Back-Focus 101: Celestron RASA 8

[@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!

[@sanjeevjoshi6873]

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.

[@AstroBlender]

Yes, that’s very true! Excellent point.

[@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.

[@AstroBlender]

You’re welcome! Enjoy the RASA, I saw your 2021 plans video. Should be good stuff. If you have any other Q’s let me know. The RASA is awesome!

[@abdulthomas1063]

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

[@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).

[@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.

[@AstroBlender]

Glad you enjoyed them! Haha I’m surprised the worry about such things. I’ll just continue pronouncing it how I do!

[@wadeprunty]

@@AstroBlender I know right! Of all the things to discuss you'd think they wouldn't even worry about that. I think they were just trying to placate all the angry Aussies.

[@jimtaylor5802]

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).

[@Wolfie6020]

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

[@AstroBlender]

You’re welcome, I’m glad you find them helpful.

[@AstroDenny]

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

[@michaell1473]

Whatever you do don't mess with the factory collimation.

[@AstroBlender]

You’re welcome!

[@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

[@harveyrichard2007]

I'd like to try to remove the optical window, but it is too tightly screwed in. Do you happen to have a trick to unscrew it ?

[@mikeregimbald8225]

i really like the way you explain things..great video

[@AstroBlender]

Thanks, good to hear!

[@robertbennett6410]

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

[@AstroBlender]

Yep, especially now that I’ve found a dual band filter that works! (Video coming soon.) I’m glad you find them helpful.

[@brucelavin8486]

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

[@AstroBlender]

L-eNhance works well. The IDAS NBX though is amazing! (Video coming soon.) Glad you found the video helpful.

[@brucelavin8486]

@@AstroBlender Looking forward to that! Do you need an IR filter to be used with it "piggybacked"? If so, would that replace the clear glass filter in the RASA (of course Dylan pronounces it RAZA............ as in Raspberry!)

[@jimtaylor5802]

@@AstroBlender Given the gloomy outlook for clear skies (and receiving the RASA8 for Christmas), and since I didn't have the starizona filter drawer, had to improvise - there's an IDAS filter designed to fit inside the 11mm space (IDAS NGS1-ZF). Installed the filter and had a spacer that gave me the 25mm backspace and was very pleased with the results. None-the-less, this video was very helpful.

[@peeb2896]

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 drawer.....??? Great video as always.

[@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?

[@joefarar]

extremely helpful, thank you!

[@nightskyimaging]

Well done and very helpful!

[@CarpeNoctem42]

Thanks again for a very clear video about my next telescope (i'm sooooo 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

[@AstroBlender]

Hmm I’ll have to check in editing next go around!

[@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

[@MrViagrande]

Hello Asteoblender, I have question i purchased a different filter adapter where the filter gets mounted on the inside the original flange, it's not a filter drawer, I also have the 7.5mm spacer, do I still remove the clear glass or leave it into get to the 25mm, thanks in advance

[@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.

[@Tink-GB]

agreed, the math does not add up. The Zwo needs 25 not 26 period

[@Jam_66]

Great video! You should do a video on the right astro camera to choose for your certain focal length next haha

[@AstroBlender]

Already done!

[@Jam_66]

AstroBlender just found it, thanks!

[@DS-zg3sw]

After watching this video I purchased a Starizona Filter Slider for the RASA 8. My advise to anyone thinking they are also going to purchase one is don’t do it. The reason is when you screw the filter slider on you cannot open the slider as it fouls the retaining ring. Starizona are fully aware of this issue but they fail to mention it on their website. The issue is there is a gasket inside the retaining ring and if this gasket is greater than 0.40 then you have to obtain another gasket....not easy. Folks on Cloudy Nights have found a solution by 3D printing a thinner version. Would suggest you look for alternative filter sliders.

[@jimtaylor5802]

So, the gasket adds some distance to backfocus. I saw a post that said to put the filter drawer “side with screws” on the camera side to solve this issue - but don’t have first hand experience.

[@TelcoKen]

@@jimtaylor5802 I received my Starizona yesterday. Their product description does have instructions (maybe recently adjusted?) saying to mount the side with the 3 screws toward the camera. BUT, mine has those 3 screws on BOTH sides! Not sure if that's a recent modification or what, but I emailed them asking. Will watch this and other videos carefully to see if I can determine correct side by the shape or another way.

