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alan687

Telephoto Vs Telescope

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Both telescopes and camera lenses have their place in astro imaging. There's a crossover point at ~400mm FL and below where telescopes end and telephoto's take over anyway.

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I've done some lens imaging. Can't claim to be too disappointed. I think that, to produce something that you have in your sights, you need a bit of advance planning but, that said, just have a go!

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-gfQmRVJ/0/X3/ORION%202014%20reprocessWEB-X3.jpg

Olly

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@laser_jock99, my understanding is that this problem is caused by an incomplete mod. After the filter is removed, it must be replaced with a piece of optical glass of the same thickness or there will be focus issues. Was this done?--Jack

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I noticed that my modded DSLR doesn't like standard camera lenses so much anymore.

It can even be impossible to focus as the lens just doesn't have enough focuser travel to reach infinity focus with longer wavelength light.

The lenses are also not designed to compensate for aberrations in IR. Hot spots and field curvature are more apparent.

Of course this isn't an issue if you don't modify the camera ;)

Hi

I believe sometimes it's necessary to add a shim in order to maintain infinite focus with long lenses. I know Cheap Astrophotography do this.

Louise

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No, that makes no sense at all. As Olly said focal length is focal length. The crop factor only applies to field of view compared with 35 mm film, it can't be used for anyting else.

It doesn't make any sense, and that's why "crop-factors" shouldn't be used, they're meaningless, and that's why I finished with "...only it doesn't, it's still a 200mm f/2.8", however people do use crop-factor multipliers, but selectively and in a way which introduces even more errors!

If you do elect to compare the field of view by multiplying the FL by 1.6 (or 2, or whatever) then you also have to multiply the maximum aperture if you're going to describe the optical characteristics of the lens, this is something which the camera manufactures and many users fail to do!

A 200mm f/2.8 lens has a particular set of optical characteristics, a specific depth-of-field and a specific compression of perspective.  If someone elects to apply a crop-factor multiplier for an APSC sized sensor for example of 1.6x, and then state that they have a 320mm f/2.8 lens then they're completely misrepresenting what they have.  In depth of field terms and apparent angular field of view terms the lens may appear to be equivalent to a 320mm f/4.5 (both multiplied by the 1.6 crop-factor) but perspective compression will still be the same as a 200mm lens.

Crop-factor multipliers shouldn't be used at all, but unfortunately they are.  If they are going to be used then they should be more accurately used, and accepted for their shortcomings.

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@SJP ,  "Telephoto lenses result in horrendous (or artistic if you're that way inclined) diffraction spikes unless you use an aperture-mask instead of the inbuilt iris diaphragm, ....."

You must be using the wrong telephoto. Different manufacturers use different blade configurations. Yes lenses can produce "sun stars" which are the same effect, but a result of much higher light levels than any but our local star can produce. I've not seen diffraction spikes produced by my telephotos, even when shooting Vega (Lyra).

There are few lenses available which offer true rounded iris diaphragms, and whilst multi-bladed square edge blades improve upon versions with less blades they do still exhibit spikes when stopped-down.

Here's an image taken at f/8 with a Sigma telephoto-zoom, its 9 iris-blades have given rise to eighteen-point stars

post-35316-0-51335200-1408012399_thumb.j

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While not technically accurate, that's still a pretty cool photo :)

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@SJP I see what your saying, but I thought (excuse if I'm wrong) that standard practice when doing astro with a telephoto is to stop down no more than required to eliminate CA. This is because the aperture appears more round at a larger aperture. For my lenses one whole stop is sufficient. I've also heard that prime lenses are more suitable than zooms.

As I stated above, a purpose built device in it's parameters should always outperform a general purpose device operating at the extreme of it's. In general photography the higher f numbers are to be avoided due to diffraction. I'd like to see the same shot with the same lens using one stop down from wide open and a longer exposure.

