Hello, all. I probably can't even call myself a newbie, because I may well not even end up buying a 'scope. But anyway, I was googling for opinions, reviews, etc of GSO mirrors and found the following thread:
Interesting, but what caught my eye was the following post, by raymo, which was sort-of OT, but pertinent to the earlier post by barx1863
"One factor to consider is that the skywatcher dobs despite their unpopular focuser and adaptor setup, are suitable for prime focus work, which is not the case with some other brands.[without either minor or major mods]. Worth checking the scope you select to see if it will reach focus, ... "
Could someone expand on the details / implications of those two posts, maybe point me to some further reading...
Roger,
Prime focus - this is the natural focus point for the telescope optics - in the case of a reflector the focus point without using additional optics (Barlow lens etc.)
The issue mentioned refers to the clearance distance between the focuser (at minimum extension) to the focus point - sometimes called the "back focus"
If this distance is too short then you are unable to fit say a DSLR camera body in the "gap" and get the focus point onto the CCD chip.
It can also have some impact when some eyepieces are used...again insufficient (generally inwards - towards the tube) space to bring the eyepiece to focus.
Hope this helps.
Hi Ken. Hmmmm.... ok, (I think). Let's see if I understand you correctly (remember, I know very little about the subject, practically). I assume the focuser is just a mechanism to which an eyepiece can be attached, and which is used to position the eyepiece wrt the focal point of the primary. Then, if an image sensor (DSLR) is to be positioned at that point, it is necessary that the mechanism (without any eyepiece attached) can be moved 'inwards' towards the tube sufficiently far for the focal point to be out 'in thin air'.
Not very technically expressed, I know, but is that the gist of it?
Roger,
Yes and No.
You are correct that the focusing mechanism must be able to "travel" through a distance to allow the eyepiece/ camera to be positioned at the focus.
The "body" of the focuser is attached to the tube of the telescope, and a "tube" moves through the focuser body to achieve the above.
If the space between the body and the focus is very small then it causes problems - the eyepiece/ camera may not fit in the available space.
Likewise if the space is too large the "tube" may not be long enough to position the camera/ eyepiece at focus.
A telescope with a good "prime focus" position would normally allow the camera and or eyepiece to be easily positioned between the focuser body and the focus.
Pic A - shows the optimal position for best focus with an eyepiece, the round circle represents the focuser knob.
Pic B - show what happens when a DSLR is plugged in and the focal point does not achieve focus on the camera.
Pic C - shows the focuser tube wound all the way in to get the stars focal point onto the camera chip.
What can happen with some telescopes (usually newtonians), the focuser cannot wind the tube in far enough for a DSLR to achieve focus. In this situation the remedy is to move the primary mirror closer to the seconday mirror to shift the focal point further out.
The last part of raymo's post ("Worth checking the scope you select to see if it will reach focus, ... ") is certainly good advice, but I imagine it wouldn't just be a 2-second check on the showroom floor. Is there typically something in a 'scope specification that would pertain to this?
Firstly, any reputable retailer will know whether their product has
sufficient back focus for prime focus work. Many manufacturers/agents/retailers don't mention the subject in their
adverts, or on their websites.
Any scope defined as an astrograph will by definition be o.k, but may not
be able to focus for visual work, so if buying one of those you would need to check that.
You could take a DSLR with you to the shop, and see if the desired scope will
focus on an object hundreds of metres away, and still have some focuser movement left.
raymo
Its really only an issue if you intend to use a DSLR on your scope and even then only if the focuser can't wind inwards enough to reach focus with the camera attached. But DSLR is not the only or even good way to take photos through a telescope. Take a look at ZWO cameras, this is the sort of camera often used and there are tons of options and no limit to cost. Essentially though they are all the same : just an imaging sensor inside a housing shaped to just slot into a telescope focuser just like any eyepiece and typically they connect via USB to let you record using a computer /laptop.
If the focus of a telescope is far out from the focuser you can buy spacer tubes to let you get a camera out to the focus point.
There is also afocal projection where you leave the eyepiece in the scope and use your smartphone or camera held to the eyepiece to take a photo. Some eyepieces have threading to allow a camera adapter to mount securely to the eyepiece for this purpose. There are many options and ways if photography is the direction you want to take.
Also be aware the cheaper telescopes, especially refractors, may not have focusers that are strong enough to hold a dslr without bending which ruins the focus possible.
As Raymo mentioned, any reputable retailer should be able to advise you if you provide them with your needs. If astrophotography is your #1 requirement, if visual (just looking at stuff) is most important, if you only want to look at the planets, if you only want to look at galaxies, if you are studying any particular astro-phenomena for work/course, list it down. Newbies often want to know "what will do everything?" the answer is everything and nothing will. Refractors, reflectors, cats all have their strengths and weaknesses so realistically knowing what you want to look at should be a strong influence on your purchase. People will tell you you cant use XYZ scope if you live in a town or city, thats not true. You can't get the BEST performance out of any scope if used anywhere near civilization and EVERY scope will improve when you take it out to a dark sky site .