[@jimtaylor5802]

@@TelcoKen filter drawer was suppose to arrive yesterday but USPS didn’t make it…will be interested to see the configuration…the new model has the option for different front and black plates…will have to do some testing to see which size is facing camera and drawer easy to remove while mounted…

[@TelcoKen]

@@jimtaylor5802 Thanks Jim. It will be interesting to see if yours has the silver screws on only on side or both.

[@paulwilson8367]

It seems to say now on RASA 8 specs that back focus needs to be arournd 28MM instead of 25mm as you say in your video. Did this change? Why is it now 28mm?

[@rclrodrigues]

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

[@wingshum7551]

Nice video. How can I fit ASI2600MM to filter drawer? Do I need another apadtor? But worried affect back focus. Please help!!

[@rclrodrigues]

Meu RASA veio com uma poeira branca no vidro. Atrás do vidro. Será que dará diferença? Devo trocar?

[@dondavies2507]

I enjoy your videos. I purchased the zwo asi 183 mc. I"m using the star-azona slider drawer set up for this camera and rasa 8. I have good back-focus, I get good images except for the extreme amp glow. I can't calibrate the glow out. Do you have any guidance? I've made new darks, bias and flats, I don't optimize darks, but haven't haven't had any luck. Any help appreciated.

[@cratervanawesome]

Not sure why it wouldn't calibrate out? That's what the darks are for. Did you take enough of them at the same temp and exposure time as you are taking light frames? I have a mono and color asi183 and never have issues calibrating it out.

[@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!!!

[@Marmo720]

As a new rasa 8 owner, I am looking forward to your IDAS NBX video before deciding which direction to go with this scope. I already have Antlia 3.5nm filter set for my redcat but they won't work with the rasa Hopefully the IDAS video is posted soon? :D

[@AstroBlender]

Yep, it’s coming soon!

[@harveyrichard2007]

Hello Cody, I'm pretty sure I followed your instructions to the letter with my Starizona RASA 8 filter holder, yet I end up with about 2-3 mm *too far* from the flange of the retainer ring... not sure what to do next. The only difference with your setup is that I have an Artesky RASA 8 camera adapter instead of the stock Celestron one, but I checked and the thicknesses are the same so it shouldn't change the back focus distance. Stars at the edges are blurry of course.

[@timothymaurer6720]

The only Artesky RASA 8 adapter I know of is the Artesky 2" Filter Drawer adapter (oddly named for sure since there is no drawer). It allows you to screw in 2" filters and then attach directly (with no additional extensions or filter drawers) from your RASA to your camera. It is designed to be used specifically with cameras where you cannot remove the 11mm T2 adapter (like the ASI2600) and are stuck with the cameras 17.5mm back focus. Also, I would think the Artesky is something like 7.5mm compared to the ~4mm of the Celestron adapter so I don't doubt your camera is too far away from the RASA. What camera are you using? How thick is the Artesky adapter?

[@jimtaylor5802]

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 …

[@sandb1867]

"...but do you gain (lose) anything by removing the factory glass?" The factory glass effectively lengthens the optical path since its index of refraction is higher than air. I would think it would be slightly advantageous to remove it to minimize scatter and reflections but then you'd have to find a way to compensate for the lengthened optical path.

[@jimtaylor5802]

@@sandb1867 will test the difference between the optical train with and without the factory glass...but my guess is that Celestron worked through the optics to but I may be overly optimistic of their optical engineering and measurements knowing they want folks to us their proprietary LP filter too...

[@sandb1867]

Thanks for the video. Since the ZWO camera + Starizona Filter Slider has a 26mm configuration (6.5+19.5), then am I correct in assuming that the filters to be used should be 3mm thick (since filters increase the optical path by 1/3 the thickness of the glass.)?

[@dominikhahn2351]

Same question. The filter I want to use is 2.5mm thick and, given the 1/3 of the filter thickness is the correct value, I have backfocus distance requirement of just 25.833mm and the slider is basically ~0.17mm too thick.

[@OigresZevahc]

So, if using a filter ON the holder and drawer one should leave the clear glass on the front of the RASA8; but what if I want to use a Celestron LP or NB filter on the RASA8, how do I achieve the correct backfocus? Would using a UVIR on the holder and drawer be the solution?

[@ehtube100]

I have been enjoying your videos since you began making them for the 8 inch Celestron NexStar SE telescope. According to your video, I screw my ZWO ASI 294 to the Starizona filter drawer assembly and then to the RASA 8, but, because of the shape of the Celestron T ring provided with the RASA 8, the filter drawer sits to low preventing the drawer to slide out. I flip the filter drawer assembly in the other direction and I can slide the drawer out, but with difficulty, scratching the T ring. Am I missing a step here?

