Celestron RASA Back Focus - How to Calculate Back Focus - What are my Options.

[@larrydrum1842]

I came here for the pet pics and I was not disappointed, great video. I'll be sure to send you some more pics for the next video and I'll get Bella to pose with some of my astro equipment.

[@AmyAstro]

Awww thank you! We had a great pet turn out!

[@billneedham5785]

Thanks Amy. I been confused on the clear glass filter. I think i will take it out and add the 1mm spacer. Lots of pets. LOL Astrobill

[@AmyAstro]

The glass troubled me also. I got Celestron's input on this also. They have tried it both ways with the filter holder with good results. They feel without the glass was slightly better, but not enough to stress over it. Choose which way makes you happy.

[@billneedham5785]

@@AmyAstro Thanks Amy for this tip. Astrobill

[@JackieO74]

I really appreciate how you break down these complex subjects! And you are so transparent with issues you encounter and how you solve them :) Love love love all of the AstroPets at the end and thanks for including my Hailey bear ♥️

[@AmyAstro]

Thank you so much! It was so much fun seeing everyone's pets. I hope to keep doing this so please send more.

[@rtpman1953]

Thanks for the video. I was hoping you'd show the complete assembly (including the ZWO camera and spacer). Will you be doing so in an upcoming video? Also, do you have a source for buying spacers?

[@AmyAstro]

Check out my website www.amyastro.com. I posted a blog this week with a diagram how to put it together with the math. You can find spacers at OPT and High Point Scientific. Look for other brands also like altair, stellavue, farpoint

[@paulwilson8367]

Still working on figuring out RASA 8 back focus. If the camera adaptor is 3.5mm, and the Starizona Filter Drawer is 19.5mm, and the camera is 6.5mm, we are up to 29.5mm. If we are trying to hit 28.73 plus about .65 with the thickness of the filter (thickness x 1/3) that is 29.39 and we are slightly over. So I am not understanding why you added a spacer. The component I am not sure about is the M42 adaptor thickness. The manual spec diagram you showed is 3.5mm.

[@CDigata]

and your are ment to leave the celestron glass in to bring the bf down

[@Lord_Whiteley]

Hi Amy - the Starizona filter draw makes it really easy :) The confusing thing is that on the Starizona site, it says remove the Optical Window - so Celestron says retain while or out Starizona says remove - who is right ? A filter will add approximately 0.83mm to the back focus. I think I will do som measurements and calculations. TBH I get really obsessed with back focus and end up 3D printing M42 and M48 washers, to get it spot on !

[@AmyAstro]

Back focus drives me nuts also. I hate losing images due to star drag. Eventually I will go down the path of 3D printing. One learning curve at a time. It is odd that they contradict each other. I will follow Celestron's manual first.

[@Lord_Whiteley]

@@AmyAstro As I posted on your facebook page - I Calculated a backfocus of 30.33mm? ie Celestron M42 Adapter (supplied with the RASA8 - 3.5mm), Starizona Filter Drawer for RASA (19.5mm), LRGB filter (2mm thick / 3 = 0.83mm) and ASI294MC Pro (6.5mm). I would say that I do not get perfectly round stars but I cannot reduce the back focus unless I remove the filter drawer and add short spacers. Interesting the normal M42 Starizona Filter drawer has a back focus of 17.5mm (but 18.5mm with M42 F both sides). Measuring my actual hardware from the M42 adapter face to camera face = 22.5mm, adding 6.5mm for the camera gives 29mm and adding filter gives 29.83mm.

[@rtpman1953]

@@Lord_Whiteley Did you design the 3d spacers yourself or does one already exist to use?

[@paulwilson8367]

The clear window changes the back focus. So Starizona says leave it out for their drawer, which was calculated not using it, so that the filter is in the drawer only and you don't have 2 filters in the optical path.

[@jefff8106]

Typical astro equipment conundrum. Filters that fit in others equipment, what were you thinking? Be nice if there was an astronomy conformity board so everyone made the same sizes 🙂

[@AmyAstro]

That would be amazing!!!