starbursts – Does this telescope only have a 4 blade aperture?

What you’re seeing isn’t the result of an iris aperture like in a camera. The 4-point diffraction spikes in the telescope are caused by the four struts holding the reflector in the mirror telescope. This diagram from the Diffraction spike Wikipedia article shows the diffraction pattern (below) created by the corresponding strut arrangement (above):

Comparison of diffraction spike patterns of various strut arrangements
Comparison of diffraction spike patterns of various strut arrangements by Cmglee, via Wikimedia Commons. CC BY-SA 3.0

The image in your question is a composite of several images and data from the Subaru Telescope in Hawaii, and the Hubble Space Telescope. Interestingly, the Subaru Telescope has a 4-strut arrangement, but they are not 90° apart. However, in this composite image, it is likely that the data for the bright stars came from the Hubble.

Hubble’s 4-strut mirror support configuration is famous for generating long, narrow diffraction spikes on bright stars. From the Hubble FAQ:

Why do stars have a cross-shaped distortion in most Hubble images? Why do galaxies not?

The cross shape visible on bright objects (such as stars) in Hubble images is a form of distortion that is visible in all telescopes that use a mirror rather than a lens to focus light rays. The crosses, known as diffraction spikes, are caused by the light’s path being disturbed slightly as it passes by the cross-shaped struts that support the telescope’s secondary mirror.

It is only noticeable for bright objects where a lot of light is concentrated on one spot, such as stars. Darker, more spread-out objects like nebulae or galaxies do not show visible levels of this distortion.


In your question, you said,

If I recall correctly, on my DSLR I get one “ray” per blade.

If by “ray”, you mean a single line from the center of the star outwards, then no. You get two per blade. You get horizontally-opposed “rays” from each edge in the aperture.

In the above diagram from Wikipedia, notice that there is no difference in the number of rays between the single strut and the double strut arrangement. Similarly, there is no difference in the number of rays between the 2-strut (ell), 3-strut (tee), and 4-strut arrangements (3rd–5th arrangements): there are 4 rays.

In those cases, because of the presence of edges within the aperture that are 180° opposed, the half of the generated rays are overlaid on each other.

But in the 3-strut (“Y”) arrangement on the far right, no struts are in 180° opposition, so you can clearly see the six generated rays, two from each strut.

From the same Wikipedia article, this diagram shows the diffraction spikes created by non-circular bladed iris apertures:

Comparison of diffraction spikes for apertures of different shapes and blade count
Comparison of diffraction spikes for apertures of different shapes and blade count by Cmglee, via Wikimedia Commons. CC BY-SA 3.0

In general, an aperture of N blades will create:

  • N-point stars, if N is even;
  • 2*N-point stars, if N is odd.

This is why 7- and 9-bladed aperture DLSR lenses create beautiful 14- and 18-point sun spikes at small apertures.

astrophotography – Why can’t I see anything in the viewfinder or via Live View when my camera is connected to my telescope?

You will need to refocus after removing the eyepiece and placing the T-mount adapter and camera directly on the telescope’s focusing tube. When it’s severely out of focus you will not see anything because the light from any stars in the field will be spread too thin.

One way to get a new combination of pieces in the ballpark quickly is to use the moon as a target. It’s large enough and bright enough that even when out of focus something can be seen through the camera’s viewfinder or on the LCD when in Live View. Then rack the focus in and out until you find the point of focus.

Since your telescope is a Newtonian reflector, the optical distance from the camera’s sensor to the primary mirror should be the same as the telescope’s focal length. If you can measure the distance from the center of the mirror to the center of the secondary mirror, subtract that from the telescope’s focal length of 1,000mm. The difference is how far the camera’s sensor, which is 44mm behind the camera’s lens flange, should be from the center of the secondary mirror.

Why do I need a telescope for astrophotography?

I understand visually that through a telescope, deep space objects, planets, etc, look much “better” than through a DSLR lens. However, I fail to find a good, technical reason why is this true.

Indeed, telescopes have long focal lengths. But an 150-600mm lens with a 2x teleconverter can reach to 1200mm. On a crop sensor, this can get to 1800mm.

It is my understanding that, when doing astrophotography, the telescope becomes the lens of the camera (and no eyepiece is needed). Then why is a 1600mm telescope better than 150-600mm lens with a teleconverter, for example?

astrophotography – lunar photos with telescope

So I have a Nikon D5500 and want to take an image of the moon, I also have an 8inch telescope too. My issue is when the camera is coupled to the telescope the moon takes up more than the frame, so its impossible to get an image of the full moon.

Can anyone recommend a tutorial, or is it possible to take a few images of the moon and then stitch them together on a computer afterwards? Or will it always look fake since the lighting wont be uniform?

Astrophotography – How to mount a Lumix G7 with a Panasonic H-FS 1442A lens on a 1.25-inch telescope

I have a Lumix G7 with a Panasonic H-FS 1442A and want to mount it on a telescope with a 1.25-inch eyepiece.

I know I can get a Micro Four Thirds adapter and attach it directly to the telescope, but that means the lens is not affected.

Is there a way to mount it including the lens?

This would allow me to still have autofocus and to be able to zoom a bit.

The lens has a 46mm internal filter thread and some kind of external attachment that I couldn't identify.

Camera Settings – How can I adjust the aperture of my DSLR when attached to a telescope instead of a lens?

Is it possible to adjust the aperture without a lens?

When you consider that the aperture is a Part of the lens, not the camera bodyNo, this is not possible.

Telescopes usually do not have variable apertures-it is not necessary to confine the incident light (which is absolutely the opposite of what is desired for star photography), and depth-of-field control is meaningless if all objects are no closer than the moon ,

The opening of the telescope is the diameter of the front element. In terms of f-numbers, this is the ratio of the focal length of the telescope divided by the diameter of the front element. Make sure you use the same units (ie, inches or mm / cm / m).

What filter size for the Pentax 100SDUF telescope?

I'm looking forward to using a Pentax 100SDUF telescope as a long focal length lens. I would like to protect the front element with a clear filter. There are no marks on the lens regarding the filter size. There are some threads on the front of the front lens cell that I can thread on. The distance between the ridge combs of the treads is 107.1 mm, measured to the best of our ability. My best Internet search has not revealed which filter size corresponds to this. Being a telescope, it can not be guaranteed that the size of the lens cell actually corresponds to an available filter size. Since this area is no longer produced, I can not ask of any company.

Astrophotography – Connecting a camera to a telescope

In astrophotography with small telescopes there are basically 3 configurations:

  1. "Main focus" = NO camera lens + NO telescope eyepiece
  2. "Eyepiece Projection" = NO camera lens + YES telescope eyepiece
  3. "afocal" = YES Camera Lens + YES Telescope eyepiece

What about this fourth situation:

YES Camera Lens + NO Telescope Eyepiece

Make it useful? If no, why?

Thank you in advance,
Carlo