convert 8 inch F6 newt to F4

[alistairsam]

Hi
I have a Bintel BT-202A 8inch F/6 dob mounted reflector and was thinking of changing the mirror to an 8 inch F/4 as I'm primarily interested in astrophotography.

I've built a fork mount for the F/6 which is still to be completed, but preliminary tests show it works fine and tracks well, but thought I'd explore the option of using an F4 mirror.

could someone please explain what the difference would be for visual and astrophotography between the F4 and F6, any disadvantages, how to calculate the size of the secondary.
would there be a change in limiting magnitude or size of objects when imaging or using for visual observation?

[mental4astro]

Hi there,

f/6 scopes are much less critical to maintain collimation and give a good image. f/4 scopes are ruthless with collimation. If it's not right, the image will have 'comet' tails.

f/4 scopes (and faster) have an issue with coma. This is because of the stumpy light cone the primary forms. This means that the object doesn't focus at the same point across the entire field of view. Not a defect, but an artifact of the combination of a flat CCD chip, and the ability of eyepieces to deal with the cone of light. This is seen as little comet tails radiating out from the centre around the edge of the field of view. This is different from the above collimation issue as in the alignment issue, the comet tails are all in the same singular direction, not radiating out. This phenomena is delt with a field flattner, also called a coma corrector.

The image size in an f/4 scope is smaller than that of f/6 by virtue of the shorter focal length. This means nothing for the limiting magnitude, photo or visual, as this is purely a function of the area of the primary mirror. This took me a little while to understand, but it works like this: if you have two scopes, one f/4 the other f/6, and both the same size, for argument sake, here 8", and you use eyepieces to give the exact same magnification in each, the image brightness in both will be the same. If you use the same focal length EP in each scope, of cause the images will be of different size and brightness- larger in the f/6, and 'dimmer', only because of the increased magnification.

Photo wise, the f/4 scope would make exposures shorter, though smaller.

Determining the size of the secondary mirror, there are several newtonian design programs to help sort this out. Someone else may be able to give some current links to the better ones. Or you can use the good book 'The dobsonian telescope' as it contains the necessary info, algorithums, and tables.

I have an 8" f/4 scope for visual use. It is a fantastic rich field scope, and does really, really well in light polluted area, like my home, out performing 10 " & 13.1" f/5 scopes side by side.

[alistairsam]

Thanks.
does this mean that with F/4, you always have to use a field flattener even with prime focus or visual observation?
i have'nt seen one, but i presume its a lens, would'nt that additional glass reduce the brightness of objects?

i used a 9mm plossl eyepiece with my F/6 to view jupiter and saturn and they were'nt too big. so was thinking of getting a wide field 4mm or 6mm rather than using a barlow.

how does the F/4 perform visually for planets, i'm guessing you'd need atleast a 3X barlow to see some details.
now with venus, saturn and jupiter visible, I was hoping to get some good views or pics.

with the other types of scopes such as RC's, or SCT's, the focal ratio is usually quite high with folded optics, same for refractors. how are they able to achieve good results for DSO's if faster optics generally are considered better for DSO's?
do you have any pictures of your F/4 scope?

[bojan]

Here it is:
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

Faster F means shorter exposure time for extended objects (nebulae) but it has no effect on spot-like objects (stars, and planets, whose image is smaller than pixel size).

Yes, field flattener is mandatory for F4.
It will not reduce the light significantly... couple of percent is really not something to worry too much about.

The bigger issue is secondary, which needs to be significantly larger (to avoid vignetting).

[alistairsam]

hi
what does resolving power mean. the bintel 8 inch F6 reflector has a resolving power of 0.58 arc sec. not sure what it would be for the F4 or how to calculate that.
if Low profile focussers are used in an F4, can the size of the secondary be reduced, that would mean moving back the primary thereby reducing the size of the cone on the secondary, not sure if this is what is called intercept.
saw a few scopes with internal baffles. are these required? i've lined the inside of the scope with black felt from spotlight after reading some posts on flocking, and i would've thought that would have sufficed.
just trying to assimilate all info required before I get that F4 mirror and a secondary.

[bojan]

Resolving power is dependent only on mirror diameter (assuming the mirror is perfect) and wavelenght.

http://en.wikipedia.org/wiki/Angular_resolution

[alistairsam]

Hi
was looking at the "Meade f/6.3 Focal Reducer/Field Flattener". description says it reduces the focal length by 0.63.
so is this a viable alternative to use on an F6 to reduce the focal length rather than changing the primary itself?
if the field flattener is a focal reducer, then if it is used on an F4, would'nt it reduce the focal length even further?

[mental4astro]

Mmmm, now we are getting into my pet mission- finding good inexpensive eyepieces!

No, I don't use a field flattner for visual use, you can though. It's more important for in photo applications.

In visual, astigmatism in eyepieces is more of an issue. This is seen in the stars around the edge of the field of view as forming little 'seagulls' co-centric to the FOV. Well corrected eyepieces deal better with astigmatism, and then only reveal the coma effect, which is then delt with by the coma corrector.

I've started a thread just on this topic recently in the Eyepieces Forum, looking for well corrected eyepieces, of which there are few. I suggest reading this thread rather than putting it here:

http://www.iceinspace.com.au/forum/showthread.php?t=63184

Planet wise, here good collimation is critical. Other than that, I find that a decent high power eyepiece/ barlow combo does the job. Remember that the magnification capacity of an 8" f/4 scope is the same as an 8" f/6 one. For this purpose I have a 2X barlow, which I only use with the 8" f/4 scope. Unnecessary with my 17.5" scope which has a 2m focal length. The high power EP I have is a TMB Planetary 6mm- I love it, great eyerelief and 60* FOV good with the drift time in a dob.

