Quote:
Originally Posted by ausastronomer
Hi Tareq,
Alex has summed things up pretty well.
F4 has the advantages of giving you a wider TFOV for a given aperture and a shorter telescope. The disadvantages are that you will need to use a coma corrector and expensive high quality eyepieces, which better handle the steep light cone of an F4 telescope than cheaper eyepieces do. This of course does depend on how "picky" you are about the quality of the views.
As Alex has also pointed out, it is no harder to accurately collimate an F4 telescope than an F8 telescope. However, an F4 Telescope is far less tolerant of slight miss collimation than an F8 telescope, and when collimation moves off a bit the views deteriorate. With an F8 telescope the collimation can move off a bit and you don't notice it. As Alex also pointed out in cheaper telescopes they tend not to hold collimation all that well. This will mean that you may need to check and re collimate 2 or 3 times a night to maintain the best quality views.
The other thing which also changes with the faster F-ratio is the "depth of focus". This is the focus range that the image will stay in best focus. With an F4 telescope it is very shallow and you will find you continually need to re focus to keep the view sharp when viewing the moon and planets at higher powers. With slower F-ratios the depth of focus is much wider and the image will stay in sharp focus a lot longer without the need to continually re focus.
While the modern trend is to go to faster and faster telescopes because they are more portable and give a lower eyepiece height, my personal preference based on all relevant factors that are important to me, is that for telescopes over 12" aperture the "optimum" F-Ratio is about F4.5 and for telescopes under 12" the ideal F-ratio is between F5 and F5.5. All of my telescopes have been custom built and my 2 Newtonians over 12" are both F4.5; and my 10" is F5.3. With apertures over 22" you seriously need to consider faster telescopes between F3.5 and F4, as the eyepiece height and telescope size can get both dangerous and impractical. A friend of ours in the US (Larry Mitchell) has fallen from the ladder while using his 36"/F5 Obsession on more than one occasion and has broken several bones in the process. I have been up that 15 foot ladder and its a long way down and not really where you want to be on a cold windy night 
Cheers
John B |
Hey John B,
Thank you for your answer as well, so appreciated!
I chose F4 at 6" because of some targets in my mind, for my camera i use, i wasn't planning about visual really, but in 2018 and 2019 i did started or asked a lot about eyepieces and i mentioned in specific about Tele Vue eyepieces as that what i want to buy first and later i add less expensive, i like to start with finest or top quality eyepiece as i do have several eyepieces cheap affordable that came with the telescopes themselves and i bought like 2 so cheap and they were fine, so i better buy very expensive first then i buy something in between to have good collection, and that will be after i buy a LARGE scope later.
Your point about the speed or focal ratio according to aperture is interesting, in fact i am thinking about getting 12" Dob next month and it comes with F5, or i buy 12" F5 or 12" F4 to be used with my AZ-EQ6, but this is a big risk i may do, i told all people it is about planetary imaging only, so although many said it is possible but they keep bringing that it is too much and not good for DSO, well, i saw some used 12" on EQ6 for DSO, so planetary should be even easier as we use videos not still long exposures, but for that purpose i was thinking maybe i better go with 12" Dob or 14" dob so i can learn with visual first until i can afford EQ platform for it to do planetary imaging, i do have high quality 5x so that is a bonus, i can also buy that high quality 4x to pair with a dob to go long FL, a Dob is always cheaper than SCT/Mak of same aperture size, and i already saw rnough results mind blowing from dobsonians, so i will go with the cheapest LARGE scope and worry later.
I requested 20" last year but couldn't continue, so i hold it longer or delayed, i was planning to go with F/4, because it was mainly for planetary imaging, so i wanted to have slightly longer focal length if i will use a Barlow with it, F/3.6 will be a BEAST for visual, so it was like i have to choose what is this scope exactly for, and when i said imaging then f/4 rings the bell, but i have to wait and see until i buy something like 12" or 14" to give it a try in both and then see if i may change my mind or not.
Until now i don't have any autofocuser, i am really thinking about buying those for my main or most used telescopes, not sure if i will buy that for my 6" F/4, but definitely i will do for my 8" F/5, unless i can get that autofocuser which i can swap between scopes without issues, or if i can find cheap affordable autfocusers [or motors] so i can get one for each telescope and not worry about swapping, i know that re focusing will be another NIGHTMARE i will face if i will image a lot under several nights and using several filters, i want to minimize issues as much as i can as i already wasted 2-3 years without having enough good results while people are already producing amazing results, so far the moon is my winner shots and some planets, but for DSO i only managed to make guiding to work was in last November, and after that i got busy and sky got changing and my mind went off and now this Corona and also i keep thinking too much about equipment mainly after i successfully figured out the guiding, i now started to think what i need really to make it better, and i hope this 6" F4 is one of them.
Thanks again for your answer!
Tareq