Focal length of Telescopes vis-a-vis Telephoto lenses

Discussion in 'Astrophotography' started by Cruzan80, Feb 28, 2014.

  1. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    So I am a bit confused here. I know that there are some super long tele-lenses out there for multiple mounts, as well as telescopes you can mount a camera on (through various means). Are the f-stops and focal lengths directly comparable, or do they have some other bit that means that comparisons will be off? Just seeing these shots that look amazing, and wanting to start getting into it a bit, but trying to find a great "starter" deal to make sure I like it before investing. I understand that f-stops dictate exposure times. Telescopes tend to be faster, so as to lessen exposure time, lessening star drift? Or it is easier/cheaper to make a fast telescope as it can be a mirror lens design, and the donut bokeh never gets seen? I guess what I am asking is what are the benefits to each, and how you would do a comparison as far as FoV and exposure times between the two. Is it just like comparing lenses?
     
  2. Petrochemist

    Petrochemist Mu-43 Top Veteran

    655
    Mar 21, 2013
    N Essex, UK
    Mike
    Yes the focal; lengths & f stops are comparable.
    Telescopes are not necessarily faster. My budget refractor has a fixed eyepiece and came with a T2 adapter to mount a camera. It works out as a 1000 f/16 to 4000 f/64 zoom. Effectively for photography there is no point in zooming in.
    my Newtonian is a 560 f/5.6 not significantly different from the long end of the Bigmos (500 f/6.3) but it allows eyepieces to be fitted to increase image size significantly at the expense of focal ratio (f stop).

    Most telescopes are optimised for visual work rather than photography so can have a little shift in focus towards the corners. Mine are visual ones and I've not really noticed this downfall as yet though.

    For astro work all the subjects are effectively at the same distance (Yes the moon is MUCH closer than he stars, but all are near infinity) so Bokeh is not an issue.

    Like mirror lenses telescopes have a fixed aperture, no autofocus, and (some designs) can have doughnut bokeh for terrestrial use. Unlike mirror lenses they are usually big enough to be a pain to handle, and need a VERY solid tripod.

    For astro work expect to pay as much for the support as for the scope, and don't expect to get results as good as aloysius till your spent a great deal of time & money on it. Pleasing results can be achieved with relatively cheap telescopes (my Newtonian should be OK) but I've not yet put in the time to get them, and even as an expert my hardware wouldn't be capable of matching that Andromeda shot!
     
  3. Iconindustries

    Iconindustries Mu-43 Hall of Famer

    Telescope focal ratios or f stops are a bit different to lenses, lenses as you would obviously know are mainly measured by there focal length, but telescopes are measured by the objective lens diameter. So for instance, a 100mm telescope with an f ratio of 6 would have a focal length of 600mm, (objective X focal ratio) and a 100mm telescope with an f ratio of 10 would have a focal length of 1000mm.

    Cheers
    Jo
     
  4. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    So since aperture is measured by length/opening, would the f-ratio would equal fstop, or is the objective lens where the aperture would be measured? Is the f-ratio usually printed on the telescope itself, the box, or just in the specifications (if buying used, and/or sight unseen)?

    As far as tripod support, I know that it can be key. For some of the extra long lenses, is it possible to provide two areas of support, so as to spread out the weight each would carry? I know that we don't have to worry about mirror shock, and if I use a release of some kind, it should help reduce vibrations.

    Edited for grammer.
     
  5. Petrochemist

    Petrochemist Mu-43 Top Veteran

    655
    Mar 21, 2013
    N Essex, UK
    Mike
    The same ratio is the case for lenses. Scopes are normally quoted by their objective diameter as light gathering power is based purely on this. Most scopes allow a large range of different eyepieces to be fitted which change the effective focal length of the combination and hence also the f ratio, so the physical aperture is a more useful measurement. F numbers in photography are the effective objective diameter (smaller than the true diameter when the aperture is closed down) and are a more useful measure in photography as the f ratio, shutter speed & sensitivity together control the exposure.
     
  6. aloysius

    aloysius Mu-43 Regular

    57
    Dec 18, 2012
    Nevada City, California
    My real name is unreal
    Astronomers view imaging and lenses (telescopes) differently from we ground-huggers. A 100mm lens to us is a short telephoto. A 100mm telescope to an astronomer is one with a 4 inch (100mm) objective lens (refractor) or mirror (reflector). Each group thinks in terms of what is most important to them.

    Astronomers need to capture as much light as possible, so they think in terms of aperture. And they NEVER stop down a telescope because, as Mike points out, everything is at infinite distance, so everything is in focus or out of focus at the same time. Why throw away photons by stopping down?

    We ground-huggers often stop down lenses for more sharpness. That would work with telescopes too, but those scarce photons are so valuable that it's better to put up with a little edge softness.

    It's all the same. Focal ratio (f-stop to a ground-hugger) = focal length / aperture. Photographers think focal length and f-stop. Astrophotographers think aperture and focal ratio. Specifying any two of the numbers fixes the third.

    Also as Mike describes, most telescopes are designed for visual observing, not for astrophotography. Visual scopes are extremely sharp in the center of the field, where our eyes and brains pay attention, but usually show field curvature so stars near the edges of the FOV can appear egg-shaped. It costs more to make a telescope with a flat field across a large aperture. These are called astrographs. My telescope is a 4-inch (100mm) aperture, f/5.8 focal ratio, 580mm focal length astrograph, so it gives up a little sharpness in the center. I spent way more on my astrograph than I ever spent on any camera gear, including some of the best Four Thirds lenses.

