How do I get full-spec focussed picture on a fast reflector, no coma?

[RayGood]

I have a question about astrophotography. Here is my dilema:

I will be doing Astrophotography with a modded DSLR (UV-IR filter removed) and the SW 130PDS newtonian reflector f/5 speed.

The coma corrector that comes with the OTA. Is it not a refractive element, and if so, should I be imaging without a UV-IR cut filter? Wouldn't my images be out of focus, because of the coma corrector mis-focussing the near-IR and UV wavelengths?

I need to capture IR, H-Alpha, vissible etc. in my pics, using the telescope, but do not want to catch any coma. Hence, the need for the corrector lens. So how should I take my full-spectrum pictures without coma and avoiding out-of-focus shots?

How do the pros do it with a fast newtonian reflector telescope? How do they avoid the coma and get full-spec (vissible, H-alpha, near IR, etc.) focussed pictures with a DSLR.

I have read that an IR-UV cut filter is needed if the optical train contains any refractive elements. If this is so, then I would have to have my modded DSLR slapped on at the focal plane with no coma corrector. Then I would end up having pin-point stars at the centre and seagulls towards the outer portions of the FOV.

You see my dilema? How would one get round this using the gear I have propposed above?

Any experienced advice would be much appreciated. Thankyou.

[Capricorn]

Cannot supply a specific answer but a few ideas:

They probably use a scope that cost 20x the 130PDS, most likely a Ritchey Chretien type, if a reflector.

They do not attempt to take all wavelength at one time as it appears you are thinking of. They will purchase the filters for each band they want, then image at that band and summ the images afterwards. In effect take an image at the IR , then one in the visible (probably in R, G, B seperately) then in the other wavelengths. Possibly could use narrow band filters which extends the exposure by large factors. The camera therefore needs only be a mono camera, preferably cooled.

Also optically there will/need be no eyepiece/focuser. Throw away the secondary and place the camera just above the place that the secondary sits, reduce the components in the path. The secondary amplifies errors by simple laws of reflection, so remove the secondary and reduce errors.

In effect the equipment you are considering is not the equipment that the "pro's" use, neither I suspect is the approach taken to get the images.

[rfdesigner]

I think you'll find the chromatic performance from a coma corrector will be fairly negligable compared to a refractor. The point is, a refractor does all the bending with lenses. so a slight change in bend with colour shows up, so UV/NIR is a problem. With a coma corrector what it is doing is making a slight correction to the bend put in by a mirror. The correction will have chromatic variation, but it's doing much much less than the lenses in a refractor. Result: your 'anti coma' will have chromatism, but it will still be much better than a newt without a corrector or a normal refractor (I'm not including the megabuck refractors here)

Take a look at 'Dream Telescopes' coma corrector. This is how some of the more wealthy amateurs do it: http://www.dreamscopes.com/pages/07/4inCCC-01.htm

Derek

[Merlin66]

Your modded camera also contains the original anti-alias filter which acts as a UV-IR cut filter, this will further supress any residual chromatic effects from the corrector.

[RayGood]

Your modded camera also contains the original anti-alias filter which acts as a UV-IR cut filter, this will further supress any residual chromatic effects from the corrector.

I am very surprised to hear that. How is that so. When the IR-UV cut filter is removed, I thought the sensor would be completely openened up to these wavelengths. What is this "original anti-alias" filter that you mentioned?

[Merlin66]

In all the Canon DSLR >400D there are two filters:

A colour correction filter (this is normally removed to improve the red response)

and

at the front an Anti-Alias/ dust shake filter. This actually acts as a UV-IR cut filter.

The attached transmission graphs tell the whole story.....

Filter #1 is the anti-alias

Filter #2 is the colour correcting

[RayGood]

Thanks people for the input. That kind of answers my question mostly. I have also posted a similar question in the Spectroscopy forum about prefferences to using and not using a coma corrector in my circumstance when imaging spectra. And where I should position the star in the fov, to capture both the zero order and 1st order spectra. And wahtever position in the fov, would more accurate results be obtained with or without the coma corrector. I am also worried about post-calibration of the spectra if using a corrector in the optical train, because the spectrum might not be linear across the sensor and a complex formula might be required to correctly calibrate the spectrum. So what method would be best to yield the easiest and most accurate spectra? Star centered, off-centered, with or without the corrector lens?

[Merlin66]

I assume you talking about a Star Analyser or Rainbow Optics grating used in a nosepiece of the DSLR mounted on a scope??

The distance between the CCD chip and the grating defines the dispersion (and resolution to a degree) for a DSLR the maximum distance is about 75mm

The zero order star image should be placed close to the edge of the field and the bright first order spectrum as close to the centre as possible.

You don't need to use the CC.

Set the camera so the spectrunm is sitting horizontally across the frame - the closer the better. Focus on the spectrum, not the zero order image. Try to get the absorption lines as well defined as possible.

Depending on the capabilities of your software - VSpec and RSpec allow you to calibrate non-linear spectra - this is not a serious issue.

Start with a A type star - Vega You'll find it much easier to see the necessary lines and it's a standard calibration star.

(It's all in the book! ;-) )