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sp900nc magnification?


willcastle
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when viewing saturn through my sp900nc (without barlow), the magnification seemed higher than when i used my 25mm eyepiece (my lowest magnification)!

how does the webcam magnify? iv herd its to do with the distance of the ccd chip from the eyepiece or something? but i dont have a lens in the eye piece so its all rather confusing to me!

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I can't see how you can define magnification when viewing an image on a screen. How big is the screen?? How zoomed in are you? Etc.

In imaging you normally talk of plate scale, which is arcseconds on the sky in terms of mm on the chip.

Olly

Edited by ollypenrice
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You are quite right Olly, but I assumed that the OP had noticed, as in perceived, like I have, that the proportion of the view filled by an object with a 25mm lens is much less than the proportion of the screen that is filled by an object with his webcam. The webcam affords a similar view to a 6mm EP.

James, this is where magnification/zoom factor with camera lenses comes from, with 35mm film cameras a 200mm lens gives you x4 because 35mm film is 50mm diagonally. 200/50=4

Barry

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ahhh thankyou! very interesting! and yes thats totally right bazzaar! i was indeed talking in terms of proportions! however, if that is true... a 6mm eyepiece would give me 167x magnification and the most useful magnification of my telescope is 400x

my intention is to buy a 4x imagemate to increase my f5 focal length... but if the webcam itself is already at 167x and i add the barlow would that not zoom beyond the useful magnification of my telescope (667x)?

would this mean my only option is to buy an eyepiece projection adaptor to use in conjunction with the imagemate and an eyepiece to effectively cut down the magnification?

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Whooooo

It doesn't work like that!

Forget magnification and think about the size of the image "projected" by your scope onto the CCD chip.

The size of the pixels and the focal length/ focal ratio of your scope will define the outcome.

Generally for detailed imaging ie moon/ planets etc a focal ratio up to approx f30 works well. So if your scope is an f5 then using a x4 barlow would give you f20....

The plate scale is measured (as Olly said) in arc sec/ pixel. This will define the best possible resolution.

When you start to consider imaging.....magnification gets left behind.

Google CCDcalc for a freeware program which will allow you to see the image on the CCD for various focal lengths....

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on another note... i have seen other users with x4 imagemates and an sp900nc... which would surely magnify way beyond their maximum useful magnifications... iv even seen people doing this on tiny 5inch scopes...

so do you disregard the highest useful mag when doing ccd?

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so with a skywatcher 8" 200p explorer (newtonian) using a 4x barlow (imagemate) and my sp900nc, i will be able to get decent planetary images?

iv downloaded the software but im still in the process of getting my head round all this arcsecond business! thanks for the help so far though

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As others have said, dont get hung up on the max magnification number.

What I would say about barlows though, the number of opportunities to use a x4 are few and far between, I borrowed an imagemate x4 and a Meade x2 to try and most of the time the imagemate didnt reveal anymore detail because of condidtions.

When conditions allowed it was stunning!

Ideally you need both and use as conditions allow. If you cant have both, get an x2, you'll get more use from it.

I ended up getting a second-hand Antares APO barlow, thats really nice, no colouration, great for imaging.

Barry

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ah barry iv only just noticed you have a 200p aswell! my first impressions have been fantastic! shame about my house having total cloud cover for the last 4 days!!

but luckily my scope came with a x2 barlow so im fortunate enough to have one already! have you upgraded your focuser?

and the antares? was it a x3 or x5? would i get even less from the x5 than the x4? i have heard about a little bit of discolouration on the AE imagemate.... but at the same time i have a good feeling about it and i have seen some incredible results across the net from using them!

im also about to order motor tracking! as i have noticed its quite literally essential! is it worth paying the extra £20-30 for the dual axis drive?

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i did quite a bit of imaging with a c8n similar to your 200p also with no tracking.its certainly possible to get some good images with the x2 Barlow but keeping the object on the chip with x4 is a challenge,but it can be done just remember which way to turn the slo mo knobs!

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The standard Skywatcher x2 barlow is very poor, the Antares x1.5 is a big improvement. I dont know why I would need to upgrade the focuser, so it still standard. I would say the dual motor drive is not essential, you only ever need to tweak the dec axis occasionally.

