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Thread: Refractors for Astrophotography AP?

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    Default Refractors for Astrophotography AP?



    Hi,
    Ive been doing some rough AP with my 10" newtonian recently and im not really happy with it. The coma is pretty horrible and the f/l is to long for me to get M45 and M31. I was thinking of going down to an f5 refractor but I have a few questions:

    1) if my newtonian was an F5, will the brightness of the image be the same, just a wider FOV?

    2) Do fast refractors suffer from Coma?

    3) Is there a huge difference between Apochromatic and non-apochromatic refractors? If so what?

    Thanks for your help
    Lee
    Last edited by admin; 02-19-2011 at 12:52 AM. Reason: changed title hope you don't mind - thanks admin

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    Hello,

    can not give you advice on 1 and 2 but for question 3 where is a difference thats why I'm looking for 80ED Apo refractors focus all colors in one point while non-apo focuses them in different points and you get wrong colors like violet around the moon and so on.

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    Hi and welcome to our forums.

    1. FOV is to do with focal length and magnification. f5 is brighter than f10 , f3 is brighter than f5.

    2. Refractors do not normally show Coma, thats a trait of Reflectors ( Newtonians )

    Svetimas gave good explanation regarding 3.

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    So will the sharpness of the image be better with an apo?

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    Hi,

    I don't know about your Newt, most folks use coma correctors of some sort, like the baader MPCC and there are others. Might better ask about Newts on the Newt forum? One thing often neglected in discussions about really fast imaging optics is the narrowness of the focal plane for critical best focus. For this I mean below f/6, can be a good or bad thing depending on you imaging rig and focuser placement, out/in focus, drawtube, 10:1 or helical and all that. Gets more complex if adding field flatteners and coma correctors.

    I'll try a stab at the 3 questions
    1) Image brightness depends on exposure time and pixel size to the fov, and will vary somewhat depending on the specific imaging sensor. Most modern CCD's can get whatever you need as far as that is concerned. As far as light per pixel it's all about the pixel size for the aperture, and the pixel resolution of your seeing and tracking. Many folks consider 2 arcsecond seeing as a sort of optimistic good point. Here is one discussion for DSLR's that popped up in a google search I just did. Another good short tutorial here...The sort of standard is R. Wodawki freeware CCD Calc.

    2) That's difficult, but in general for doublets and triplets 'yes', but most imaging scopes need a reducer and or reducer and field flattener to get down to that f ratio. There are Petzval refractors, a 'quadruplet' type design that can be both fast and flat... but the more lenses and surfaces the more difficult the QC and etc... but a great sort of Petzval type gold standard is the Takahashi 'Q' series. So answer becomes, yes bit it depends... more important question is in any steep light cone where the center to edge light path difference is increasing - can the issue of coma be addressed easily? I'd say refractors in general are great for theis as the pocuser by nature and it's placement in the light cone allow for greater versatility and flexibility to put what is needed into the path and still retain focal requirements. If you look at the Takahashi and Astro Physics brands both off complete dedicated solutions including reducers and flatteners made specifically for their scopes.

    3) Yes, but again there's lots to think about. There is some talk of apochromats dedicated to visual and apochromats dedicated to visual. This has more to do with CCD's and their sensitivity WRT data and imaging CCD can collect vs. human eye - sure there is a huge overlap and to many it's 'hair splitting'. why? well what really matters is are the 3 primary wavelengths the designer figured optics to all in focus at the focal plane enopugh that the light energy is in the critical focal plane. This out of focus light makes for blurry, fuzzy, bloated, or maybe soft image. (but there's a little more to it than that and I'm over simplifying) CCD are very sensitive to this and the saying is that 'imaging takes no prisoners', my experience is that is true.

    But there is way more to it. You can monochrome image and image at critical focus for the particular wavelength(s) and not have to deal with the 'Bayer Matrix' deconvolution or interpolation. I've seen some great work done with quality doublets.

