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# Mistake calculating sagitta- 6" as an f/6 or f/8 ?

6 replies to this topic

### #1 Maggie Murphy

Maggie Murphy

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Posted 27 May 2013 - 06:28 PM

Hi everyone, this site is so helpful and has been fun perusing so far!
I've recently embarked on making my own 6" newtonian reflector, in hopes of machining components to make a german equatorial mount.

I have ground my 6" mirror to possess a sagitta of 0.055". Initially I had intended to go for a focal ratio of f/8 and somehow calculated the desired depth for an 8" (grinding carelessly away in hopes for .083")- I cannot believe how foolish that was! I still have #80 grit, I can put tool on top and make centered strokes and try to remove the 8 thou, or alter my f-number to an f/6 and continue grinding for another 7 or so-

So I spose my question is what are the advantages to a larger focal length? Do I even know my focal length before testing with sun, being wrong to assume accuracy using a straight edge and calipers to measure depth at center? Will this glass really reflect light? I have read that coma is reduced with a larger f-ratio, that a smaller f-ratio will give a wider field of view and eyepieces can work to make up for a smaller focal length? To have a 36in focal length would be far more convenient for using/moving my telescope without help.

I post in desperation for some enlightenment! Sorry for my lack of knowledge on the subject. Will continue to research, thank you!

### #2 mark cowan

mark cowan

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Posted 27 May 2013 - 06:55 PM

Measure at the center with a straightedge and feeler gauges, just add them until the tiniest rocking occurs. Measure the exact diameter to where the straightedge contacts the bevel and do the math - this'll get you closer than a sun test will with a rough ground curve. You can wet the glass and test the FL with a sun spot focus to within an inch or so also.

In that range there's little advantage or disadvantage to longer or shorter.

Best,
Mark

### #3 Mirzam

Mirzam

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Posted 27 May 2013 - 07:10 PM

Hello Maggie and welcome to Cloudy Nights!

An f/6 will make a nice widefield scope and will not have serious issues with coma. You may need to use a larger secondary mirror with the f/6, so if you already have a secondary you should run the numbers to check.

JimC

### #4 Maggie Murphy

Maggie Murphy

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Posted 27 May 2013 - 10:47 PM

Thank you, Mark Cowan and JimC very much!!

### #5 planet earth

planet earth

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Posted 28 May 2013 - 05:47 AM

You should be able to get it back to f8 with #80 grit, using tool on top using a centre over centre and W stoke in minimal time. Your basically close to f7 now.
and a relatively tall focuser.
http://www.loptics.c.../diagonals.html
Main advantage for the beginner is with the f8 the tolerance when parabolizing is less critical then with a f6.
Your chances of getting a better quality mirror is increased.
Here's a link to Fig XP for when you are doing your final correction.
http://stellafane.or...t/software.html
Sam

### #6 Jon Isaacs

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Posted 28 May 2013 - 06:19 AM

So I spose my question is what are the advantages to a larger focal length? Do I even know my focal length before testing with sun, being wrong to assume accuracy using a straight edge and calipers to measure depth at center? Will this glass really reflect light? I have read that coma is reduced with a larger f-ratio, that a smaller f-ratio will give a wider field of view and eyepieces can work to make up for a smaller focal length? To have a 36in focal length would be far more convenient for using/moving my telescope without help.

Maggie:

The coma free region of a parabolic mirror can be computed using:

CF = 0.022 x F^3 mm

To convert that to an angle you need to divide by the focal length:

CF = 0.022 x F^2/D radians = 1.26 F^2/D degrees where D is the aperture in mm

For a 6 inch F/6, the coma free region is 0.30 degrees, for a 6 inch F/8 the coma free region is 0.531 degrees, both are reasonably generous, at a normal planetary magnification, 200x, these represent a coma free/diffraction limited apparent fields of 60 degrees and 105 degrees respectively.

I think F/6 makes a nice all around scope, F/8 would be leaning more towards the planets and require a somewhat more substantial mount though the classic 6 inch F/8 Criterion RV-6 OTA only weighs about 8 lbs.

Jon

### #7 Ajohn

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Posted 29 May 2013 - 05:07 AM

Jon makes an interesting point. The acceptable field angle goes up with slower focal ratios but as the focal length also goes up the size of the field increases which in turn means a bigger 2ndry mirror if you make use of it. 2ndry mirrors reduce contrast so in many ways it's best to keep them small.

Personally I feel F6 is a better compromise for an all round scope but more glass has to be removed when the mirror is figured. Some people feel that the extra effort and problems with doing this aren't really significant. My 1st mirror was an F6. In fact the few mirrors I have made have all been around that F ratio.

There is a book that you can download that helps on this aspect and gives sufficient info to design your scope.

https://ia600706.us....reau-HowToMa...

Fortunately no one has any interest in the copyright of this version. One thing you may notice in it is mirror F ratio's that meet Rayleigh's criterion without any figuring at all providing it's a perfectly spherical. For a 6in mirror it's F8.2 which is an indication of how little glass has to be removed to make it perfect. I think this is why many older books mention 6inF8 mirrors. Providing people go some way towards correcting the sphere and not going too far it will be a decent mirror. An F6 mirror on the other hand ideally needs more precise testing gear and for some that is a drawback.

John
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