[@AstroBlender]

Hmm not sure on that one. I don’t have that issue on my RASA 8 model. It’s possible the spacer ring on the inside of the securing ring is too thick. On earlier RASA models I know this was a problem for some.

[@steventranchina]

Your videos are so helpful, thanks for taking the time to make them!!! I was wondering if you knew about attaching the asi2600 and the RASA 8? From what I can tell it would just require a 7.5mm adapter like your first example in the video. If this were the case would an additional 1mm spacer be needed when adding a filter in the place where the clear glass is?

[@AstroBlender]

If you swap out the clear glass for a filter you would not need to add any back-focus as the integrate glass/filter is included in the back-focus calculation. Yep, with the 2600 you would just need to add 7.5mm! I hope this helps!

[@steventranchina]

@@AstroBlender it does thanks!

[@davejenkins8524]

I just got my RASA and was wondering. If I have a Baader filter tray like the ZWO and it seems extremely close to correct distance, Can I move the Celestron filter from the screwed in position to the filter tray. does the 1mm distance change stay the same? Then swap it for dedicated filters. The Baader has a ring to replace the original mount. I am hoping to get this right.

[@pekwalker]

When I install my filter drawer, it hangs on the ring on the RASA. Do you have that problem?

[@oldfilmguy9413]

Very informative, thanks!

[@AstroBlender]

You’re welcome!

[@AstroBlender]

You’re welcome!

[@paulwilson8367]

Still there? Well I have terrible (as in, "extreme") tilt, and I want to rule out back focus problems. So, I have the big and heavy ASI2600MC/MM cameras on my RASA 8.. Starizona filter drawer won't work for it. I now have the Starizona filter HOLDER, and it says "6.0mm". The camera is 17.5mm. This includes the tilt plate right? The tilt plate is pretty useless. So now I am at 23.5mm? If I leave in the RASA "clear window", that's another 1mm. So, now 24.5mm. But if I then ALSO add a filter to the filter holder, I would appear to be at 25.5mm? Too much. Not sure what I need to do here?? As you know, it's very confusing for users that Celestron says to NOT use 2 filters in the optical train. So I haven't been, but this would seem to leave me at 24.5mm.

[@eloniewordan8890]

Hi, can you connect your camera direct to the RASA 8?

[@davejenkins8524]

Is there a 3d printed cover for the OTA with the camera attached?

[@Ben_Stewart]

Does this apply to the Baader UFC system for RASA 8?

[@backyardskies7307]

Your Starizona filter drawer has M42 on both side right? Mine has M48 on the scope and M42 on the camera side but measures 19.5mm total...

[@AstroBlender]

Yes, M42 on both sides, 19.5mm thick, measured using calipers.

[@MrWakeupUSA]

Is there a EQ mount that is similar to the Evolution Alt/Az? (Built in battery, Wifi, etc)

[@scottrk4930]

I still can't wrap my head around this backfocus topic . Do you pre-adjust the standard Focus knob so that it is half-way in its travel ? Is this the position required after physically setting the camera backfocus ? Everything I've seen on this topic make it sound like there isn't an integral Focus Mechanism on the Scope or that I'll never achieve Focus if everything isn't within a millimeter or less . What am I missing here ? Noob Question I know . /SRK

[@TelcoKen]

They are separate issues. You need to get to the right back focus for initial set up, I think if not stars away from the center of the view will be distorted (not clear if that's the only problem it causes, a bit of a noob here too). Then the telescope focus knob is used separately to focus the overall view. You don't need to worry about where the scope focus is when setting up the back focus or attaching the camera.

[@harveyrichard2007]

I can't unscrew the clear glass, it is toooo tight ! Any tricks to solve that ?

[@AstroBlender]

Carefully use a spanner wrench or the pointy ends on calipers. There should be two small notches for a spanner. I’d use a spanner wrench as a last resort - the corrector could be damaged easily if you slip.

[@mitraravenjohn9926]

Nice telescope

[@astrocapetown4955]

Nice video. Interesting. I've seen conflicting discussions regarding the clear piece of glass. Did you get this info direct from Starizona?

[@STOMP1E]

Does anyone else make a similar product? Like ZWO?

[@espacioaficion3123]

🤩👌🏻

[@chaotician23]

surely this is front focus, not back focus?