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RE: [APML] solution on "backfocus" problem when doin astrophotography.
Thanks for your advise.
i really appreciate it.....
Do someone got any good opinion or comments on how to solve this problems.pls
e-mail to me or we all discuss on the net...........
Thanks.
tommy
-----Original Message-----
From: MIME @ Jaring {georgea@cam.org}
Sent: Friday, December 07, 2001 1:09 PM
To: -@ Jaring {astro-photo@seds.org}
Subject: Re: [APML] solution on "backfocus" problem when doin
astrophotography.
Tommy
You want to avoid cutting the tube if at all possible.
How is your mirror cell mounted to the tube? Are there screws throught
the side of the tube into the mirror cell?
If you are not sure how much more backfocus you need then set your
focuser at the mid point. Then point your scope at the moon.
Then remove the screws securing the mirror cell. Have a friend who you
trust very much help you with this. Place your camera with the adapter
in the focuser. Then have the friend slowely push the mirror cell up the
inside of the tube until the moon is in focus. Now measure the distance
from the back of the tube to each of the points where a screw is used to
fasten the mirror cell to the tube. Average this distance, then add to
that number the distance from the back of the mirror cell to the centre
of the screw hole.
Drill the new holes in the tube based on this distance and mount the
mirror cell.
You should now replace the secondary mirror with a larger one, and then
collimate the entire scope.
George Anderson
Montreal Canada
Clear skies and good health
Tommy Lim wrote:
>
> 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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