    That being said, we are not research astronomers. We can get great images with telephoto lenses and ordinary telescopes optimized for viewing. Often a field flattener (an add-on lens) can be added between the telescope and the camera to give a flat field, and they typically cost 'only' a few hundred bucks.
     
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  7. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    So how do different eyepieces fit into this? Started this thread as we had some great shots (and shots of rigs) but little to help the budding astrophotographer (like me) put it into terms they understand.

    I have since found out that I may be able to get my grandfathers Bushnell starmaster II, which seems to get nice reviews. There is one eyepiece with a 22x WA (wide angle?) and another one that says 40x60, along with the original velbon tripod. Loos like also made various camera mounts, but I am unclear if they go behind the eyepiece or directly on the lens. Found one that would convert to MD, and can then use my conventional adapter, but unsure what that would do to the focal length/aperture. One nice part is the size, though.

    Sent from my LG-P769 using Mu-43 mobile app
     
  8. aloysius

    aloysius Mu-43 Regular

    57
    Dec 18, 2012
    Nevada City, California
    My real name is unreal
    There is a name for astrophotography through the eyepiece that I can't remember, and there are adapters for attaching cameras to the eyepiece tube, with or without an eyepiece. You'll have to research that because I don't know any more.

    With my new equipment I've started doing 'prime focus' astrophotography. That's the same as using the telescope as a lens. With no other optical elements in the light path, the optical quality is higher than eyepiece AP. You've got only one focal length, although focal length reducers and extenders can be added. Eyepieces will allow different effective focal lengths (fields of view), but at the cost of less light / longer exposures.
     
  9. Petrochemist

    Petrochemist Mu-43 Top Veteran

    655
    Mar 21, 2013
    N Essex, UK
    Mike
    There are 4 standard ways of connecting a telescope to a camera:
    1. Prime focus - no eyepiece or lens - generally gives best results
    2. Negative projection - a barlow or teleconverter but no eyepiece or lens.
    3. Positive projection - using an eyepiece but no camera lens
    4. Afocal - using both eyepiece and lens - his works even with camera phones.

    The last two are the only options it the scopes eyepiece is fixed, the last one the only option if the cameras lens is fixed. Many scopes have to little focus travel for SLRs to focus on infinity with prime focus, negative projection get round this.

    My 130mm aperture Newtonian is one of the many with limited focus travel. Prime focus can't be dome with my Pentax cameras, but ยต4/3 does not have this problem, it can be coupled easily via an eyepiece tube to c mount & a c mount adapter. FWIW I recently posted some terrestrial comparison shots to the spotting scope thread. (Though I din't have a suitable adapter for Positive projection with me at the time) Link
     
  10. OzRay

    OzRay Mu-43 Legend

    Jan 29, 2010
    South Gippsland, Australia
    Ray, not Oz
    A bit off-topic, but some years ago I bought a Meade Autostar and GoTo kit to complement my 600mm mirror lens to do some astrophotography etc, but then moved to a place where we are surrounded by 100'+ gum trees. I've been wanting to start up again, but it looks like Meade has completely dropped the GoTo line of mounts and I'm not sure I want to go through the pain of trying to install the Autostar software on my tablet.
     
  11. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    So I understand more optics means degredation, but is the amount generally thought to be large enough to be noticable? I understand that it may be a case by case thing. If in the case of the bushnell, they made a MD mount, is it likely that goes behind the eyepiece, due to travel?

    Are the numbers listed on the eyepiece magnification amounts? 20x, 40-60x would be a zoom? What is the difference between a barlow and an eyepiece?

    Sent from my LG-P769 using Mu-43 mobile app
     
  12. Petrochemist

    Petrochemist Mu-43 Top Veteran

    655
    Mar 21, 2013
    N Essex, UK
    Mike
    A barlow is like a teleconverter, it's designed to change the properties of the eyepiece rather than work on it's own.

    Yes the 40-60 is probably a zoom eyepiece (I have a 8-24 zoom eyepiece)

    It's not just a matter of more optics giving more distortion. at higher magnifications the total system focal ratio can rapidly get to the point where diffraction total ruins the image. Many cheap scopes sold with claims of the magnification ignore the diffraction limit.
     
  13. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    All of the below is general, as I understand that very few things are absolute across all tele-things.


    I know diffraction is a slowly building thing, and that the fstop at which it start being unsusable is different for different focal lengths, but is there a limit that you should try and stay below? Or is more of the issue with trying to track moving objects with a tiny aperture? Do most quality manufacturers try and make sure that the telescopes/mounts are designed to work around this (aka don't sell things that cant deliver)?
     
  14. Petrochemist

    Petrochemist Mu-43 Top Veteran

    655
    Mar 21, 2013
    N Essex, UK
    Mike
    My understanding is that the defraction limit is constant for all focal lengths, but is dependant on the sensor size & resolution.
    In cameras it's taken to be the point where diffraction blurs a point image to cover more than the width of a pixel. But it is also relevant in visual optics, and unfortunately many companies boost the numbers in their equipment's stats with 'Empty magnification' This is most common at the budget end of the market where customers don't usually know much about what they're buying.

    See the section on maximum usable magnification in Wikipedia