Back-Focus 101: How Filters Affect Back-Focus
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- Опубликовано: 5 июл 2024
- In this episode of Back-Focus 101, I describe how filters change the back-focus requirement of your optical system, both visually and mathematically. Filters don't change the back-focus distance by much (in most cases), but it is critical to compensate for the change as your focal ratio decreases (or, as the "speed" of the optical system increases). In other words, it is not absolutely necessary to add the extra back-focus when using a filter at f/10, but it becomes very critical at faster focal ratios.
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![Telescope Back Focus: The Ultimate Guide [TAIC Short]](http://i.ytimg.com/vi/5J654T0UpBc/mqdefault.jpg)
If the background was black and the marker was some bright color, this could easily be a Kahn Academy video! Fantastic explanation!
Well that’s good to hear because Khan Academy is a great resource! I’m glad you found it well explained!
This is a re-upload of an older video. I have re-filmed the video with better audio quality, better illustration, added subtle music, and corrected a mistake from the previous version (thanks @John Silver). If you're interested in a variable spacer set, this one works pretty well: bit.ly/3za1BKp Thanks for watching - I hope you find this episode of Back-Focus 101 helpful!
Hi Cody - Great clear video on filter back focus :) I 3D printed off some M42 and M48 rings using PETG with varying thickness to get the back exactly right:). Nice job !!
Another very clear and helpful video. Thanks for taking the time to produce these, it’s really appreciated.
My pleasure, I am glad it was helpful!
Thanks so much for putting this together and sharing it. I have been having issues after adding a filter to my imaging train for my EdgeHD 8 and this is exactly the info I needed to help me solve the focus issues I was experiencing
You’re welcome, I’m glad you found it helpful! :)
I've always had problems with backfocus and this video has certainly enhanced my understanding of it.
Great to hear!
This!
Simple, easy to understand. And to the point. Thank you
Glad you think so!
That made total sense, great show Cody!
Awesome, I'm glad it was helpful!
I have a 115mm refractor with 640mm focal length when adding a reducer of 0.8. I added a 2 mm thick Antlia Quadband Filter, just after the focal reducer and i adjusted slightly the back focus from 55 mm to 55 + (1/3*2mm). Not much but it can be seen in a F5.6 set-up when zooming in :-) Thanks for the video.
Awesome, you’re welcome. :)
Great to know how these figures are derived 👌 I've a filter on the way and at least I now understand why I've to add a third to my backfocus. Thanks for the info.
You’re welcome! :)
Great explantation!!! Thank you very much!!!
I’m glad you found it helpful!
Great video, thanks for sharing your knowledge on the subject with the community.
I’m glad it was helpful!
@@AstroBlendergreat video! Question. Should we have our filter drawer as close to the sensor as possible? I have mine directly screwed to the front of the camera without the spacer. Didn’t know if that would drop dust on the sensor being that close. Thank you!
Cody. With your help I just figured out that I can change filters and ‘’Almost’’ not have to refocus by doing the simple math you explained and increasing the backfocus by filter thickness/3. It really works. Actually I’m now able to calculate the diffraction index of different filters by not changing the focus and measuring the exact backfocus ajustement required to obtain perfect focus. My English might not be that great but anyway thanks a lot.
By the way, I use the Askar 18mm +- 2mm backfocus adjuster to exactly dial in the required adjustment.
That’s a smart way of running the calculation. I’m glad the video helped!
Great video Cody, very clear, concise and helpful. Can I ask what spacer rings and tubes you use in your image train?
Hi Paul, There are quite a few brands that make little spacer rings. I like to use these color coded ones: bit.ly/3za1BKp
It includes a bunch of rings which makes different combinations easy.
Dear Cody, thank you very much for your videos! I've got really lot of useful information about astronomy in general and Celestron SE8 and different accessories for it in particular (I have Celestron C8-SCT, so it's pretty much the same).
Can I ask your advice please (on a topic different to this video)?
Regarding the brightness of picture and deepsky objects applicability:
- In online shops & forums there are a lot of positive reviews about the applicability of f/6.3 reducer for visual observation of deepsky object - that it gets more visible, brighter etc.;
- It is interesting to compare two options (for Celestron C8 SCT):
- If I use the eyepiece [ES 82° 14mm without f/6.3 reducer] then I come to something like this:
[145x magnification] / [0.56° FoV] / [1.40 mm exit pupil];
- If I use [ES 82° 8.8mm with f/6.3 reducer] then it should give pretty much the same:
[146x magnification] / [0.56° FoV] / [1.40 mm exit pupil];
So the question is:
- If I come to absolutely the same characteristics (magnification, FoV and exit pupil) in these two options - does it still make a difference in visual perception of deepsky objects?
