Wednesday, April 16, 2014

SKETCH: M104

M104 with 4 inch telescope

Telescope: Astro-Physics Traveler EDF 105mm F/5.8
Eyepieces: Televue Ethos 8mm
Transparency: 3/5, 21.3 SQM
Seeing: 1/5
Date: 05/04/14

Notes: Very high humidity, had to observe inside porch to finish sketch with the ota not fully thermally stable, so everything was boiling.

Thursday, April 10, 2014

SKETCH: Leo Triplet M65, M66 and NGC 3628

Leo Triplet with 4 inch telescope

Telescope: Astro-Physics Traveler EDF 105mm F/5.8
Eyepieces: Televue Ethos 10mm, Pentax XO 5mm
Transparency: 3/5, 21.2 SQM
Seeing: 2/5
Date: 23/03/14

Notes: Clouds passing by, far from good conditions. I only carried with me two eyepieces that night, so I didn't have all the magnifications I needed to do the sketch.

M65 had some strange S shape with the Pentax XO 5mm. Looking at the photographs and sketches from others, it seems it's a internal arm that it's slightly brighter than the galaxy.

It's really hard to get details from the galaxies, so patience is absolutely needed here.

Tuesday, April 8, 2014

SKETCH: M51

M51 with 4 inch telescope

Telescope: Astro-Physics Traveler EDF 105mm F/5.8
Eyepieces: Televue Ethos 10mm, Pentax XO 5mm
Transparency: 3/5, 21.2 SQM
Seeing: 2/5
Date: 23/03/14

Notes: Clouds passing by, far from good conditions. Some hard to see faint bright structures at high magnification. Bridge can be seen as a faint streak with lateral vision. Only one star visible tonight.

Thursday, April 3, 2014

ARTICLE: Effects of aperture in brightness (II)

Continuing Effects of aperture in brightness (I)


Aperture and exit pupil relationship


I'm going to avoid complex formulae, because this is not the object of the article, neither the blog. If you want some brain burning things, you can check the links at the end of the article.

If you keep exit pupil fixed and you change the aperture, the brightness of the object will not change. Let's say, for example:

10 inch dobsonian F/5, 13mm Eyepiece. Exit pupil = 2,6mm
20 inch dobsonian F/5, 13mm Eyepiece. Exit pupil = 2,6mm

(I have selected the same design to avoid contrast differences derived from stray light control, flocking and so on).

The aperture is doubled, so one would say that the light that enters is 4 times increased, but also the focal length is doubled, so the magnification is doubled again!. If we double the magnification, we also spread the light over a larger area, so it's 4 times decreased.

The result is that the brightness is the same regarding the aperture!.

As an exercise, try to compare different instruments with the same exit pupil. Look for the moon and the sky background.

Another consequence is that light pollution affection is independent of the aperture. Your big mirror will not catch more light from artificial light.


Why we see more with bigger telescopes?


Good question. Because if you are using a CCD, the aperture only increases image scale. That's why some larger instruments are matched with bigger pixels CCD, to get better SNR in less time.

But your eye and your brain work in a different way. To detect an object or a feature inside an object all that matters is contrast and size. There is an optimum angular size where something it's more visible. The fainter the contrast difference, the bigger it needs to be.

Increasing the magnification makes the object larger, but also fainter (up to a point of no gain). There is a sweet spot for each object or feature where it will be most visible. Some magnifications will show some features better than others, so don't forget to change eyepieces with the same object, some surprises will arise.

Bigger aperture let's you increase the angular size of deep sky objects, so more features and more small faint objects are visible.

Useful links


Tuesday, March 25, 2014

ARTICLE: Effects of aperture in brightness (I)

Why this post?


Everywhere I go, every forum I read, there is always people talking about the same things over and over again:

- "If you observe in a light polluted site, do not go above 4 (8, or whatever) inch aperture, since you want black backgrounds, and aperture catches more light pollution".
- "Look at that monster, the moon will be super bright and can even burn your eye!".
- "Aperture improves SNR, because it catches more light".
- "That galaxy will look super bright with more aperture".

Well, all of that is false. Aperture does not make things brighter at all. Extended objects, like nebulae, galaxies, etc. will look exactly the same with independence of the aperture used. This is a surprising statement, because we are used to see more with bigger telescopes.

There is a lot of background here, so let's start with some key concepts.

Exit pupil is king in visual


Exit pupil is the image of a virtual aperture produced by the eyepiece field stop. It's dependant of the focal ratio of the telescope and the focal length of the eyepiece.

Exit pupil formula


Bigger pupil creates brighter images, but there is a limit, the size of your eye pupil, which tops at about 7mm.

