Back-Focus 101: Celestron RASA 8

The Celestron RASA 8 has very specific back-focus requirements. This video shows a few different methods of obtaining the proper back-focus using ZWO cameras that have 6.5mm of back-focus built-in, from sensor to flange. These methods will also work with other cameras with similar back-focus, as long as you meet the requirements described in the video.

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[AstroBlender]

Thanks for stopping by! I hope you enjoyed the video and found it helpful! Feel free to subscribe to this channel for more tutorials, reviews, and astrophotography videos. Have a great day!

[sanjeevjoshi]

Astroblender, good video. I must point out the simplistic approach of shooting for 25 or 26 mm may work for beginners but to really get the full quality with RASA, it is essential to nail back focus to account for variations in equipment.. the actual Celestron spec + filter adapter + filter + manufacturing tolerance for camera + accuracy of any spacers used etc. It does not take much to throw off the best performance. I ended up using no clear glass, Starizona drawer, an Optolong Pro filter + 0.3 mm Baader spacer to get the best backfocus as measured in Pixinsight and CCD Inspector. Cannot emphasize enough what a big difference this makes! Thanks for doing these videos.

[PeterZelinka]

Thanks for the clear explanation on how to set everything up! I got a RASA, but wasn't sure how to attach the Starizona filter drawer properly.

[abdulthomas]

Thanks to your tutorial I was able to get the right spacer I need to reach backfocus on my Rasa this month. All of your tutorials are easy to understand. Keep up the good work

[wadeprunty]

Hey, both of these back-focus videos have been great. For this video, I've had a lot of discussions over the clear filter in the RASA and now have a definitive answer for it. One thing though, the World Astrophotography Congress met in 2019 and declared the pronunciation of RASA to sound like Casa, not NASA.

[TheMalbroughs]

Great video! Note: if you currently own 2 inch filters, you can swap the filter glass with the Celestron clear glass. The glass rounds are the same size round (48mm) but the Celestron filter frame is just stupid big at 54mm. Just remove both filter glass retaining rings and swap out the rounds. I used my plastic dial caliper as a spanner wrench to unscrew the retaining rings, which came out easily. Note: once removed, the Celestron clear glass filter measured 2.3mm thick, where my SVbony CLS filter measured 1.9mm thick. That's 0.4mm more spacing, so I switched from the 7.5mm spacer to a 7mm spacer (available on Amazon or eBay in some cheap M42 spacer sets).

[peeb]

Hi Astroblender, I use the Starzona filter drawer with my RASA 8 and have removed the glass filter....My stars from edge to edge on my images are perfect??? I use an Optolong L Pro filter in the Great video as always.

[Wolfie]

Thanks again. Your videos are always well explained and demonstrated.

[TheAndyoesq]

Great video. What if you get the filter drawer, but don't want to use a filter? For instance, I got the filter drawer, but I don't always use a filter. But I guess that isn't possible, since the back focus spacing will be off?

[jimtaylor]

Thanks for removing (for me) the confusion between the various ways to meet backfocus using the Starizona filter drawer (designed specifically for RASA8) and without the filter drawer. For those using the ASI2600 and ASI071 cameras with 17.5mm of distance between the sensor and front of camera, you may need to consider the Starizona solution that allows you to use other 2" light pollution filters like L-pro (eNhance or eXtreme).

[markice.s]

Just followed up with Celestron’s tech support on the debate surrounding the optical window. They say that it is best remove the optical window when using either their LP filter or when using starizona’s filter drawer. I also reached out to Dean at starizona and he recommended the optical window to remain in place (I followed up on this and Dean couldn’t specify beyond this). I’m leaning Celestron on this matter bc I’ve been using the starizona filter drawer in place without the optical window in place for almost a year now and it doesn’t seem like I am sacrificing quality. The consensus on many forums also seem to suggest the optical window should be removed

[AstroDenny]

I'm about to snag an 8" RASA. This cleared up a lot of confusion! Thank you.

[rueffy]

Thanks so much for sharing this invaluable knowledge. It's great for beginners like myself looking to establish our first setup.

A couple of questions from this videos:

I cannot make the maths add up for the Starizona setup,
Retain Clear Glass (+1mm)
Remove 11mm spacing ring from Camera [-11mm]
Install T adaptor and retaining ring [+7.5mm]
Install filter drawer (+19.5mm)
Install Camera (+6.5mm)
Install custom filter into drawer slot (+1mm)
1 - 11 + 7.5 + 19.5 + 6.5 + 1 = 24.5mm (not 26mm)

Can you explain where I've gone wrong?

Second question, can you use the LPF instead of the clear glass in addition to other filters in the Starizona drawer simultaneously?

Thanks again.

[SinCityDarkKnight]

Stupid question.. how do you attach the ZWO 30mm finder guide scope to the 120mm mini cam? How far does it need to be in? Does it need the extension piece? :(
Your celestron videos have been a GOD SEND. Thank you 1000x over

[CarpeNoctem]

Thanks again for a very clear video about my next telescope (i'm getting it). Only piece of advice i'm gonna give is your right audio channel seems misaligned (in the middle). Could it be you removed a filter and did not compensate for it? :-D

[robertbennett]

Loving that Rasa 8 still huh?? Your videos help me SOOOO much, THANK YOU!

[FamiliaColl]

Great video, very informative, quivk question to your knowledge can a ccd be front mounted on the CGEM II 9.25? in the specs I can't find any details on back focus.if yes what's the back focus mm distance?, thanks!!!

[rclrodrigues]

Meu RASA veio com uma sujeirinha no vidro. Será que vai fazer diferença na imagem? Devo trocar o RASA?

[brucelavin]

Love your videos! You have really helped in my decision making. Remove Glass or keep it in. Others said to remove glass if using a filter. I wanted to avoid doing that. I specifically purchased the Starizona Filter draw for my ZWO 533 and Rasa 8. Now you helped solve the problem. Plus, I learned from your experience not to use the Optolong L-eXtreme with the Rasa. How about the L-eNhance? Thank you for your straight forward instructions

[jimtaylor]

Great video in the AstroBlender series. Perfect timing, since I snagged a Rasa 8” - was shocked to find one in stock - and then found a dedicated used filter drawer and extra drawer. Also purchased the Starizona 7.5mm adapter (if not using the filter drawer and at great price) and I use a fine tipped sharper to label this for the future use.

So, my question…if you remove the factory glass while using the filter drawer, then you need to add 1 mm to the optical train - presume the reason for the Starizona 1mm Filter Slider RASA 8 Adapter to compensate for removing the factory glass. Understand the reason for the glass - but do you gain (lose) anything by removing the factory glass? Would seem that less glass is the better optical train but not sure if it’s a material advantage (besides the glass keeps the inside of the tube clean and protected).

What is your best performing Zwo camera and filter combination - sadly my driveway observatory is a Bortle 6/7 - so the Optolong filters are my friends. Are you using a 2” filter to can you get by with a a 1.25” filter? Will be checking this out on Astronomy Tools…

Thanks for this great video …