M42 - ED80 vs 180ED comparison

[tooth_dr]

I ran both stacks in APP, did the lightest DPP stretch, and corrected LP.  Same camera used for both data sets.  I was going to combine my 2018 data with my 2019 data of M42, but instead I ended up just doing a comparison of the two.

The ED80 data is a little cleaner but it's also got 10x the exposure, at 500 minutes versus 51 minutes.  Both have virtually the same FL, at 510mm versus 500mm.  Since I got the 180ed I havent really had a successful full night of imaging due to permacloud.  Heres hoping for a good run in 2020.

 

CS

Adam.

 

 

 

Edited December 27, 2019 by tooth_dr

[vlaiv]

Very interesting comparison.

Same camera and same focal length (almost - but let's say it is) - ED80 image is less noisy as is expected - aperture alone is x5 (without accounting for double mirror coating and central obstruction of 180ED) but exposure time is x10:

vs

I believe that ED80 image is sharper - better resolution, which is very strange because of aperture difference. It looks like seeing dominated imaging and was in favor of ED80.

vs

stars look tighter in left image - ED80.

[tooth_dr]

There was high cloud / poor transparency when I collected the 180ed data on Xmas eve.  And I’m not totally sure I’m exactly in focus. Still doing it by eye, and my focus step is 17um but my CFZ is 10um. 

[vlaiv]

10 hours ago, tooth_dr said:

There was high cloud / poor transparency when I collected the 180ed data on Xmas eve.  And I’m not totally sure I’m exactly in focus. Still doing it by eye, and my focus step is 17um but my CFZ is 10um. 

Less than perfect focus can surely contribute to star softness.

[tooth_dr]

Just now, vlaiv said:

Less than perfect focus can surely contribute to star softness.

I’m looking at all my old ED80 images and the stars are smaller and nicer.  

[vlaiv]

4 minutes ago, tooth_dr said:

I’m looking at all my old ED80 images and the stars are smaller and nicer.  

In comparison to this ED80 image or 180ED one?

[tooth_dr]

1 minute ago, vlaiv said:

In comparison to this ED80 image or 180ED one?

I prefer the stars in the ED80 images over the 180ed

[lux eterna]

Could it be a minor collimation issue with the 180ED ?

Ragnar

[Stardust1]

Adam, really nice comparison. Which camera did you use?

[tooth_dr]

39 minutes ago, Stardust1 said:

Adam, really nice comparison. Which camera did you use?

Thanks I used an Atik 383L+ (Kodak KAF8300).

[Louis D]

Visually, I find stars are tighter in my refractors versus my Newts.  I've always attributed it to the CO pushing light out of the Airy disk.  It's one of the main reasons I like them despite the smaller aperture.

[ollypenrice]

Here we have the antidote to the F ratio myth conversation because we have different apertures at the same focal length, meaning that the F ratio becomes a truly meaningful number. The increase in speed is pretty spectacular, as we'd expect.

Personally I'm less bothered by the slight softness of the Epsilon stars than I am by the spikes, but people feel differently about this. For me spikes say, 'This is a picture,' which stops me from mentally losing myself in space, so to speak. Nothing can be done about this and not everybody feels it anyway.

Setting aside the seeing on the Epsilon's night, it is always going to be much harder to nail focus, tilt and collimation on a system as fast as this. To my eye you're doing very well with the instrument. Tweaks won't hurt but they don't hit me as a screaming necessity. If you need to pixel peep to see it then it isn't a valid problem in my book.

So, dual refractors or fast reflector???  (Remembering that, by definition, doubling your refractors only makes a difference of one F stop. I always find this a scary thought!)

Olly

[vlaiv]

12 minutes ago, ollypenrice said:

So, dual refractors or fast reflector???  

Oh, I know answer to that riddle - quad fast reflectors

6 hours ago, Louis D said:

Visually, I find stars are tighter in my refractors versus my Newts.  I've always attributed it to the CO pushing light out of the Airy disk.  It's one of the main reasons I like them despite the smaller aperture.

It is common misconception that central obstruction is the main reason for size of the stars in images. In images stars are usually number of times larger than size of airy disk.

This is even true for visual - let's compare 80mm unobstructed scope with 130mm obstructed scope (25% CO) to see which one is going to give tighter stars (if CO and aperture are only difference).

First a bit of math - from this wiki article we can see that first minimum in Airy pattern (where disk ends) corresponds to first zero of Bessel function of first kind of order one. Other zeroes represent dark divisions between rings.

Now let's compare where "disk ends" for first aperture to disk+first ring for larger aperture (we can take lambda to be the same):

First minima of small scope: 3.8317 / PI * 80mm = 0.015246 (some units or other)

Second minima of larger scope: 7.0156 / PI * 130mm = 0.017178 (again same units as above)

This tells us that even if central obstruction is pushing light into rings - airy disk + first ring of larger obstructed scope is almost the same size of only airy disk of smaller unobstructed scope (in this case difference is ~12% larger).

To get the idea of how it looks side by side, we can do images of both and compare them as they would look visually if there was no atmospheric interference and we were observing them on same magnification:

[Kev M]

1 hour ago, ollypenrice said:

Here we have the antidote to the F ratio myth conversation because we have different apertures at the same focal length, meaning that the F ratio becomes a truly meaningful number. The increase in speed is pretty spectacular, as we'd expect.

 

I read that as the other way around.

F ratio would only be meaningful at "the same aperture" surely ?

[vlaiv]

1 minute ago, Kev M said:

I read that as the other way around.

F ratio would only be meaningful at "the same aperture" surely ?

F ratio is meaningful at same pixel size and without binning.

If you fix pixel size then aperture / focal length ratio determines "aperture at resolution" because at fixed pixel size aperture = aperture and focal length = resolution, so ratio of the two determines aperture at resolution.

 

[ollypenrice]

15 minutes ago, Kev M said:

I read that as the other way around.

F ratio would only be meaningful at "the same aperture" surely ?

Quite the opposite, I think. The significance of F ratio arose in the camera lens world in which a diaphragm allowed the aperture to be varied while the focal length remained unchanged. Problems arose in AP discussions when F ratio was reduced not by increasing aperture but by reducing focal length.

tooth_dr's comparison is interesting precisely because FL (and pixel size) are fixed (almost) and only the aperture has changed, as is the case with camera lenses at different F stops. When the F ratio is reduced by increasing the aperture then we know we have more light so we can be sure the exposure time will go down. They key player, here, is the extra light.

Olly

[andrew s]

I suspect seeing and sky transparency are dominant in the comparison. With out control of these variables scientifically robust conclusions are impossible to draw without a much wider statistically sample.

Regards Andrew 

[Kev M]

6 minutes ago, ollypenrice said:

They key player, here, is the extra light.

Olly

That was my point ?

 

[ollypenrice]

5 minutes ago, Kev M said:

That was my point ?

 

Fine, I think we're just at cross purposes in the use of words here. It seems we agree.

Olly

[tooth_dr]

6 minutes ago, andrew s said:

I suspect seeing and sky transparency are dominant in the comparison. With out control of these variables scientifically robust conclusions are impossible to draw without a much wider statistically sample.

Regards Andrew 

I own two KAF8300 cameras and so I could do a side by side if I get a mount capable of carrying both scopes at the same time. 

[tooth_dr]

If I get a chance later I’ll fire up a few mire comparison images.  It’s not scientific but i think it’s interesting (to me and others I hope).

Thanks for all the comments. 

[tooth_dr]

I've went through my images - although I have comparative images of the Iris neb, Eastern Veil and HH, it was before i collimated the scope, and the focus is a little off, so I'll not post them as the comparison isnt really that great.