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Question for SCT Owners

ZWO ASI290MM Equipment Reflectors Focuser
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astropical avatar
astropical avatar
Hello SCT Owners,

Hope you are all doing well.

Quick question, please: With the ASI290MM attached via Baader Clickstop, I feel as if the FL of my Celestron 8
is notably under 2000mm, assumingly because the main mirror moves for focusing.
Can't test for a while because of overcast. Please let me have your view if I am missing anything.

Thanks a lot and CheerS

Robert
Thomas avatar
Thomas avatar
Hi Robert,

what exactly makes you fell that the FL is shorter?

CS
Thomas
Torsten Daiber avatar
Torsten Daiber avatar
Hi Robert,

the FL of a SCT depends on the distance between primary and secondary mirror, which changes when focusing. Thus to get the nominal FL you should place the focal plane at a certain distance behind the scope. Only then will the separation of the mirrors give the nominal FL.
HTH

Greetings
Torsten
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astropical avatar
astropical avatar
Torsten Daiber:
Hi Robert,

the FL of a SCT depends on the distance between primary and secondary mirror, which changes when focusing. Thus to get the nominal FL you should place the focal plane at a certain distance behind the scope. Only then will the separation of the mirrors give the nominal FL.
HTH

Greetings
Torsten

Hallo Torsten,
That ties it, thanks a lot!
CheerS
Robert
astropical avatar
astropical avatar
Thomas Klemmer:
Hi Robert,

what exactly makes you fell that the FL is shorter?

CS
Thomas

Hallo Thomas,
When I compared actually taken images with an FOV simulator, such as
https://www.astropical.space/fovslider.php
Sorry, the word "feel" was not accurate, I checked it out :-)
CheerS, Robert
Thomas avatar
Thomas avatar
Hi Robert,

got it. I was just courious if you are familiar with the optical path of an SC. smile
Torstens answer was perfekt in your case.

cheers
Thomas
andrea tasselli avatar
andrea tasselli avatar
Having had a C8 in the past I feel I can interject here. If I recall right the nominal 2040mm focal length is achieved at 150mm from the exit flange,
John Hayes avatar
John Hayes avatar
The critical number for mounting any camera on a SCT is the back working distance (BWD).  That's the distance from a reference datum to the focal plane as defined by the optical design.  For Celestron SCTs, the reference datum is located at the rear surface of the baffle nut.  If you have the camera sensor properly positioned, a plate solve should show that the effective focal length of the system is close to the published spec.  I've found that it can vary a little depending on temperature but it should measure to be within 1% - 2% of the spec.  The BWD for the C11 and C14 Edge systems is 5.75" [146.05 mm].  Off the top of my head, I believe that the C8 Edge might be around 105 mm but you should check to make sure that's right.

John
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Lorenzo Siciliano avatar
Lorenzo Siciliano avatar
As a C11 owner, I can say that the real focal length is a function of the distance between the primary and the secondary mirror.
The closer they are, the longer is the equivalent focal length. In a practical use, if you have a standard external focuser, such as a crayford, in most cases you'll end with a focal length of about 10% longer than the nominal one.
The opposite if you place the sensor inwards with respect to the nominal backfocus.
The right focal length occurs when the sensor is at the correct backfocus, as Andrea and John said before.
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Lorenzo Siciliano avatar
Lorenzo Siciliano avatar
And the spacing between mirrors has impact to the spherical aberration too.
The more the difference with respect to the right spacing, the more the amount of spherical.
John Hayes avatar
John Hayes avatar
Lorenzo is absolutely correct; however, there is more to it than just SA.  Achieving the correct sensor spacing is critical to minimize field aberration across the sensor.  The Celestron systems balance astigmatism across the field to achieve a flat focal plane, which means that small spacing errors will cause astigmatic (i.e. elliptical) off-axis star images.  Celestron publishes a spacing tolerance spec for the C14 Edge of 0.5 mm but that's not quite right.  With a 16803 sensor on a C14 Edge, the spacing tolerance is on the order of about 100 microns in order to achieve round stars all the way out to the edge of the 52 mm image circle (into the extreme corners.)  The actual tolerance on any given system depends on the size of the sensor, but it's a good idea to get it as close as possible--both for minimizing SA and for optimizing field performance.

John
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