secondary too small

asimov (John) · #1 14-07-2005, 05:27 PM

I'll keep it brief..Let's say my secondary was not collecting the full amount of light from the primary. IE: can't see the full primary when looking down the draw-tube.

What would be the visual consequence's of this....What would be the optical effect....WHAT would one see looking into an EP at Jupiter, for instance?

And could this be remedied by say, moving the primary AWAY from the secondary, bearing in mind that if you take it back too far, you will not come to focus.

Or is it even critical?

Thanks.

mch62 (Mark) · #2 14-07-2005, 05:52 PM

I take it this is a DIY Newtonian?
If you think your secondary is to small the only way around this with the same secondary will be to lower the profile of your focuser so the image plane is closer to the optical center line.
Then you will need to move the mirror back to attain focus.
Most probably cheaper and simpler to get a larger secondary than buy a low profile focuser and moving the mirror.
Do you have a copy of newt2.5 , if not down load it now and feed the figures in.

http://home.att.net/~dale.keller/atm...t/newtsoft.htm

Read the help files on scope design first to understand the paremeters and terminology.
It's not a difficult program but will give you all the figures you need to see if your in the ball park.

You want to obtain a fully illuminated field of a bout half the diameter of the largest eyepiece field lens (bottom lens) you have or a bit bigger.
The 75% illumination should be the same size as or larger than that field lens.
If the scope is soley for CCD imaging than you need to fully illuminate the diagonal of that CCD chip.

If you want as small as diagonal as possible for a planetary Newtonian carefull designing and using a truss tube and 25mm low profile focuser will get quite a small secondary of under 20% and maintain a fully illuminated field big enough for even a 31mm Nagler.

Have fun

Mark

mch62 (Mark) · #3 14-07-2005, 06:01 PM

P.S. if your fully illuminated field size is to small for a given eyepiece size objects towards the out side will drop of in brightness.
The 75% drop off is used as anything above that 75% to 100% you won't notice the difference in brightness.
Below 75% things will suffer in brightness.
Imaging the 75% is noticable so you need to fully illuminate that CCD chip.
If ýour only going to use the Newt on the planets and the largest eyepiece is say a 12mm for high power than you could work on only a 6mm FIF but i would not be going to that extreme.

Mark

asimov (John) · #4 14-07-2005, 06:05 PM

Thanks for that Mark, quite an interesting little prog. there!

asimov (John) · #5 14-07-2005, 06:13 PM

To be more specific, It's an existing reflector (my little 4.5") I cant actually see the whole primary when looking down the draw-tube. Seeing it's factory built, I would assume it to have the correct size (or near about's) diagonal. Iv'e been mucking about with it, adding bigger focuser's etc etc.

Was just wondering as to the effect visually that's all.

mch62 (Mark) · #6 14-07-2005, 06:17 PM

Depending on how old it is it will more than likely have more than big enough secondary.

What is the f ratio , tube diameter and focuser hight fully racked in. Also the diagonal diameter across the diagonal not the long side..

asimov (John) · #7 14-07-2005, 08:40 PM

Actually, Ken answered this question a few week's back...

asimov (John) · #8 15-07-2005, 04:16 AM

Mark. First thank-you for trying to help, I appreciate it.. Second, your information go's WAYYY over my head, lol....I just want to know if it's a serious prob. & what problems would I see visually, keeping it like it is.

Now, using my 'common sense' (aaahahaha!) In order to see more of the primary, I would move the primary away from the secondary. (because it's easier than moving the secondary/focuser) Yes, I have at least 30mm of more 'rack in' movement left in the focuser to compensate.

mch62 (Mark) · #9 15-07-2005, 03:16 PM

OK if you have 30mm of spare intravel I could move it back say 24mm.
It's a good idea to keep at least 1/4"or 6mm of spare intravel.
I would put all you eyepieces in especially the lower power ones and double check the in travel once it is focused.
To be honest I am not real sure what you will see if your mirror is not fully visiable.By the sounds of things you have a fairly long travel focuser on the scope .
how much spare out travel would you have and again check all eyepieces and go from the one with the longest distance out for focusing.