Calculating Antlia Filter Offsets with a Bahtinov Mask and Evaluating Hyperbolic Curve Focus

[@LogansAstro]

Good to know that when Antlia say they are parfocal, they are. Great test.

[@Aero19612]

Very pleased so far. I'm finally doing some SHO imaging with them and my ED102. Looking forward to seeing if I can tell the difference between the Antlia and the ZWO filters. Thanks for watching, Logan!

[@PatriotAstro]

Another great video. Your attention to detail is unparalleled as always.

[@Aero19612]

Thanks Chad. I had a mental image of you pounding your keyboard: "what are you doing, there's a NINA plugin for that!"

[@PatriotAstro]

@@Aero19612 HaHa. Nah. I love that you test so thoroughly. It’s sort of a “trust but verify” approach. You learn so much more by doing it yourself. It forces you to really understand concepts. There is way too much assumption in this hobby. Thanks for all you do. Clear skies.

[@dalepowell7353]

Great interesting video as usual! I had heard that the Antlia filters are parfocal. Nice that you have verified. My focus position changes over night's session. I had assumed temperature of the scope was driving the changes. You've made me wonder if seeing also is a contributor. Thanks for all your video's! I learn a lot from you!

[@Aero19612]

Thanks for watching, Dale! Yes, temperature change is the big effect on focus. Target altitude has a big effect on half-flux diameter (or FWHM) that we generally use to assess focus. Sometimes loss of focus is measured as increasing HFD. But HFD also increases as you point closer to the horizon. And, yes, seeing also bloats HFD. Other than that, this hobby is easy!

[@billblanshan3021]

Excellent job Jay!

[@Aero19612]

I'm glad the Antlia filters are so consistent. Can save a lot of time by focusing only with Lum. Just did the same test with my ED102 and got about the same results. Thanks for watching, Bill!

[@JoesAstrophoto]

Wow, thanks for all the work involved Jay! This is a very informative video.

[@Aero19612]

Haha. Some people take pictures of galaxies. I take pictures of diffraction spikes. I don't think I'll get an APOD for this...

[@johnposavatz]

This is awesome research, thanks so much for sharing! Did you ever get around to repeating the steps with a refractor? I recently purchased this same full set of filters (though in the 2" variety) and have them in a wheel connected to a 416mm FL refractor. I was planning to do my own analysis (just sitting down one night with my ASIAir Plus and letting it run through it's hyperbolic curve AF routine multiple times for each filter). But if you've already done it, I'll save myself the effort. :)

[@Aero19612]

Hi John. I did, but I didn’t make a video about it. Basically, I found the same to be true for the RedCat 51. I perform focus with the lum filter and use zero offset for the filters. But, going through this exercise isn’t that painful. Just rattle through all of the filters with the Bahtinov mask on for two positions on each side of focus. Or trust me and see what you get…

[@astrobath]

Thank you for the video and the research! I have Optolong SHO 3nm filter set and L-Pro+RGB, so I need to perform the same adjustment myself. They do not seem parfocal :(

[@Aero19612]

You may not get away with "0" for the offsets, but a consistent set of offsets will save you a lot of time otherwise lost in focusing with the SHO filters. Good luck!

[@starsips]

Brilliant work! I really enjoy how you present your findings. Is it possible to use Bahtinov Grabber with a triBahtinov mask? For collimation purposes?

[@Aero19612]

Thanks Rowan! Good question about the triBahtinov. I would guess not, However, if you have a circular cover (black paper or posterboard) cut to the size of the mask and cut out a 1/3 "pizza" slice from it then it may work. Align the opening with 1/3 of the mask. That should keep the other three spike sets from appearing. No promises! If you give it a try, let me know it if works.

[@starsips]

@@Aero19612 Thank you James! I'll give that a go 👍 Looking forward to your next video

[@jameshe2023]

I have ZWO LRGB filters and Antlia 3nm Ha, Oiii and Sii. my offset is 0,0,1,-3 and -10,-18,-19. I have an 8" F3.8 Newtonian reflector.

[@Aero19612]

Very interesting, James. Seems reasonable that the Antlia filters would be offset from the ZWO filters. Thanks for watching!

[@quiquetorres5040]

Great info as usual. I really enjoy your videos. Shouldn't the final test be to get an image of the same field of stars (or a series of them for statistical validity) at the measured focuser position for the Bahtinov mask and another for NINA's and then measure the actual FWHM in Pixinsight in those imagenes to see which one has the smaller value? That would indicate which method is more accurate.

[@Aero19612]

Hmm. First, my real objective is to measure the filter offsets. As a "bonus", I compare Bahtinov and Hyperbolic focus. There's too much variability to get a definitive answer...if there is a definitive answer. As I say in the video, these are two different focusing philosophies: (1) get focus on one bright star in the center of the field of view or (2) get an image-wide "average" focus using multiple low-signal stars. I guess the way to do this (probably with a wider field of view refractor) is to focus on a bright star in the center of the field with the Bahtinov mask and then slew off the bright star and perform a hyperbolic focus limited to the center of view. Also, Pixinsight's FWHM has variability too. It's very complicated! In this hobby, it's best not to let the perfect be the enemy of the good enough. I'm satisfied that the hyperbolic curve fit method does a good enough job (as good as Bahtinov). Thanks for watching, Quique!

[@ricardoabh3242]

Question those filter offset re good for the primary camera, what about an off axis guiding camera?

[@Aero19612]

It's better to put the OAG in front of the filter wheel so that you're not trying to guide on filtered light. That would be a real problem with narrowband filters. But, to answer your question, the offsets should work for the guide camera as well