    One thing I want to mention about imaging and refractors in general is that it's not as simple as getting an f/7 or 8 refractor and grabbing an off the shelf one size fits all reducer or flattener. There are companies as mentioned above that offer complete systems for the specific lens prescription used for the main aperture lenses, and there are some that require tolerances in mm's for critical hardware placement! There are some reduces or flatteners that can be mix/matched - but it's fairly specific on a scope by scope basis.

    Going back to the overall picture. As far as f ratio and aperture, what is most important is the quality of the lens figure and the initial starting conditions of the entire optic including focuser design and placement. Any out of focus light is essentially image information 'lost' or contributing to a noisy image. Some of this can be overcome with the modern signal processing workflow of the raw data, some can be overcome, reduced, elimenated by post processing to point of nearly creating a painting of acceptable portions of the image data.

    One way around the chroma issues is to go with a dedicated DK, modified DK, RC, modified RC, or other reflector or astrograph designs as the light isn't refracted, just reflected - reducing issues to coma and coma correction.

    Whatever the case you have a lot, lot of flexibility these days in choosing an imaging setup - it's getting the overall system to fit at what you need it to do. You'll see fantastic images made with all types of gear. So I like to think of the focal ratio question of having more to due with the field of view exclusively and the matter then to find a good pairing of CCD pixel size to it.
    Last edited by klaatu2u; 02-18-2011 at 03:29 PM.

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    What sort of camera and what sort of mount do you use? If it's a DSLR and some tracking mount - try camera's own tele lens first. Some of them are sold quite cheaply here http://www.overstock.com/search?keyw...rchType=Header They are APO and give you nice wide field photos. As I can see on your Profile your second telescope is a MAK - it's perfectly suitable for larger magnifications AP, and might be much easier on coma.
    Last edited by mplanet62; 02-18-2011 at 11:45 PM.

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    1) At a given aperture, a shorter focal length generally gives a wider field of view.

    2) Refractors are inherently corrected for coma, but "fast" also means more field curvature unless you go to an advanced design like a Petzval or add a field flattener. CCD sensors and film are flat, and unlike the human eye they can't compensate for changes in focus on the fly. Field curvature that wouldn't be much trouble for visual use will make the stars out of focus away from the center of the image.

    3) Achromatic refractors are not much use for astrophotography because the far ends of the spectrum, violet and red, are out of focus. Our eyes don't see these colors well, but a color CCD sensor will show them as bloated stars with halos. An apo refractor controls the colors over a broader spectrum, even at low f/ratios, and is ideal for AP.

    Some people do narrow-band imaging with a monochrome CCD camera and filters designed to pass only certain wavelengths. I suppose an achromatic refractor might be OK for this kind of AP as long as it is focused properly at each different wavelength.

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    If you want to image wide field Targets, such as M45 or M31, a 400mm to 500mm FL Refractor will work well, provided the Camera also has an apropriate sized pixel array. You get to the Focal Length by adjusting the (aperture size) x (focal ratio).

    Download the free CCDCalc from Ron Wodaski and check, Scope to Camera combinations with various Targets. It' allot to absorb at first, but it's also a good starting point. AlanP
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    Non apochromatic refractors are called achromatic and usually have two focussing lenses(called a doublet)and therefore only focus two wavelengths of light at one point and the 3rd wavelength is not focussed at the same point and leads to color fringing or 'chromatic abberation'.This usually presents as a coloured fringe on bright high contrast objects like the moon or Jupiter etc and doesn't affect deep sky objects.You can use a minus violet filter to reduce this effect although they tend to introduce a slight yellowing of the image.
    I use the Baader "fringe killer" filter on my 6" achromat and find it almost eliminates the fringing effect on bright objects.

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    I would imagine that a Achromat could also work well for Ha imaging. And would be a less expensive way to get a large Scope and great images, that work work well regardless of the Moon phase. AlanP

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