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ED Doublet LRGB

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#1 Douglas729

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Posted 23 May 2020 - 06:42 AM

I have an APM 140 ED refractor I purchased because it is excellent for visual observation but not being a triplet it is not ideal for CCD color images.

 

Does a CCD mono camera with an LRGB filter wheel negate that deficiency and create images that capture the color spectrum accurately like a good triplet refractor?



#2 sg6

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Posted 23 May 2020 - 07:24 AM

Should do a good job as you are in effect working with narrower bits of the spectrum, so CA on the RGB items should be not applicable. There is no R and B to form a purple halo around the G objects.

 

Might be slight refocus between R and G and B. Green likely being a fraction longer. But it is a "fraction", as in a really tiny fraction.

 

The L is the question, since that is still all the colors = white in effect. Shouldn't cause anything but unsure. Not enough experience I am afraid.



#3 alphatripleplus

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Posted 23 May 2020 - 11:53 AM

Moving this to B&II for a potentially better fit.



#4 Madratter

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Posted 23 May 2020 - 12:44 PM

I have an APM 140 ED refractor I purchased because it is excellent for visual observation but not being a triplet it is not ideal for CCD color images.

 

Does a CCD mono camera with an LRGB filter wheel negate that deficiency and create images that capture the color spectrum accurately like a good triplet refractor?

Not entirely. Many ED doublets perform acceptably well. However, even with a good ED doublet, you are likely to find spot sizes in some of the colors better than others. How much different they are will depend a lot on the glass used. To generalize (always a bit dangerous), the slower the f-ratio, the more likely you are to get away with the doublet but then you are looking at longer integration times.



#5 ks__observer

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Posted 23 May 2020 - 01:10 PM

Would perform fine for narrowband.



#6 Stelios

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Posted 23 May 2020 - 01:40 PM

Unless you are a very advanced imager, you would find the APM plenty good enough for astrophotography, even with a DSLR or OSC camera. You would have slightly more bloated stars perhaps than with a triplet, but you would probably do better by sinking money into mount and OAG and autofocus before worrying about the doublet vs. triplet difference. You are *not* going to have color aberrations.

 

Not trying to talk you out of mono LRGB imaging--that's the way to go for many reasons. But don't go that way because you worry about the scope. 




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