Image scale is king in astrophotography


Image scale is the amount of sky that covers a pixel. The image will be brighter if the F number is lower for a given scale.

Image scale formula

Take note that aperture is not in any formulae, but it's implicit, since we are talking about F ratio and focal length.

Point source objects are out


Stars will get brighter with aperture, since their size will always be dependent of the airy disk size, and the flux of light depends on aperture.

So they are out of the article, bigger aperture will show fainter stars.

But when we talk about extended objects, everything changes.

Surface brightness of extended objects


Surface brightness is the magnitude of the object in an area. It depends of the amount of photons an object emits as a whole. The sky background is an extended object, like nebulae and galaxies.

If the object is magnified two times, the light will be dimmed 4 times, since the flux is the same, and the image scaled is doubled in an area (square of the magnification). But it's surface brightness will be the same. If we get far from the object, the light will diminish at the same amount rate as the object gets smaller, so it's brightness per area is fixed.

Friday, March 21, 2014

ARTICLE: Analyzing tilt and curvature in Focusmax

Usually we see a lot of posts in the forums asking why their stars in the corners are not round. There are nearly infinite causes for this problem, but I will try to summarize some steps to analyze what's happening.

Remember that no optical system is free of curvature, tilt, miscollimation or flexure, but it's our mission to improve as much as we can.

Why Focusmax?. Because it's free, and it's much more precise than any other tool in the market.


First step: measuring with your eye


Many people just put the CCD in place and start to collimate. There is a strong belief that if you use the CCD, the system will end much better using this shortcut.

This is a terrible error, since all CCDs are tilted some microns. Some manufacturers are better than others in this respect. If your F ratio is high, this will make things simpler.

1) Collimate with a wide field eyepiece, at low power. Then use a high power eyepiece, about 50x per inch. A last round with more magnification to leave everything perfect is better.

2) Slew to other stars. Does collimation hold?.

3) Measure optical defects that will impact image:

- Astigmatism
- Turned down edge
- Spherical aberration (1/4 for deep sky imaging is ok)

4) Put the same weight in the focuser as the whole image train. Does collimation hold?.


Second step: use your CCD


If all the above is ok, then you can forget everything and start using your CCD.

1) Check that all your image train sits flush. Use a flashlight to check every adapter. Move the whole image train with your hand. Does it flex visually?.

2) Now point the telescope to zenith, this will make the gravity push the image train perpendicular, so no altitude tilt here. Then measure with Focusmax.

Focus a star in the center, take note of the focuser position. Make sure that it's repeatable, and not a random value. This value will be marked as zero value for now.

Example: 32456 microns -> 0

Then put the star in one very corner of the image, and focus on the star. Take note of the difference:

Example: 32397 microns -> 32456 - 32397 = +59

Do this for the rest 3 corners, and put the results in a matrix:

+59   +62
      0
-45   -35

Now you have a 3D representation of your image in that particular point of the sky. You can print the matrix with Excel to see it more clearly. Here we see a tilt in the optical axis from top to bottom.

3) Check for other points in the sky. Does tilt vary?.

4) Try to correct the tilt:

- By shimming: One cheap way to shim is use Aluminium foil. There are a lot of sizes out there starting from 8 microns. Put the shim and measure again.
- By collimation: If your tilt is small enough, you can correct it by miscollimation. A bit won't show in the image.

5) Check for field curvature, the matrix of field curvature will have this look:

+123   +128
        0
+125   +119

6) Try to correct field curvature moving the corrector, the secondary, or the way your instrument corrects it.

7) If unsuccessful, just modify your program to focus at the middle of the CCD. In the last example: +60

8) Enjoy your round stars!

Thursday, March 20, 2014

NGC 6543: The Cat's Eye Nebula

NGC 6543 by ManuelJ, 2013

NGC 6543 is a famous planetary nebula in the constellation of Draco. It's one of the top 3 surface brightness nebulas, and has a strong OIII dominant.

It is surrounded by a faint big halo of matter that was ejected during its red giant phase.

This nebula was discovered by Herschel, and so it's included in its first 400 compilation catalogs.

It's not exactly located in an easy zone of the sky, but its high brightness will make it really fast to find.


Finder chart for refractors
Finder chart for reflectors


Observing reports



Recommended exit pupil: 0.75mm
Minimum aperture: Small binoculars
Filters: UHC, OIII

50mm binoculars: Object appears star like, prepare a good finder view.

4" refractor: Use high magnification and wait for good seeing. You will see some structure inside, but as all planetary nebulae, is not easy to see.

Bigger aperture observations will be added in the future.