Various focal length scopes for deep space imaging
So i'm wondering what I should consider next for expanding the range of options for deep-space imaging with my 350D.
Currently:
- I have a Saxon ED80, focal length 600mm f/7.5. Good for fairly large objects or wider starfields but not so great for smaller objects, as they cover a very small area on the camera chip.
- I'm getting a WO 0.8x reducer, which will give me a focal length of 480mm and a focal ratio of f/6. Again, good for large, wide objects or wider starfields.
But what if the object is small and I want a closer view..
- I could get a 2" 2x barlow, giving me a focal length of 1200mm, but slowing the scope down to f/15.. meaning longer exposures are required.
- I could use a 2" 2x barlow in combination with the WO reducer to give me around 960mm focal length but still very slow at f/12.
So what about a newt then? A 6" or 8" newt would give me similar focal lengths (800-1000mm). The advantages include a larger aperture, and a faster focal ratio (f/5 or f/6 usually) which means I can use much shorter exposures. (and look at what Garyh and Ezystyles (Eric) do with their 6" and 10" newts!! )
Sounds all good?
I guess there are some disadvantages. I'd also need:
- An MPCC to reduce the coma,
- A set of rings/dovetail to hold it on the EQ6,
- Some sort of piggyback system for the ED80 or cheapo refractor to guide with (although I still need to do this with the ED80 on its own for guiding).
So, more cost. Probably more hassle. And do I really want to delve deeper into deep space imaging? I'm not sure yet
Anyway i'm just thinking out loud so your opinions and suggested are most welcomed.
hahaha sounds like me Mike, although you have been in the business much longer then me. at the moment im not too fussed about longer exposures hence why im looking at the Meade ETX-125PE due to the goto capabilities (unless there is a better one out there for around the same price??) so i can learn the sky. but on the other hand, im also looking at the 10" Dob due to the aperature and focal length for the deeper space stuff.
personally id hit up a cheaper refractor piggybacked to a 10" newt and use that to guide it with (not sure if its possible, but would look cool nonetheless and it would be a decent finderscope haha).
anyways thats my two cents....
What i find most important particularly with the 350d if your f ratio, the lower you go the deeper and more detail you capture. the 350d from my experience is good for about 6 mins of exposure after that color balance goes red and no noticeable improvement in depth
Dont go the barlow as it will be too slow exept for maybee really bright stuff M42 etc.
i cant see the eq6 holding the 12 inch plus your ed80 for guiding i use this on a g11 and i reckon its as heavy as i can go.
Like many do, don't lose sight of the arcsec/pixel combination Mike. Your telescope and camera are one. They should compliment each other, not work against. As the focal length changes, so will the arcsec/pixel value. By all means you can image DSOs at .25 arcsec/pixel, but will be wasting sensitivity with little gain to resolution (due to seeing limitations). A higher arcsec/pixel such as 2 or so will capture more light per pixel increasing sensitivity. Too high and you'll lose resolution. This is the reason why on the end of many long focal length instruments, you'll find cameras with 16u - 24u+ pixel size. They are a better match for the telescope focal length.
Mike, to use the chart, simply draw a straight line from your right to left. i.e start with your pixel size (right) and draw a line through to the left for DSO arc/sec pixel (left). As the line intersects the middle virtual line - you will know the focal length in mm to achieve this desired combination. You can of course calculate from left to right - what ever way, the result is the same.
I should point out that this is only a reference. There isn't any hard and fast rules. For example, the Tak FSQ I use has a arcsec/pixel of 3.501 when coupled to the SBIG STL11k 9ux9u pixel size and makes an excellent all weather wide field instrument that is not troubled by seeing. Nyquist Theorem indicates you should sample at twice the frequency you want to resolve.
Arcsec/pixel = pixel size in microns / (0.00485 x telescope focal length in mm).If your pixels aren't perfectly square you can simply square root the two - sqrt(W x H)= X microns). The most accurate way to determine the arcsec/pixel combination is to plate solve.
The downside of a DSLR is the lack of support for binning. Binning provides greater flexibility when trying to reach the optimal arcsec/pixel combination.
Mike, download the CCD Calculator from The New Astronomy Press website. It is great for modifying FL, Pixel sizes, FR and gives results of pixel resolution, images size in arcmin. You can even input and save cameras and scope dimensions that are not included in the supplied list.
I find it fantastic and would be lost without it when trying to calculate the best options for camera/scope/object.
given the chart then a 350d for deep sky would be 600-1000mm range.
so whats wrong with 1500mm as it seems to work ok,( im not trying to be difficult)
There is nothing stopping you from using the 350D at 1500mm. How optimal is the question. If we take the example of a C9.25 @ F/6.3 - it delivers 1480mm. Coupled to the 350D pixel size you get an arcsec/pixel combination of .90 arcsec/pixel.
Now ask yourself, how many nights a year will the seeing get below an average FWHM of 1 to take advantage of this arcsec/pixel combination?
But Jase, OK .90 arcsecs/pixel doesnt make full use of the available res, but given the huge leap to an astrocam, its not a big disadvantage, surley. Better that than too far the otherway;-).
Im not sure about this, but wont a barlow give massive vignetting with a DSLR?.
Mike, the ED80 DSLR is a nifty combo, seems to me you could get some increadible images 1st before you move to a longer FL. A longer FL especially over 2000mm is fraught with pain to get the image quality I suspect you will eventually hope for, youd really need to spend big for a good mount 1st methinks.
Fred I have no intention of going over 1000mm focal length. You're right - I'd need a much more solid mount that my shaky EQ6 which has been driving a 12" newt around on it.
Plus, at longer focal lengths, perfect polar alignment is much more critical and I am too lazy to properly polar align.
Only if the seeing will allow it Fahim. There is no point in having a 0.5 arcsec/pixel resolution when the seeing will only allow 2 aresec/pixel. You are sacrificing sensitivity for unachievable resolution. Better to reduce your focal length or increase your pixel size (binning) to increase sensitivity rather than attempt high resolution images.
This is applied to DSO imaging rather than planetary imaging. By using many images as is done with planetary imaging improved resolution can be achieved by interating bits of repeated detail from multiple images.
I have spent many hours looking over photocollector and Ezystyles images and from what I can gather they are using sub to arcsecond setups. How do they get such great images? Paul is using a 12" F5 1500mm and Eric is using 10" F4.7 1175mm these would produce ~1 arcsecond. Whats the secret? I am considering a SN6 + ED80 setup (ideally i would like a SN8 hard to find), perhaps later a custom made Newtonian like Bird's. Right now I can go 8" F6.3 (with FR) on my LX90 and I have seen good results from this combo from 5ash who has produced some great images with this setup.
And I have read somewhere b4 that binning is possible with DSLR images in photoshop, is this right?
Various focal length scopes for deep space imaging
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