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Redcat 51 III Focus

William Optics Redcat / Cat 51 III WIFD William Optics Redcat 51 ZWO EAF Focuser Troubleshooting
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DavesView avatar
DavesView avatar
Last night I put 1st light on the Redcat 51 WIFD and could not reach focus with an EAF. I had 26.5mm of extension and the 6.5mm of camera sensor depth. The focuser would not travel past 23mm (assuming that is mm) on the scope focus indicator, which has a scale of 0-30. I pulled the scope off the mount, brought it inside and added 22mm of extension. Attempted again to get to focus and found focus at 9.5mm on the scope focus indicator. Can someone explain this to me?
kuechlew avatar
kuechlew avatar
Can you please specify what exactly confuses you? You reached focus at a total distance of 26.5 + 6.5+ 22 + 9.5 = 64.5 mm. Your first attempt had a maximum focus capability of 26.5+6.5+23 = 56mm which seems to be insufficient in your case. Even letting the focuser reach the maximum extension of 30 - resulting in a total extension of 63mm would not have been sufficient.

Best regards
Wolfgang
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andrea tasselli avatar
andrea tasselli avatar
The focus is 64.5 mm out.
Brian Diaz avatar
Brian Diaz avatar
redcat 51 is a  petzval refractor ( don't need special back focus )   just add 55mm back focus for zwo camera and filter back focus 

matthew.maclean avatar
matthew.maclean avatar
Hi.

If I understand your spacing values, I think that the distance makes sense. The back-focus diagram for that version (2nd image hereon the Agena site) says that the correct image plane is 64.2mm behind the back edge of the step-down adapter ring.

So, if you had:  26.5 + 6.5 + 22  = 55mm, then another 9.5mm of focus travel would yield a total around 64.5, so very close to the target value. Sometimes those spacers and adapters aren't perfect and depending on how tightly you get them assembled, there can be small rounding differences.
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DavesView avatar
DavesView avatar
Can you please specify what exactly confuses you? You reached focus at a total distance of 26.5 + 6.5+ 22 + 9.5 = 64.5 mm. Your first attempt had a maximum focus capability of 26.5+6.5+23 = 56mm which seems to be insufficient in your case. Even letting the focuser reach the maximum extension of 30 - resulting in a total extension of 63mm would not have been sufficient.

Best regards
Wolfgang

I see the reasoning on the focus and I did not have that info to start. The remaining confusing part is why would the focuser not travel to 30mm. Is it possible it is the EAF not making the trip or the focuser, i.e.: will the focuser travel to the indicated 30mm mark? I didn't manually try the focuser when I received the scope. I went straight to installing the EAF. Lesson learned!
DavesView avatar
DavesView avatar
Brian Diaz:
redcat 51 is a  petzval refractor ( don't need special back focus )   just add 55mm back focus for zwo camera and filter back focus 


Cameras don't have backfocus.
Brian Diaz avatar
Brian Diaz avatar
Brian Diaz:
redcat 51 is a  petzval refractor ( don't need special back focus )   just add 55mm back focus for zwo camera and filter back focus 


Cameras don't have backfocus.

***of course , i mean  most telescopes have a 55mm back focal length included the red cat 51

 This means that you are free to add any accessory in your image train as you'd like, such as a rotator, as long as you can reach focus. No need to conform with 55mm back focus standards like on other telescopes.
Mike Webster avatar
Mike Webster avatar
I noticed the same, that the distance the tube would travel to focus didn't reach the full distance available in the window when using an EAF. I haven't taken off the EAF to test so I am not sure if that is the full travel distance available or not.
Richard Witzke avatar
Richard Witzke avatar
I have an ASI183MC-Pro and use the T2 extender ring (usually is already on the camera), T2 Extender (21mm) and T2-M48 Extender (16.5mm) that came with the camera. Maybe you are trying to focus on stars in the dark and that can be quite tricky first time around. Best to focus during the day on a distant object and then fine tune at night.
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Johnny_Utah9 avatar
Johnny_Utah9 avatar
DavesView:
Last night I put 1st light on the Redcat 51 WIFD and could not reach focus with an EAF. I had 26.5mm of extension and the 6.5mm of camera sensor depth. The focuser would not travel past 23mm (assuming that is mm) on the scope focus indicator, which has a scale of 0-30. I pulled the scope off the mount, brought it inside and added 22mm of extension. Attempted again to get to focus and found focus at 9.5mm on the scope focus indicator. Can someone explain this to me?

@DavesView I have the Redcat 51 iii. The focuser does not go past 23mm and is normal per William Optics, though they assured me it does reach 30mm. I had the same question when I first started using that scope. I use a zwo asi533 dedicated Astro cam and use the attachments provided to reach 55mm though the back focus shouldn’t be an issue on this scope. I usually reach focus for most targets around 9.5mm using an EAF & manual focusing.
DavesView avatar
DavesView avatar

@DavesView I have the Redcat 51 iii. The focuser does not go past 23mm and is normal per William Optics, though they assured me it does reach 30mm. I had the same question when I first started using that scope. I use a zwo asi533 dedicated Astro cam and use the attachments provided to reach 55mm though the back focus shouldn’t be an issue on this scope. I usually reach focus for most targets around 9.5mm using an EAF & manual focusing.

Good to know.
kuechlew avatar
kuechlew avatar
What focuser are you using? Usually the focuser works with a stepper motor and has an upper and lower limit for the number of steps in software. So it is possible that you can't use the full extension range of the lens because the focuser reached the maximum number of steps. However, normally you can set any position of the focuser as zero position and then the focuser will be able to move further. So if your focuser refuses to move further just set the position it reached to zero and it will be able to move on again. Be careful though: If you reach the maximum extension of the scope and the focuser tries to move on you may damage the focuser. Of course good focusers will turn off when facing the resistance but cheaper ones may break.

Best regards
Wolfgang
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Jonathan avatar
Jonathan avatar
DavesView:
Last night I put 1st light on the Redcat 51 WIFD and could not reach focus with an EAF. I had 26.5mm of extension and the 6.5mm of camera sensor depth. The focuser would not travel past 23mm (assuming that is mm) on the scope focus indicator, which has a scale of 0-30. I pulled the scope off the mount, brought it inside and added 22mm of extension. Attempted again to get to focus and found focus at 9.5mm on the scope focus indicator. Can someone explain this to me?

@DavesView I have the Redcat 51 iii. The focuser does not go past 23mm and is normal per William Optics, though they assured me it does reach 30mm. I had the same question when I first started using that scope. I use a zwo asi533 dedicated Astro cam and use the attachments provided to reach 55mm though the back focus shouldn’t be an issue on this scope. I usually reach focus for most targets around 9.5mm using an EAF & manual focusing.

Seconding this, I also tried without the EAF first and the focuser goes to about 23mm. No issue reaching focus with ~55mm of spacing and an ASI183mm.

Despite being a petzval and not needing exact spacing, there still is a min and max spacing, you just don't have to be *too* precise as long as you're close to 55mm.
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SouthWestAstro avatar
SouthWestAstro avatar
As long as you can reach focus with your cameras there should be no problem.

Odd that they would configure it that way. It reminds of the musician in the movie "This is Spinal Tap" who has an amplifier where all the dials go from 0 to 11.

I have one of the new AT60EDP Petzvals. The focus travel is only 20mm and matches what's on the scale.
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