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Old 17-04-2020, 01:47 AM
TareqPhoto (Tareq)
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6" F/4 Newtonian, ... is it a regret?

Hey all,

I ordered 6" F4 Newtonian today with Bob's Knobs for secondary as i can't wait to have a good serious telescope with good colors, refractor i must buy an APO that good, or even semi APO and not sure it will be well corrected for colors as well, so i decided that the Newtonian will take away this color issue thing, and then i only have to worry about collimation, we will see how difficult it will be collimating F4 once i receive the scope.

What else i have to add with the scope beside collimator and coma corrector? Are you happy with F4 Newt or you feel it is the nail in the head to be used?
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Old 17-04-2020, 07:43 AM
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mental4astro (Alexander)
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I have two 8" f/4 Newts. One I use exclusively for visual, the other for both visual and photo. With visual I can get a massive 3 true field of view and make for stunning rich field scopes. With photo, it is a versatile focal length for large and small chip sizes.

I don't know why people say an f/4 Newt is difficult to collimate. It isn't any more difficult than f/5, f/6 or f/8. What is more critical is the need for the OTA to hold its collimation, which is the problem with mass produced OTAs. These need to be modified so that they behave, not misalign so easily. One of my f/4 Newts is a mass produced solid tube OTA, but it has been heavily modified so it holds collimation really, really well. The other scope I built myself and was appropriately designed and made from the start. I also have other Newts that range from f/4.5 to f/5, and they all are the same when it comesto the collimation process.

A coma corrector is necessary, especially for photo as coma is significant with fast Newts, and the faster the focal ratio the more significant coma is.

There are a variety of coma correctors, and one optimized for f/4 is best for an f/4 Newt. They also have a range of properties, such as some have a magnification factor, others don't, some significantly put the focal plane further outside the tube but others bearly have a shift in focus. What they all have is the need to place the correct spacing between the camera or eyepiece to the corrector lens. Carefully investgate these and make a choicebased on facts, not Brand prejudice.

An f/4 Newt is a very fine imaging and rich field scope. Their fast focal ratio is not the problem any more than for slower ratios. But the fast focal ratio does expose the short comings of mass produced instruments a lot more than slower ones. While relatively inexpensive, you will need to invest some more money, reading and time to fix these short comings.

I love mine! They are also my grab'n'go scopes at home. Very well figured mirrors that also perform really well at high magnification as much as low. And stunning for photo.

Alex.
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Old 17-04-2020, 01:30 PM
ausastronomer (John Bambury)
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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
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Old 17-04-2020, 06:49 PM
TareqPhoto (Tareq)
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Hi Alex,


Thank you very much for your answer and reply, that is what i need or wait, thanks a lot.


Let me comment in points too so you may need to add something later or not up to you.


1. I am deciding to keep this 6" For Photography/Imaging only, i don't need to make things complicated by putting it for many tasks, i will give it a try anyway.


2. In fact with all those negativity or cons about F/4 i felt like it is a BIG mistake to buy an F/4 Newt or even faster, i don't know what people can do about F2.8 Epsilon for example or some new products with F3.2 Newt? So i try to give it a try and see, and i want o know what kind of modification you are talking about that can hold the collimation that long or it behaves, it will be a big appreciate if you can tell me how or what.


3. I already bought a coma corrector before in the past, to be used with my 8" F5 Newt, it is an F/4 Aplanatic CC, so a great one, i can think about Paracorr later in future as so many told me this is the top quality or correctors out there, it has that Barlow effect, i did read about correctors so i do have the idea, and i know you mean about back focus, i will work on that, it won't be easy task but it won't be an atom producing as well, the fun is in the challenge and the success.


4. I bought this one after i saw enough results and images by others with this kind of scope, i also used calculations from sites for the combo of telescope and cameras, and with the camera i have for certain targets i felt like i want something in between 550mm up to 700mm at most, and i found that 600mm is just that right and perfect, and if i use a reducer i may go little wider than 600mm, and if i use the Paracorr that will give me a slightly longer FL, so 600mm is like a sweet spot i was thinking about, and with F4 it is also flexible, it is just when i asked i felt people are against F4 or 6" F/4 in general.



Thanks for your answer, this is what i need so i keep going and learn more, i really can't wait to use my 8" F5 first and/or 6" F4 with it time to time according to targets i aim at.


Tareq
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Old 17-04-2020, 07:16 PM
TareqPhoto (Tareq)
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Quote:
Originally Posted by ausastronomer View Post
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
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