I.e. - will [145x / 0.56° / 1.40mm with f/6.3 reducer] give a nicer experience with deepsky objects than [145x / 0.56° / 1.40mm without f/6.3 reducer]?
- PS: Of course, 145x is too high for many deepsky object - I used it just as an example, because the characteristics of [14mm without f/6.3] turned out to be absolutely the same as [8.8mm with f/6.3]. But the question is still valid in general: for any common [magnification / exit pupil] which can be reached both with and without f/6.3 reducer - does it make sense to use f/6.3 option to have a better experience with deepsky
Best regards,
V
Thanks for the derivation and simple explanation. I’m a retired engineering graduate school prof. You’ve got skills!
Glad it was helpful!
Great video! Thanks :)
Glad you enjoyed it.
Great video..I am soon about to try mono with filters
Good luck, mono is awesome! The detail is fantastic. Now all you need to do is look up the thickness of the filter glass you’re using and add a little bit of spacing if you’re using a fast-ish scope. :)
Thanks Cody. Very clear and very helpful ( esp for a math phob like me)
Haha no problem, I’m glad you found it helpful!
Cody, thank you for this video. I had a question. Can you please do a video on Focal Length of a telescope vs Back Focus? For example, if I had a 72mm doublet telescope with a 430mm focal length, then I put various eyes pieces in or an Orion Starshoot Autoguider, does that change the focal length or just the focal point? Or Back Focus? I'm confused about focal length vs. focal point. If I put my starshoot autoguider in and the draw tube is at 2.5mm, is my focal length 430mm or a new value because I had to adjust the draw tube to bring the image into focus?
Thank you.
Thanks for the explanation and great teaching! I added an Optolong L-enhance to my setup and soon found out that my focus was off and now understand why. I have gone back and forth on focus if I'm not using it so wondering if that is not recommended and should use a spacer. I'm running at f/7 on a 9.25 SCT with the focal reducer.
No problem, I’m glad it was helpful! The L-eNhance is 1.85mm thick, so I’d add a 0.6mm spacer to your imaging train. The link in my comment above to the Astrodymium spacer set has a 0.6mm spacer. It’s amazing how sensitive back-focus can be, especially at the edges of your images. Take care, Cody
Hello, can I ask where you download registax for editing? I can’t seem to find it anywhere or a link would be helpful
Great explanation! Thanks! If I add a 0.6mm spacer to accommodate the Optolong L-Enhance (in a filter drawer), should I replace the filter with something like the Optolong Clear Focusing Filter when not in use? That way I don't need to also remove the 0.6mm spacer?
Yes, you could certainly do that as long as the thickness of the glass filters is the same.
Thanks Cody for this very helpful explanation. I believe I now understand why filter drawers are usually 21mm in an otherwise 17.55mm (camera) + 17.5mm spacer + 20mm spacer = 55mm back focus system. I was wondering why the filter drawer was 21mm (giving a back of 56mm in the above system) because of the focal point shifting by 1mm. (I hope I have the correct!) Thanks! So when used with my 11" RASA which has a 55mm back focus from the camera adapter I will now be at 56mm (when using the filer drawer of 21mm).
I suck at math and am confused by back focus. Your explanation is terrific! Thanks :)
Awesome, I’m glad it was helpful!
Very helpful thank you. However I thought we could just adjust the telescope focus to make up for these minor adjustments? I don't understand why w can't?
Thank you for the clear video. Does it matter if you put the extra spacer before or after the filter?
Try to get your filter as close to the camera sensor as possible.
i like this video
extremely useful
I’m glad you found it useful!
I see that this is a little bit older video, but I’m just getting to it, myself.
Just wanted to say that it was helpful in helping me to get my answer. On a side note, I enjoyed getting into the math behind it as well! Had fun with this. I love math as well! 👍
I’m glad you found it helpful!
Hi. I got a TS Optics 70/350 quadruplet with flattener integrated in the system. I do have "weird" stars in the corners without a filter. Does not change any when adding the ZWO IR filter or Optolong L-ultimate. What do You recon, should I try to add some spacers to see if anything changes? Both filters are 1,85mm, so 6mm should do the trick according to Your video.
Thanks for the video. When calculating the back focus do you need to take into account the thickness of the camera’s glass as well or is that already calculated by the manufacturer when they give is the backfocus such as zwo says 55 mm?