However, if photographing the planets is your thing, an 8" f/4 scope isn't the best choice- just too short a focal length unless you use a really, really good quality barlow lens.

I've uploaded a couple of pics of my 8" f/4 scope. It is a commercially made one, to which I made the dob mount for it.

[bojan]

Yes, it is.



Yep, it would.

[alistairsam]

Hi, is'nt magnification a function of the focal length of the primary and the eyepiece, so how would the magnification of an F4 be the same as an F6?
how do you calculate magnification when using prime focus photography as there is no objective lens, and in this scenario, does the magnification change between an F4 and F6?

my primary interest is with DSO's and astrophotography, but do like the odd visual planet observation.

any recommendations on reasonably priced field flatteners, I might try it on the F6. would it then reduce exposure times?
still don't get how the shorter focal ratios facilitate shorter exposures as the speed of light, number of photons hitting the primary would be the same (not considering larger secy). have to read up on it.

[bojan]

The magnification is the ratio of FL of primary and eyepiece..
Obviously you have to have shorter eyepiece for F4 to achieve the same magnification.
However, the apparent size of the Airy disc (resolution) will be the same for the same magnification and the mirror size.

[bojan]

BTW,
there is NO "magnification" when photographing.

"Magnification" as a term is applicable only for visual, and it tells you how much bigger the object appears between looking at it with naked eye and through the telescope.

But there is a number called "scale", and it tells you how many pixels makes one arcsec on the sky for particular sensor/lens combination, or how many arcsec makes one pixel.
For example, for 1100mmm primary FL and Canon400d, it is 1.1arcsec/pixel.

For the file that will do calculation, have a look here :
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=64741&d=1254174226

Or, if the link does not work, here:
http://www.iceinspace.com.au/forum/showthread.php?t=50828&highlight=crop+factor

[alistairsam]

Thanks. so would objects appear the same size through a DSLR/ccd at prime focus in an F4 & F6 ?

[bojan]

If the mirror is the same diameter, then no - extended object would appear smaller at F4, since FL is shorter.

Star image would be smaller than one pixel, so assuming the image is not overexposed, both would illuminate only one single pixel (in real world, there would be some spilling over into neighbour pixels).

[bojan]

Sorry, I made the mistake earlier, the scale for 1100mm and Canon400D combination is 1.1arcsec/pixel.
I corrected this in the post.

[mental4astro]

As bojan said, magnification is the ratio between focal length of the primary and the focal length of the eyepiece.

Hence, as an example, we have two scopes both 8", one f/4, focal length (fl) 800mm, the other f/6, fl 1200.

You can achieve the same magnification with each by using the appropriate eyepiece:

1200 / 30mm = 40 X mag.

800 / 20mm = 40 X mag.

Now, if you use the same EP in each scope, you do get different magnification:

1200 / 30mm = 40 X mag.

800 / 30mm = 27 X mag.

[RobF]

The time taken to collect photons on faint objects is related to the inverse of the square of the focal ratio. So a faint fuzzy with 30mins of data at F/4 you would have to image for a bit over an hour to get the same number of photons on your camera at F/6. Just how important this is in practice is however debatable given the power of modern CCD/CMOS detectors and the options in terms of software we now have.

Astrophotography is like building a very tall house of cards. There's so much to learn and get right in terms of the basic foundations, that is:

- Polar alignment
- Guiding/tracking/balance/camera mounting
- Focus & collimation
- Image processing

If you can't get those things right reliably and learn to troubleshoot them, then you'll never get a nice image regardless of the telescope.

I'd strongly suggest you run up at least 12months practice with your F/6 which should be about the same weight. If you still enjoy hitting your head against a brick wall for hours every night getting those items above to work reliably and sweetly, then is the time to worry about changing scopes - whether you go F/4 Newt or Tak FSQ106 on an Astrophysics mount....:)

[alistairsam]

Thanks. is there any writeup on that equation and how its derived.
the 400mm difference between an F4 and F6 should'nt practically make a difference in the time it takes photons to hit the objective given the speed of light. would be interesting to understand this relationship based on light and how it behaves in both cases.

you're right about getting all those other aspects right. have been on this quest for a while, guess there are quite a few walls along the way. but then that is the whole objective, to break and learn how you break the next one.

refractors do provide detailed images (post processing). is 80mm considered a minimum and can it be compared to say an 8" reflector in some ways in terms of resolving power or what can be seen?

[bojan]

Rob,
This is not exactly right...
The total number of photons collected depends ONLY on primary area (and time, of course).
However, if we are dealing with extended objects (nebulae or, in everyday photography, anything else that is not point-like source of light), then F ratio plays the main role, and its ONLY F ratio and exposure time that will determine the surface brightness of the final image of that object. Of course, number of details will depend on primary diameter.

Alex, all this is pretty basic photographic stuff and the details can be found on many places on web (Wikipedia?).

I will try to find some useful links for you when I have some time later today.

[alistairsam]

Thanks Bojan, I forgot the link to photography, will look it up.

[bojan]

OK, here they are.. some of them :

http://en.wikipedia.org/wiki/F-number
http://en.wikipedia.org/wiki/Aperture
http://en.wikipedia.org/wiki/Focal_length

You will find everything there that you need to know :thumbsup: