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Fish Head Nebula


Rodd

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I'll clarfiy my position on F ratio since it's come up (but I normally shut up about it these days!)

When a lower F ratio reduces exposure time, and sometimes it does, it does so because the aperture has been increased. This is the situation with F stops and camera lenses and it's perfectly logical. Increase the aperture, get more light, expose for less time. The active ingredient is aperture.

When you use a focal reducer you don't do this, you reduce the focal length and so you get no more light from an object small enough to fit on the chip originally and any reduction in exposure time simply arises because you've poured that light down fewer pixels. You can do that by software binning without a reducer.

When you use a focal reducer to widen the field and bring in new objects of interest then the whole image will reach a satisfactory S/N ratio faster in accordance with the camera lens rules but the small object which previously filled the frame will now be only a part of the image at lower resolution.

Olly

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9 minutes ago, ollypenrice said:

When you use a focal reducer to widen the field and bring in new objects of interest then the whole image will reach a satisfactory S/N ratio faster in accordance with the camera lens rules but the small object which previously filled the frame will now be only a part of the image at lower resolution.

This is the part of the position that applies here. Naturally I am interested in the image as a whole.  But this begs the question why an epsilon 130 is so fast with 130mm of aperture.  It goes to reason that using a reducer on a 130 mm scope should reduce exposure time in similar fashion to the epsilon.  There is nothing inherent in the optical design that creates the speed (refractor, reflector  etc).  You agree that SN ratio will grow faster for the image as a whole when a reducer is used--That is what I was trying to capitalize on.  

I must say that I saw no difference between 152 Ha subs and 54 Ha subs (very little)--so maybe I am collecting needless data.  Maybe at a truly dark sky site, I would be able to use 4 hours of data and achieve the results I am after.  A key factor with the ASI 1600 is that one needs al lot of subs to overcome the 12 bit thing.  So it becomes imperative to shoot subs at a short enough length to accomplish this in reasonable time.  So--maybe its not the reducer/scope I am up against--maybe its the camera.

Rodd

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That's one classy image Rodd.  

I am not good with understanding about pixel size, resolution and getting sharp detail, was trying to understand what you said, but in the end what is your conclusion as regards the best camera/scope combination?

Carole 

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2 hours ago, carastro said:

That's one classy image Rodd.  

I am not good with understanding about pixel size, resolution and getting sharp detail, was trying to understand what you said, but in the end what is your conclusion as regards the best camera/scope combination?

Carole 

Thanks Carole.  Well, I can't speak to all scopes and cameras of course, but i will say the the FSQ 106, and I assume any quality refractor, works extremely well with 3.8um pixels.  I would think the same would be true of the CCD sensors that have small pixels (ATIK 1, 460 and 490 come to mind).  As far as the CMOS-CCD comparison, I have had no difficulty calibrating the asi 1600, and as far as I can see, the 12 bit-16bit issue is a non starter as long as you collect sufficient amounts of data.  Then again, I have not tried printing any images taken with the asi 1600.  The dynamic range portrayed on screens is a lot less than that depicted in print form I am told, so it remains to be seen if the images will print satisfactorilly.  I tend to think they will, and at 16mb vs 8mb (STT-8300) I think they will print better.  But we'll have to see if that's the case.

I know the answer is not much help--but I only have experience with 2 deep space cameras.  Both are nice, both work well (other than computer/filter wheel glitches with the STT-8300).  Both imaging styles are OK for me.  I suppose its all in what one wants.  I was attracted to large FOV and high resolution--2 traits not commonly seen together.   Now, if they made a full frame sensor with 3.5um pixels--I would want to try it with my refractors-though I am not sure the guidescope approach would be tenable for the TOA unreduced (their resolution would be around .5 I think--OAG territory.

Rodd

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