 PM

Have a look at the last 3 images I have on my Flickr account,  all done with my 8inch SCT and with a guide scope.

 

 

Here is what I learned thanks to this forum and experimentation:

 

You have to be good at guiding, it is much better to get practice with a small refractor before going SCT,

Limit your exposure time to 2 minutes, above that flexture shows its ugly face,

The Celestron reducer/flattener is not very good,  I bought a Starizona SCT reducer/flattener for SCT and it does a much better job,

Automated focusing is a great asset to have as SCT are a bit temperamental  to focus and you have to do it sometimes twice to nail it.  The automated focuser saves a lot of time and aggravation.

 

Hope that helps.


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