Hello, I'm new here and to telescopes in general, though I've been doing some heavy research. I'm looking for guidance with a reflector telescope we picked up in a second-hand shop some years ago now (photo attached). The only identifiable marking on it is a sticker that says "Orion Optics". No model number or anything. The measurements are as follows:
-The tube is 6.75" (170mm) in diameter, 27.5" (697mm) length.
-The primary mirror diameter matches the opening aperture, just shy of 6" (148mm)
-The distance between the primary mirror and the centre of the eyepiece slot/secondary mirror is 21.75" (550mm)
-We have two unbranded Plossl lenses (25mm and 10mm)
Now, because there is no model number, I can't look up what the focal length might be. I'm assuming it is the raw distance between primary/secondary mirror, so 550mm? I think this puts it in the f/5 ratio, which is good for wide-field viewing as I understand it. We get lovely views of the moon with both lenses, but have had less satisfying views of venus, jupiter and saturn. If the focal length is as I assume 550mm, then the magnifications we are getting with 25mm and 10mm lenses would be 22x and 55x respectively, right? I've read that to achieve good views of the planets you need upwards of 150-250x magnification (if your telescope has that capacity). If the capacity is roughly 50x the diameter, then we should be able to achieve 300x magnification (atmospherics allowing) with this scope. I've been looking at getting a Barlow to use with my current lenses (the 10mm should net us 130x with this) and buying another lens closer to 5 or 6mm to use with it also. Is this wise? Does anyone have any recommendations?
Summary of my questions:
-Are my assumptions about the focal length of this scope correct?
-Will a Barlow lens and a smaller focal length lens help achieve better views of the planets with this scope?
-Would replacing the unbranded Plossl lenses yield clearer results? (Are all lenses created equal?)
-The scope has never been collimated by myself, so I imagine it needs a tune up, what's the best way to go about this without dropping £££? I have good practical ability so no worries there.
-I can see lots of dust on both mirrors, is it advisabe to clean them?
One last thing. Last night I managed to locate Andromeda galaxy, but it was only faintly visible with binoculars (if I looked around it rather than at it), less visible in the viewfinder and totally invisible through the telescope itself. Is the only way to see deep sky objects like this to have longer focal length and wider diameter scopes to collect more light? I know increasing magnification decreases brightness of the image so more powerful lenses won't help for deep sky objects. It was fairly low on the horizon, in a well-lit town, with a nearly full moon in the sky, so yes, visibility conditions were less than ideal! I'd like to start planning to build a telescope specifically for deep sky objects, so if anyone has any suggestions please let me know.
I know I had more questions but I've run out of steam and don't want to scare anyone off from answering this beast. Thanks in advance for any replies!