Locking the mirror on a C8 (or other) SCT
Bob Koller
From time to time the comment is made in this newsgroup that, when doing
astrophotography with an SCT, the mirror should be locked to avoid
"mirror flop." My question: How does one go about locking the mirror of
a C8. What tricks have been used? Thanks in advance for the info.
bob koller
KenAli
>From time to time the comment is made in this newsgroup
> that, when doing astrophotography with an SCT, the mirror
> should be locked to avoid "mirror flop." My question:
>How does one go about locking the mirror of a C8.
Hi Bob,
It sounds to me like you're a bit confused. The mirror being discussed,
is the mirror in a 35mm SLR camera body!
Some cameras have a mirror lock-up feature, which helps reduce vibratons,
caused by the slap of the instant return mirror.
The comments do not refer to the primary mirror on an SCT.
Regards,
Ken
Long Island, NY
Bob Koller
Ken, I don't think I'm confused on this one. What I understand is that
as the mount turns in RA past the meridian, the weight of the mirror
will tend to "flop" to the opposite side. The idea is to , somehow,
prevent the weight transfer which most likely will result in a ruined
image. I understand that some SCT's have a hole where a bolt can be
screwed in so as to immobilize the mirror. I'm looking for a similar
approach to locking a C8 mirror, wghich does not have such a bolt hole.
I'm still looking for ideas.
Thanks.
Bob koller
Rockett Crawford
Sorry I misunderstood your post as well.
I have a ruined picture which I am pretty sure is a victim of this
effect.
--
Capella #5
Learn how build a knife edge focuser
or a hypering tank temperature regulator.
Also amateur pictures of deepsky objects at
Capella's Observatory
http://web2.airmail.net/capella
Rockett Crawford
Bob Koller wrote:
>
> From time to time the comment is made in this newsgroup that, when doing
> astrophotography with an SCT, the mirror should be locked to avoid
> "mirror flop." My question: How does one go about locking the mirror of
>
> bob koller
I think what was being discussed was the mirror in the camera as
opposed to the diagonal mirror. The mirror in a camera (SLR) that
reflects the view to the viewfinder has to get out of the way
when a picture is being exposed. It will bounce up and out of the
way when the shutter is triggered. This can cause vibration which
can possibly blur a shot, particularly shots of very bright objects
like the moon or the sun (through a filter).
Some cameras have a feature that allows you to lock the mirror
up before the shutter is opened which will allow any vibrations
to die down by the time the shutter opens.
Take care,
Leon R. (Bob) Koller
RichardN22 wrote:
>
>
> Sorry William, he's not asking about a camera with mirror lock up. Bob is
> quite clear in what he wants.
>
> If you lock your mirror down, you can't focus. I have a friend with a lock on
> the focus knob, and then he has a JMI focuser attached to the eyepiece area and
> makes fine focus there. The JMI is a pricey item though.
> Richard Navarrete
> Richa...@aol.com
> Astrophotography Web Page - http://members.aol.com/richardn22
Yes Richard but... I don't think that merely locking the focus knob is
going to stabilize the mirror. It appears the mirror has a mind of its
own inside there!
bob koller
William Hamblen
On Tue, 17 Feb 1998 14:05:31 -0700, Bob Koller <lrko...@koller.net>
wrote:
>Ken, I don't think I'm confused on this one. What I understand is that
>as the mount turns in RA past the meridian, the weight of the mirror
>will tend to "flop" to the opposite side. The idea is to , somehow,
>prevent the weight transfer which most likely will result in a ruined
>image. I understand that some SCT's have a hole where a bolt can be
>screwed in so as to immobilize the mirror. I'm looking for a similar
>approach to locking a C8 mirror, wghich does not have such a bolt hole.
>
>I'm still looking for ideas.
The mirror everybody talks about locking up is the mirror in the SLR
camera. The vibration caused by the movement of the SLR mirror can
ruin high resolution photographs when you are using short exposures -
those on the order of fractions of a second. For long exposures -
those that are minutes long - vibration caused by the SLR mirror
usually is not a problem because if necessary you can cover the
telescope with an opaque cover, open the shutter on the camera and
then uncover the telescope to start the exposure. The mirror on an
SCT does not shift much when the telescope crosses the meridian and
you can guide out the mirror shift if you are using an off-axis
guider.
