RE: [APML] solution on "backfocus" problem when doin astrophotography.

[Tommy Lim <Tommy.Lim@omd.com.my>]

Thanks  for you  help  Fabrizio.

really dont know what to do......
i also will think about it...............
so,my tube is focal length 1200mm............
So,how many ????MM should i cut off it?

Tommy 

-----Original Message-----
From: MIME @ Jaring {fmontauti@pointredtech.com} 
Sent: Friday, December 07, 2001 11:08 AM
To: -@ Jaring {astro-photo@seds.org}
Subject: RE: [APML] solution on "backfocus" problem when doin
astrophotography.


I am not sure and that is why i did not do it.
A possibility however could be to remove the mirror and the mirror holder
and cut the tube shorter on the mirror side. Then you replace the mirror and
the holder at the bottom of the tube.
I do not see any other way right now. I will think about it.
Fabrizio

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Tommy Lim
Sent: Thursday, December 06, 2001 6:54 PM
To: fmontauti@pointredtech.com; astro-photo@seds.org
Subject: RE: [APML] solution on "backfocus" problem when doin
astrophotography.


Thanks Frabrizio,

But one thing, how can i move the main mirror upward ????
have any idea to do that?

Tommy

-----Original Message-----
From: MIME @ Jaring {fmontauti@pointredtech.com}
Sent: Friday, December 07, 2001 10:00 AM
To: -@ Jaring {astro-photo@seds.org}
Subject: RE: [APML] solution on "backfocus" problem when doin
astrophotography.


I figure you have a newtonian. There are two ways of dealing with backfocus
in teh case of a newtonian.
The first way is to move the main mirror upward in the scope tube towards
the secondary mirror. This will move the focus out of the focuser of the
same amount you are moving the main mirror.
The drawback of this approach is that you lose effective aperture due to the
fact that the secondary mirror will no longer intercept the cone of light
from the primary. At this point you can upgrade to a little larger
secondary. As far as i know anyway the light loss should be negligible
unless the shift of the primary is too much.
The second option is to buy a low profile crayford focuser. This is what i
did. The only problem in this case is that you need to check the focuser fit
the scope tube mechanically. You need to get a focuser whose curvature
mathces the one from the tube of your scope. In my case it did not but it
worked anyway. I wasable to get the camera just on the primary of the scope.
Good luck,
Fabrizio

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Tommy Lim
Sent: Thursday, December 06, 2001 4:48 PM
To: genehorr@texas.net; astro-photo@seds.org
Subject: [APML] solution on "backfocus" problem when doin
astrophotography.


Hi,

Backfocus actually is my main problem now. So,can someone give me an opinion
or solution on how to solve this problems?

I really need help from you all expert out there.

Thanks.

Tommy

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