Using the Antlia Dual Band Filter for OSC Cameras 

Hi Aleixandrus, Thanks for following along! Yes, I need to do more processing videos. That's also an area where I need to improve. There are lots of good processing videos out there, so I typically feel I'm not adding much to the conversation. Most other RU-vidrs aren't that interested in the nerdy engineering stuff that I tend to do here. But I'll try!

@@Aero19612 Oh, take that as a suggestion! You can add processing videos but please keep publishing engineering nerdy stuff :)))

Hopefully soon if the clouds stay away...

Thanks Paul! I was surprised by the colors and brightness. Still need to work on the detail though. Good task for cloudy nights.

That's crazy about having to have the focuser all the way. Is that a design issue, do you think? As for the blue at H-beta: I just used that filter for the first time last night and do see a distinct blue and distinct red. More on that later... Happy imaging!

Hopefully, I'll see my first results by the end of this week. Thanks for watching, Mark!

Thanks for watching, Christophe!

I agree. I usually do that and thought I did here, but then this is the first pass through these data so I may have skipped some steps to save time. Thank for watching, CJ!

Thanks for watching, Guillermo!

Good to hear. Thanks Kelly! I'll try it on the Bubble Nebula - doing the Oiii-Ha now.

@@Aero19612 my current primary target is the bubble! I got 1.5hrs of Ha O3 and my next session will be S2 O3. I have the Askar D1 and D2 set as well. I will have to throw in some of the Antlia S2 Hb data maybe. I should collect more data than I have, but I have had fun exploring different targets since I started last fall. Thanks for all your work and what you share.

Yep. I bought the Antlia HbSII after this video and gave it a go along with my OIIIHa filter. See the next vid in the list if interested. I really like the combo for tru SHO(plus Hb).

Right. I'm in a race. I need two nights to get some H_beta-Sii data with the new filter and a brief opening between clouds on the third night for some experimenting with the focal reducer back focus. Then I'll be down for maybe two weeks. Keep your fingers crossed!

I don't doubt it. There seems to be a lot play in the back focus for this reducer. I just changed mine by 4 mm (101 mm back focus) and still could use a shorter back focus. May have to try a big reduction and see what happens. Thanks for watching!

Very interesting about the Antlia OAG. I'll look into it. Thanks!

If you're using a hyperstar, be sure to get the correct version of the filter. There's one for "fast" scopes and one for "slower" scopes like mine.

Hi Dan. I partly agree. An optical element like the focal reducer/corrector should have the same back focus for scopes with essentially the same F-numbers (directly related to the angle the incoming light makes with the lens). So, the reducer/corrector with the C8 at F/10 should require the same back focus as the C9.25 at F/10. Another commenter on this thread noted that he found a back focus of 85 mm (sound familiar?) is required for best results with his C8. I changed mine to 100 mm and tested and concluded that was still too high. I think you're both correct: 85 mm is closer to the true back focus for this 0.63 reducer. Thanks for watching!

I keep seeing 85mm pop up, but it seems to be drowned out by the 105mm recommendations. I have to go back and watch some of your videos on this James.

@@DrunkferretKG People keep mentioning 105mm more because that's the number for the C8, and many of them are visual observers who can't plate-solve. Celestron has clarified on multiple forums that 105mm is specific to the C8. Additionally, Starizona states C8 is closer to f/11 and their SCT Corrector IV 0.63x reducer yields f/7.1 on C8 and f/6.5 on C9.25, so there is a difference between the OTA's. If you place your sensor with a focal reducer at 105mm on a C9.25 and you get a focal ratio of f/5.6, and your stars look elongated it's clear that the spacing is too long. You will be much happier at around 85mm because in addition to stars looking better you'll also get less vignetting in the corners.

Here's the data I've collected on my C9.25 + Celestron f/6.3 reducer vs back focus distance: 106mm => FL = 1381 => FR 0.588 95mm => FL = 1436 => FR 0.611 88mm => FL = 1467 => FR 0.624 86mm => FL = 1482 => FR 0.631 84mm => FL = 1489 => FR 0.634 79mm => FL = 1516 => FR 0.645 Back focus distance is measured from the point where the diameter of the reducer drops, right before the threads start. Different adapters might thread more or less on the threads, but the flange is a fixed point.

@DanPechiu thanks! That give a good perspective for me. I just got my C8 SCT setup again after running a 65PHQ for a while. My back focus is at 106.5mm and plate solving gave me 1276-1277mm(different targets). The math says it should be 1278.9mm. If I order some more parts I might be able to shave off that 1.5mm or I could change a spacer and subtract 5mm. Stars are good with just a bit of distortion at the corners of my Asi294mcp. I have already done flats for my filters so I have to find the motivation to change it again.

Hey Tom. Weird. The video shows "520 subs @ 200 sec". Could it be the resolution you're viewing the video at? Thanks for watching!

I've imaged the Cat's Eye, so I have run into that problem. 45 sec is a long time. Are you guiding during the auto focus run? I've heard that the Starizona is much better.

@@Aero19612 I don’t guide, but mount tracks constantly. It is built in function in the mount. It doesn’t track when slew, when it has settled it starts to track right away.

Hi Ranjit. I doubt that this filter would be good for visual observing. It just lets a little light through. It might be OK for Electronically Assisted Astronomy, where images are continually added in real time to produce an image. Thanks for watching!

Thanks for the reply.@@Aero19612

Yes, the raw image is green. That's normal. Are you performing background neutralization? That will align the median background levels for all channels to the same value and you'll have a neutral background instead of green.

Thanks, Ron! The rule goes: if the stars point in toward the center of the image, you need to increase the back focus. If the stars are tangent to a circle centered on the center of the image, then you need to reduce the back focus. The latter matches what these stars are doing.

@@Aero19612 Celestron with this backfocus issue! Have you ever done a platesolve on your optical train to get an effective focal length?

@@Ronbo765 Absolutely. That's where I get such random numbers for my actual focal lengths: 1318 mm with the focal reducer and 2312 mm without the reducer. I also put these numbers in PHD2, NINA, and Stellarium.

@@Aero19612 I did not know about the orientation of stars to determine inside/outside optimal backfocus! BtW Optcorp has a nice diagram demonstrating the effect. To be clear, do you mean you get random focal length numbers on the same optical set up? Perhaps that is a function of seeing conditions during the plate solve? Does it vary during multiple exposures over a few minutes? I find it very odd that people get such big backfocus deviations. The Celestron EdgeHD white paper shows a deviation from nominal of < .5 %. Must be the focal reducer causing the variance. You could always transition to a Hyperstar !

Haha. No, it's not that random. I mean the C9.25 has an advertised focal length of 2350 mm, I get 2312 mm. With the reducer/corrector it's supposed be 1480 mm, but I get 1318 mm. The numbers are constant for a given back focus. Add or remove back focus and the actual focal length will change.