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Showing results for tags 'focal length'.



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Found 6 results

  1. Hello all, I have just joined and have been looking around, and putting in various searches to find the answer to my question(s). I have already found some valuable information, but i can't find a specific answer to a question i have relating to exposure times. I have shot the milky way several times before, from a tripod and a wide angle lens. I am aware of and understand the "500 rule" and that worked fine for me at first when i was shooting with my Canon 6D Mark II. When i moved over to the Sony A7III i noticed significant trailing using the same rule and that led me to the NPF rule (Via the photopills app incase people dot know). I am heading back to Tenerife once again in about 6 weeks time and want to buy a star tracker so i can get some really detailed images. I have done a fair bit of research and in principle, the whole thing doesn't seem to be too daunting or difficult. I have purchased the Polar Scope Align Pro app so i can align Polaris as accurately as possible, i will practise putting the unit together and familiarising myself with the different parts etc, but it is the exposure times that i do not understand. My best glass is the Carl Ziess 50mm F/1.4 Planar, the 18mm F/2.8 Batis, the Sigma 35mm F/1.4 Art & the IRIX 15mm F/2.4 Blackstone. I currently do not own, nor have i ever used a tracker, and I cannot find any information relating to which aperture, ISO and Shutter length any of these focal lengths should or could be shot at. Is there anything similar to the 500 rule or NPF rule that relates to using a tracker with varied focal lengths? or is it just a case of stepping the lens down for sharpness and then trial and error? Thanks in advance, Matt.
  2. I am thinking of getting a Lunt 50mm scope. I want to image as well as observe. I am thinking I could put my ZWO on the scope and capture video. My question is, does focal length in a solar scope relate to the FoV as it does for a non-solar scope? The field of view calculator (http://www.12dstring.me.uk/fovcalc.php) suggests I could just about get a full disc with a focal length of 350mm (the 50mm Lunt), but not with a focal length of 500mm (the 60mm Lunt). Is my logic correct?
  3. Hi guys, I received from a friend a new telescope (new for me, but the telescope is not new) - a classic Cassegrain. The telescope is made by someone very handy, I realize that when I dismantle it to fine tune it and to see how can I collimate the scope (is a new type of scope for me). My problem is I don't know how to calculate the focal length of this type of scope. Is not so easy like doing this for a newtonian, because of the curved shape of the secondary mirror. Have anyone a clue how to calculate this?
  4. Given that assembling a skeleton tube dob in the field requires collimation checking, what models of say F7 to 8 upwards, 8 to 10 inches aperture are available, which would not exceed an OTA vertical height of 72inches/6 feet? I like Portaball scopes, do they come with shrouds, or can shrouds be bought separately for them?
  5. Hello there probably a stupid question with an obvious answer but I've been lurking around this forum for weeks and can't find an answer sorry! Now I'll pre-empt this by saying I'm a total mathematical failure and that is probably the crux of the problem but..... I want to build a small dobsonian for the kids as my skyprodigy 70 just isn't cutting the mustard. Now I think I understand the basics of focal lengths and the such - but as far as secondary mirror location goes (and I've checked NEWT etc and I'm still not completely clear!), am I right in thinking that if a primary mirror has a focal length of, say, 800mm then the secondary mirror isn't actually placed 800mm further up the tube, but that 800mm distance includes the light bouncing off the secondary mirror and up into the focussing hole? All the focussing hole stuff confuses me too tbh; where exactly is that 800mm point supposed to be in relation to the eyepiece? I.e. Just below it, or above it, or at the bottom of the lens? I don't get that because I thought to focus the eyepiece you moved the eyepiece up and down anyway? Why is focussed height so important? Is it a product of the internal diameter of the tube? sorry, I've probably given myself as a total fool. I've read stellafane and a few others about 6 times and I don't get it so that probably says more about my level of intelligence than the quality of their instructions! Many thanks!
  6. Hey folks, I know this might seem like a stupid question but I am about to buy myself my first telescope and I am confused by the telescope itself being much shorter then the stated focal length (The focal length is supposedly 1000mm, but the telescope itself is like 40cm). I've spent hours on the internet trying to find out as much about telescopes as I could but I didn't find any page that would answer this question. Thanks in advance
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