--Jack

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The one advantage of a cropped sensor with a full frame lens is that some of the far edge artifacts dont appear on the image so i find less need to stop down in fact a lot of targets get shot wide open.

I have tried aperature masks but cannot make them accurate enough and close enough to the glass but am experimenting using old filters with the glass removed now, if only i could cut a round hole.

Alan

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I have tried aperature masks but cannot make them accurate enough

Just an idea to try which I thought about at the beginning of this topic, how about using a cheap UV filter screw on to the front of the lens and use a white board marker to make your mask. If you are not happy with the size of it then you can wipe it off and do another one.

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The one advantage of a cropped sensor with a full frame lens is that some of the far edge artifacts dont appear on the image so i find less need to stop down in fact a lot of targets get shot wide open.

I have tried aperature masks but cannot make them accurate enough and close enough to the glass but am experimenting using old filters with the glass removed now, if only i could cut a round hole.

Alan

It's easy. You can buy a compass cutter from a graphics outlet.

Olly

APERTURE%20MASK-L.jpg

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Thanks Olly I have plenty of suitable plastic from old cd/dvd covers so will give it a go just have to remember to cut the outer circle 1st.

Alan

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Exactly! Like not sawing off the branch you are sitting on...

Olly

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I noticed that my modded DSLR doesn't like standard camera lenses so much anymore.

It can even be impossible to focus as the lens just doesn't have enough focuser travel to reach infinity focus with longer wavelength light.

The lenses are also not designed to compensate for aberrations in IR. Hot spots and field curvature are more apparent.

Of course this isn't an issue if you don't modify the camera ;)

It sounds like you removed the sensor filter but didn't replace it with anything, which means you changed the amount of glass between the sensor and lens, and thus the light path. Therefore it won't focus to infinity any more.

It you replace the filter with glass of the same thickness, eg. just clear glass or Baader IR filter glass, you will regain normal focus. Another way is to use an EOS clip filter, which are about the same thickness and will also regain focus.

Of course, these don't fit with EOS lenses! (Unless you slice the rear protrusion off!).

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Camera lenses are good for AP as they are generally shorter focal length and more compact than scopes, making them easier to image with.

But it's hard to get a lens which gives you decent stars to the edge of the frame, particularly zoom lenses. However the Canon kits lenses to a reasonable job as does the nifty fifty.

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You might try one of these little Canon lenses - if you have $99,000 going spare....  http://www.bhphotovideo.com/c/find/newsLetter/Mother-of-All-L-Lenses.jsp

Mother7.jpg

ChrisH

thanks Chris, you know, i have been staring at my wallet for a few days now and couldn't work out to spend my 100k pocket change on, but now i do :grin: lol

On a serious note, this is a bit excessive, just imagine the scope, mount, ccd setup you could put together for 100k, it would make this telephoto look like a toy from argos lol

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Hmmmm, how much WOULD a 200mm f/5.6 APO cost?

My Marriage :)

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Perhaps I missed it if mentioned previously, but one major drawback to camera and lens is inability to use proper filters. I understand there may be some clip in filters for Canon, but in general all the really good filters are offered in 1.25" and 2" only. --Jack

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Perhaps I missed it if mentioned previously, but one major drawback to camera and lens is inability to use proper filters. I understand there may be some clip in filters for Canon, but in general all the really good filters are offered in 1.25" and 2" only. --Jack

I'm using 2inch Hutechs with my two EF 200mm Ls.

Using them as am aperture mask, 72 to 52 stepdown ring, 52 to 48 stepdown, screw filter into 48mm ring.

The filter clear aperture is approx 44mm, so this gives about f4.5, which is about where the stars are really nice.

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I just use a 52-48mm step down ring for a uhc filter on my Fz200. Just clips the corners at wide angle but otherwise great.

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I had a Nikon mount version a few years ago & for the money it was a good lens. It did need stopping down to f8 & was a little soft at 300mm though. How good it would be for astrophotography though, I couldn't say. For the price it may be worth a punt. Edited by rob0971

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