Barry

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  • 2 weeks later...

Glad I found this thread, seems to be exactly what I'm looking for :D

I have a 200p also and have recently purchased the Phillips modded webcam from Morgans.

I had read that achieving F20 was desirable but was wondering how that would work with magnification as I would need a x4 Barlow to achieve F20 with this scope.

Am I understanding correctly that this won't give a magniifcation of x800 then? Far in excess of the max power of the scope.

If so, can anyone recommend somewhere to get Barlows of this size, I can only find x2 on FLO (which I'm going to get anyway)

Thanks for any help

Justin

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I find thinking in terms of magnification makes it easier to get a grasp of the size of image that will result. After a bit of use with scopes and viewing objects at different magnifications it comes more naturally to us.

Justin, I think you have your numbers wrong, how did you get x800? although its academic really, the max power figure of a scope is a bit of an anachronism that doesnt apply for imaging, I'm not sure why, but hardly anyone worries about it. Look again on FLO there are many barlows at other magnifications than x2.

Barry

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Apparently the camera is the equivalent of a 6mm eye piece which is x200 with my scope. x4 Barlow and QED :hello2:

If it's wrong do I at least get points for showing my workings? :)

I'll have another look on FLO, maybe it's a bit more subtle than putting x2, x3, x4 etc in the titles :)

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You are still trying to compare a camera's field of view to an eyepiece's magnification. If you replace the webcam with a camera with a larger sensor, a DSLR for example, the field of view changes. The field of view will now be equivalent to a lower power eyepiece. The arcsec/pixel ratio will not change much if the two sensors have similar sized pixels. For planetary/lunar imaging you need an arcsec/pixel value of between 0.5 and 0.33 to get the best out of your camera/telescope combination. For DSO imaging the ratio is between 1.5 and 2.0

The formula to find the ratio is 206.265 x P / L. Where P is the pixel size in microns and L is the focal length of the telescope in mm. This tends to be a value that gives a focal ratio of between F20 and F32.

Peter

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Peter, what you are saying is all very well and correct and completely unhelpful.

Beginners to the art of imaging may need to take an incremental approach to getting there heads around the subject. So a rule of thumb is good enough to start with.

We routinely overlook the fact that eyepieces have different fields of view and happily except that an ep of Xmm gives magnification of Y.

Justin, a 6mm ep gives a mag of 166 in your scope, you need 5mm for 200. 8/10, and only 'cos you showed your working! :)

Barry

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Thanks guys, I do appreciate all input, however much I struggle with the theory!

Did try some imaging tonight and it all turned out academic anyway as I couldn't focus on Saturn. Couldn't seem to get the webcam anywhere near close enough to the mirror. I was only familiarising myself with the equipment and software however.

I haven't got a barlow yet which I'm hoping will sort the focus problem out???

Looked good through the EP though :)

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Peter, what you are saying is all very well and correct and completely unhelpful.

Beginners to the art of imaging may need to take an incremental approach to getting there heads around the subject. So a rule of thumb is good enough to start with.

We routinely overlook the fact that eyepieces have different fields of view and happily except that an ep of Xmm gives magnification of Y.

Justin, a 6mm ep gives a mag of 166 in your scope, you need 5mm for 200. 8/10, and only 'cos you showed your working! :)

Barry

The term "magnification" is irrelevant in imaging. The field of view or image scale changes with the size of the sensor, you don't start seeing smaller details in the image because a webcam gives a greater "magnification" than a DSLR at the same focal length. If I image with my Skymax150 at F24 (focal length 3600mm) this would give me a "magnification" of 600x with a webcam, which is of course rubbish because the practical maximum magnification for visual use is about x300. If I then use a DSLR at the same focal length the "magnification" will then be less. It is the field of view that changes. Using the term "equivalent field of view" is OK when comparing eyepieces with cameras but not "magnification" as this can lead to confusion.

The formula shows where the idea focal ratio or planetary/lunar imaging of F20 to F32 comes from. Ideally you need at least a block of 2x2 pixels per 1 arcsec of resolution, but not more than 3x3 to get the best out of a telescope/camera combination. For planetary/lunar imaging this tends to be somewhere between F20 and F32.

Peter

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