I chose an RC design scope (GSO RC8) as I felt it was a less expensive option than a high end refractor in avoiding aberrations such as chromatic. I have just run my first few images and notice that the diffraction spikes show colour.
My understanding is that the RC design provides a refraction free path and I wonder is the colouring a normal artifact of spike diffraction or something unusual.
I should have included in my post that the image was a single exposure taken with a canon M6 mirrorless APS-C. Exposure 30” at ISO 1000. Hope this might help any comments.
It may be a silly question, but were you doing prime focus, or using eyepiece
projection? If the latter, the CA would almost certainly be caused by the eyepiece.
Incidentally, do you get CA on the Moon?
raymo
Last edited by raymo; 25-02-2020 at 01:42 AM.
Reason: correction
Don't feel bad Geoff, even Hubble is guilty of such diffraction effects, as seen in this narrowband? and Infra Red image below.
Different wavelengths of light (colours) diffract slightly differently around the edges of the spider/secondary support structure. You have particularly sharp focus in your image, which shows it up very clearly.
Also in addition to the diffraction above, if there is an element in the image that is close to the camera pixel spacing there can be aliasing effects/moire/false colour created
I don't recall such colourful diffraction spikes in Newtonians. At least not in images not using any form of lens (coma corrector, barlow, etc) before the camera.
Does a single or two edges produce the same effect as a diffraction grating? Also, the direction of the spectrum dispersion in a grating is also parallel to the direction of the lines, not perpendicular as seen in the OP's pic and in the Hubble pic.
My first thinking us the spectrum that is seen is also an artefact of internal reflections in the RC lens arrangement on because of the diffraction pattern. Further more because of the repeated sequence of the spectrum. The lens & coatings are not designed to deal with the diffraction caused by the spider.
Abberations have many different sources, and some are unexpected consequences, not all anticipated.
Raymo the image was taken at prime focus and that's what surprised me as I felt there should have been no refractive effects. This was among the first shots I took with the GSO so haven't done any moon shots yet.
Thanks JA for your reassurance that even Hubble produces the effect as it was knowing Hubbles optical config that I went RC. I suppose I haven't looked hard enough at Hubble images to notice.
As to whether I like the coloured spike effect I don't really know but would be good to know if I can produce images without the colour. I do have another DSLR (a canon 60Da) which I will try when I break it out of its storage but it is also an APS-C sensor so I don't know if the effects will be any different.
Your not the only one who has seen coloured diffraction spikes in images
My images of Nebula ( eg M42 ) which have larger stars in the background and therefore very long diffraction spikes usually have RGB colours emanating about half way out along the diffraction spike until it fades into interstellar space again
I use a 6” f6 newt with DSLR and a 8” f5 newt with DSLR
It’s more prominent on the 8” f5
I’m just putting it down to the cameras sensor picking up some Ha OIII etc..in the nebulosity cloud behind the diffraction spike as the stars white light intensity diminishes outward into interstellar space
That’s my theory anyway , could be totally barking up the wrong tree !!!
Its normal to see them in RC scopes. I have them in my 6 RC GSO and seen them in 24" RC scopes using a professional CCD.
Here is my image of the lagoon nebula using LRGB HA and OIII with the ZWO 1600mm. https://www.astrobin.com/full/362145/0/
Have been torturing my head trying to read some publications on light diffraction. It seems that chromatic separation is a natural property of light when diffracted as each wavelength is diffracted slightly differently and hence chromatic effects are to expected after diffraction of white light.
I have cropped the image to examine the spikes more closely and it looks to me like a definite spectral continuum and I also noticed that the spikes actually consist of two bars side by side. Maybe this has to do with the width of the spiders.
Am learning to accept them and maybe it might tell me a bit about how good I can get my focusing.
I use the spikes to check and fine tune the focus. Here's a pick of a star with my RC6 and a cropped image. The double lines should merge when focus is sharp.
Rick
I’ve been using 2.0 to 2.5 magnitude stars near my image target with their diffraction spikes to achieve fine focus for years. It usually takes around 5 to 10 images with exposures at 20 seconds to compare and nail my focus as tight as possible , beats FWHM hands down
This is what I got purposely on Sirius as I wanted the see the diffraction spikes and it does help with focus.
By the double spikes I could tell I had 3 spider vanes that were twisted in my Saxon 10" reflector, 1 more than the other 2 which were later corrected. This was at a high ISO but short exposure around 15 seconds from memory, with a Sony A5000 mirrorless. My image was also edited to enhance the spikes and flaring.
I believe any telescope with spider vanes will give diffraction spikes to a certain degree on a bright star. The thicker the spider vane the fatter and shorter the spikes, the thinner the spider vanes the thinner and longer the spikes with the colour spectrum depending on your exposure settings, that's from what I've noticed in other peoples images online with varying scopes.
To visually see which vane is out I put a small hand spirit level on the vane and sighted along the bottom of the spirit level that's sitting flat on the spider vane, it should point directly at the image of your spider vane on your primary mirror. This was a rough guide for me to correcting the spider vanes in my 1200mm focal length Newt. Best to do it at each end of each spider vane.
If you are going to test it this way, put your telescope flat on a table, not pointing up because if you accidentally drop the spirit level you don't want it falling on to the primary mirror.
It looks like to me Geoff that the spider vane that's pointing to the top right hand corner in your image is tilted, not twisted.
The spider vane that's pointing to the bottom right looks tilted with slight twist.
The spider vane that's pointed to the bottom left looks to be slightly tilted.
The spider vane top left is close to good.
The focus looks to be a bit off so the top left spider vane may actually be pretty good so I wouldn't touch that one for now.
Last edited by Aussie_Dave; 27-02-2020 at 02:42 AM.
I don't recall such colourful diffraction spikes in Newtonians. At least not in images not using any form of lens (coma corrector, barlow, etc) before the camera.