RichardN22
Bob Koller wrote:
>as the mount turns in RA past the meridian, the weight of the mirror will tend
to "flop" to the opposite side. The idea is to , somehow, prevent the weight
transfer which most likely will result in a ruined image. I understand that
some SCT's have a hole where a bolt can be screwed in so as to immobilize the
mirror. I'm looking for a similar
>approach to locking a C8 mirror, wghich does not have such a bolt hole.
>
>I'm still looking for ideas.
<<William Hamlen replied..
The mirror everybody talks about locking up is the mirror in the SLR camera. >>
Sorry William, he's not asking about a camera with mirror lock up. Bob is
Herm Perez
Its obvious that you have to disassemble the C8, drill and tap a
1/4-20 hole in the back (thick part) of the mirror holder (without
going all the way thru and touching the mirror)and then drill a
clearance hole in the back of the SCT. Doing this will prevent mirror
flop or focus creep during long exposures. I mean to do this one day,
I despise off-axis-guiders, would prefer to use a separate guidescope.
Do not disassemble the mirror from its holder, it requires special
factory collimation.
Herm
On Tue, 17 Feb 1998 13:13:47 -0700, Bob Koller <lrko...@koller.net>
wrote:
>From time to time the comment is made in this newsgroup that, when doing
Randy-Michigan
KenAli wrote:
>
> >From time to time the comment is made in this newsgroup
> > that, when doing astrophotography with an SCT, the mirror
> > should be locked to avoid "mirror flop." My question:
> >How does one go about locking the mirror of a C8.
>
> It sounds to me like you're a bit confused. The mirror being discussed,
> is the mirror in a 35mm SLR camera body!
> Some cameras have a mirror lock-up feature, which helps reduce vibratons,
> caused by the slap of the instant return mirror.
> The comments do not refer to the primary mirror on an SCT.
> Regards,
> Ken
> Long Island, NY
Greetings,
Mirror Flop DOES happen when using SCT type scopes. When taking long
exposures crossing the meridian you MAY get some mirror flop. I have a
10" LX200 so I can't comment about the C8 but I can say that many people
blame "mirror flop" when in reality their image has been ruined by
something else (alignment, balance, etc.). Mirror flop occurs when
your SCT's mirror "flops" or shifts from the movement of the OTA,
usually when the OTA travels across the meridian. I have made up a
mirror lockdown bolt that I describe on my web site. The bolt screws
into the bracket that holds the mirror on the LX200 and thus prevents it
from shifting or "flopping". I use the lockdown bolt when taking long
exposures but I have taken exposures without the bolt without showing
any sign of "mirror flop". The Meade 10" and 12" LX200s come with a
"shipping" bolt and that is what is utilized for the lockdown bolt. I
don't believe the C8 comes with a shipping bolt so you would have to
drill and tap into the mirror holder-bracket. I wouldn't recommend the
procedure unless you are very familiar with the C8 and its mirror
assembly. Good Luck and Clear Skies
--
Randy-Michigan
"My Karma Ran Over Your Dogma"
-----------------------
Home Page http://www.look-inc.com/rrubis/randy1.htm
ru...@webtv.net
ru...@home.com
ICQ UIN 2918747
RichardN22
>I despise off-axis-guiders, would prefer to use a separate guidescope.
>Do not disassemble the mirror from its holder, it requires special
>factory collimation.
>
>Herm>>
Herm,
I've had my SCT completely apart, including removing the mirror. It really
isn't that big a deal as it turns out. A friend did it for me. We re-greased
the baffle, cleaned the mirror (there were a few spots on it, don't know HOW
they got there) cleaned the corrector. There is really only one way to put
the thing back together because of the placement of the focus mechanism. It
all works much more smoothly now and mirror-shift is not as big a problem as it
previously had been. I wouldn't neccessarily recommend the procedure, but if
you know what you're doing it's not a problem.
Richard
Herm Perez
Hi Richard, I was talking about separating the main mirror from its
metal holder, not just removing the holder/mirror from the focusing
tube. I do agree that removing the mirror assembly and regreasing does
wonders for mirror shift problems.
Herm
RichardN22
Hi Herm,
My late 80's Meade 8 Inch SCT doesn't have a metal mirror holder. Must be some
different designs out there.
Richard