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1.25" reticle eyepiece suggestions for my Duplex

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#1 alwilder

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Posted 29 July 2020 - 08:43 PM

For accurate polar alignment, would an inexpensive reticle eyepiece such as the Celestron CrossAim 12.5 mm Reticle Eyepiece work and focus accurately on my Duplex? It's equipped with the 1.25" slip-fit adapter that works with my 24, 16 and 8 mm Questar threaded Brandons and my non-Questar threaded 12 mm Brandon. If the Celestron CrossAim is a poor choice, any suggestions for those that have done this? Thanks.



#2 RobertPettengill

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Posted 29 July 2020 - 09:31 PM

An accurate polar alignment requires pointing the polar axis of the Questar at he north celestial pole.  What are you planning on lining up in a  12.5 mm reticle eyepiece that will help you do that?

 

Remember Polaris is 39 arc min from the NCP.  The FOV of a 12.5 mm eyepiece is less than this.

 

There are lots of apps that will show you the relationship of Polaris to the NCP.  Get one that handles the LR flip.  Set your scope to 90 deg dec.  With a 32 mm eyepiece put Polaris at the edge of your FOV at the angle shown in your app (e.g. 4 o’clock).  Rotate your scope in RA to see that Polaris is rotated around the center of your FOV.  .This makes sure that 90 on your dec axis is calibrated to your scope’s axis.  Not perfect, prob 20 arc min or so, but close enough for visual with your setting circles only good to about 30 arc min.  For astrophotography you will want to do a drift alignment or use something like the Polemaster that does plate solving. Those can get you to a couple of arc minutes. 


Edited by RobertPettengill, 29 July 2020 - 09:49 PM.

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#3 alwilder

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Posted 30 July 2020 - 05:18 AM

Thanks Rob, I don't have the Brandon 32, the widest I own is a 24. If the 24 isn't wide enough, I take it a Polemaster make more sense for precise alignment with astrophotography. Also, does LR flip mean left-right flip?



#4 RobertPettengill

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Posted 30 July 2020 - 06:23 AM

I usually use my 24 mm eyepiece and just nudge Polaris out of sight in the right direction. No point in getting too obsessive when your 90 dec setting on the scope is probably off a fraction of a degree. 

I haven’t used the PoleMaster myself but I believe it must be mounted to the Questar itself so that the axis rotation is around the scopes true RA axis. (R= right, L=left). Questar gives a true view out the axial port, but flipped at the eyepiece.

I do use a plate solving polar alignment tool with a William Optics RedCat 51. This is the one built into the ZWO ASIAir camera and telescope control computer (based on the Raspberry PI 4). The Air uses your ZWO imaging camera, not a separate alignment camera like the PoleMaster. It brings a song into my heart every time I use it. Better than 3 arc sec polar alignment in less than 5 minutes that is soooo easy. I’m sure it can do better, but my limitation is the mechanical Alt-Az adjustment on the StarAdventurer wedge I use with the RedCat.  I plan to try it with the Questar and it’s narrow field of view and the Astrotrac wedge I use there.

 

Where I use it with the RedCat my view to the north is obstructed by trees and a roof.  It doesn’t bother the ASIAir at all. I suspect that it will work even if Polaris is hidden, although I expect the plate solving to slow down as you move away from true north. 


Edited by RobertPettengill, 30 July 2020 - 06:26 AM.



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