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Celestron 9.25 EdgeHD focusing problem

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#1 Terroll

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Posted 08 September 2021 - 04:05 PM

Hi All,

 

I'm having a problem getting proper focus and good stars with a Celestron 9.25 EdgeHD with 0.7x Reducer installed.  When I set the back spacing, from the Reducer, to the specified 146mm I can just reach focus; the focus knob is at the end of travel.  However, after taking an image at that spacing and examining the stars, the stars around the perimeter are radiating outwards, indicating that I must increase the back spacing. But if I increase the spacing I wouldn't be able to reach focus!  Am I missing something here, or is this the way things are with the 0.7x reducer?

 

Terry 


Edited by Terroll, 08 September 2021 - 04:35 PM.


#2 mehdymo

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Posted 08 September 2021 - 04:09 PM

It would be the best if you could attach some images. I remember that the spacing should be measured from the tip of the reducer not the base where the threads finishes. I mean the length of the threads should not be included.



#3 michael8554

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Posted 09 September 2021 - 09:36 AM

I would be very surprised if Celestron designed the scope to be at the end of its focus range at 146mm.

 

Have you included the camera flange to sensor distance in the 146mm ?

 

Is the FR screwed directly onto the back of the EdgeHD ?

 

"(The FR) Maintains the identical back focus when used at f/10 and provides generous back focus to accommodate additional accessories and wide variety of cameras"



#4 Terroll

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Posted 09 September 2021 - 09:53 AM

"I would be very surprised if Celestron designed the scope to be at the end of its focus range at 146mm."

 

Totally agree, but that's where I'm at.

 

"Have you included the camera flange to sensor distance in the 146mm ?"

 

146mm is measured from the end of the threads on the reducer.

 

"Is the FR screwed directly onto the back of the EdgeHD ?"

 

Yes, it is.

 

I've been searching to web for some decent images taken with the 9.25 EdgeHD, but haven't really found any yet (at least not to my liking).  This is the fist time I've tried to use the scope for deep sky imaging; I've previously only used the scope for planetary imaging.  I'm starting to think that this scope is just not up to the task I'm trying.  If I ever get any clear skies again, I'll look into my setup with this scope closer.

 

I'm using an APC-C sensor by the way, if anyone is wondering.

 

Terry



#5 mehdymo

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Posted 09 September 2021 - 11:01 AM

Terry,

I have EdgeHD 8 and 11 and can say something is wrong as they all work well with reducer. Have you loosened the locking screws enough? May be your mirror is touching the locking screws!

#6 Terroll

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Posted 09 September 2021 - 11:05 AM

Oh yes, I've tried loosening the locking screws a lot, to no avail.

 

Terry



#7 Bean614

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Posted 09 September 2021 - 09:17 PM

If you've removed or replaced an Accessory which was mounted to the rear cell with Accessory Screws,  you may have an issue with the Screws interfering with the mirror movement!  If, for instance,  you removed the Bracket for a finderscope,  and simply replaced the Screws into those holes, you have a Problem!  The Screws are now extending further into the tube, and most likely obstructing,  and scratching, your mirror.



#8 Terroll

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Posted 09 September 2021 - 09:25 PM

Interesting.  Now that's a great design, isn't it.  Thanks, I'll check it out..

 

Terry


Edited by Terroll, 09 September 2021 - 09:36 PM.


#9 mehdymo

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Posted 09 September 2021 - 11:35 PM

You can count the number of turns that the focuser does. The number is mentioned somewhere in the scope or Celestron focus motor manual.

#10 michael8554

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Posted 10 September 2021 - 08:59 AM

"Have you included the camera flange to sensor distance in the 146mm ?"

 

146mm is measured from the end of the threads on the reducer.

 

That doesn't answer the question .........?



#11 Terroll

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Posted 10 September 2021 - 09:20 AM

The 146mm measurement is from the end of the threads on the reducer to the sensor.



#12 06AwzIyI

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Posted 10 September 2021 - 11:18 AM

What camera are you using? Most zwo/qhy cameras have a their own backspacing, measured from the mounting flange, through the camera body, and finally to the sensor. This typically is measured to be 17.5mm (11mm for the flange + 6.5mm for the camera body). Did you include this in your calculations?

#13 Terroll

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Posted 10 September 2021 - 12:32 PM

Yes, all backspacing specs are taken into account.  The measurements are taken from the end of the reducer threads to the sensor.

 

The camera is a ZWO ASI2600MC Pro, for those who want to know.




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