Are you referring to the protective window in front of the sensor? If so, I’d think ZWO would have already incorporated that, but I’m not sure, that’s a great question!
Hey cody, I am commenting for a little help. I have the same scope with the focal reducer and I just order the same filter drawer to use with my 296MCPro. I am struggling to find a way on HOW to attach this filter drawer to my cam. Here you have the 11mm spacer on the cam followed by the filter drawer (21mm) and then the 16.5mm and then SCT T adapter? did I get it right?
Thanks for the video, Cody. I finally understand clearly what backfocus is. But I have a question. I use a Redcat51 with a ZWO Filter Drawer and an ASI183MC PRO. As far as I understand, backfocus is not a problem on a Petzval telescope. Is this correct or would I need to put some spacer ring between the Filter Drawer and the ASI183?
Thank you very much!!!
On a Petzval you want to get close, and then finish with focus. I made a BF101 episode on the Petzval lens as well.
Totally new to this hobby and any ideas, hints, or words of wisdom are a big help. I have a Celestron 8sct on its way. Also, have a ZWO-IRCUT2 ZWO 2 IR Cut-Off Filter - IRCUT2 on it's way. I've been looking at the website for the glass thickness and see a thickness of 5mm + 2mm (thread). Does that mean I should just figure the 5mm or the 7mm total? Sorry for the stupidity of the question, but I am new to this and trying to do things corrctly.
That’s the thickness of the filter cell and the thread. Not the thickness of the glass. You should only add back-focus using the thickness of the actual glass.
The light ray inside the glass is not refracted parallel to the Normal. However, the explanation is valid.
Interesting video. I have been doing imaging for almost 2 years and never realized this. However, there are glass covers on all astro-imaging cameras. For instance, my 533MC Pro camera has a 2mm anti-reflective glass cover above the sensor. Presumably that will have an impact as well. But there is no way to control for that, is there? This seems a bit like going down a rabbit hole......
ZWO accounts for the optical back-focus length of the protective window in their cameras, so you only have to worry about the filter. For example, the sensor to flange distance on the 533MC Pro is 6.5mm optical distance.
@@AstroBlender Thoughtful design by ZWO. Thanks again!
Anytime! One other suggestion is to always tweak your back focus based on your images. The actual distance can vary based on the machining tolerances of all the parts in your imaging train. AgenaAstro sells an awesome kit of spacer rings of various thickness for really tuning in the back focus.
Wondering with the new fast speed filters starting to emerge, if anything changes in relation to spacing.
Hi, the “fast-speed “ filters are pre-shifted in terms of wavelength, not back-focus, so for proper back-focus you would still want to account for the thickness of your glass filter. - Cody
So my ZWO asi camera has a 2mm glass/or plastic screen over the sensor, like most of them. Does this make a difference or is it accounted for by ZWO
ZWO indicated in their online forum that the protective window is already accounted for. The sensor to flange distance is the optical distance, not physical distance (according to ZWO).
best channel for explanation! thank you. If im using l enhance clip in filter for my canon dslr, should i add in spacer too? or this applies to non clip in filter only . sorry for the noob question. Using t7i and samyang 135mm with Clip in L enhance filter
Yes, you should still add the spacer as the filter glass will push back the optimal focal position.
@@AstroBlender noted. M48 spacer?
For a DSLR whatever will fit in your imaging train will be just fine.
@@AstroBlender so astrodymium spacer ring 0.6mm should be sufficient since the the l-enhance clip on filter is 1.85mm thick (1.85/3 = 0.62). the spacer inserted between the lens and the camera. right? sorrrry for the noob question
Yep, exactly!
Коди, спасибо.
I didn't understand anything all I saw was maths and they may as well have been written in Chinese, is there a simple non math method of determining back focus? eg f 4.9 at 357mm focal length with a backfocus of 55mm and an optolong l-extreme?
The L-eXtreme is 1.85mm thick, so 1.85mm/3 = 0.62mm. You would want to add a 0.60mm spacer ring to your system for proper back-focus. The link I have in the comments above to the Astrodymium spacer set includes a 0.6mm spacer.
@@AstroBlender thanks for the info I have all the spacers cheers
Very informative. Does this apply to a Petzval lens system? My understanding is that back focus is not an issue with this design but I may be mistaken.
But how does this help if you are using a filter wheel? You can refocus the scope but changing the back focus for every filter change is not practical.
One solution would be to use the same filter thickness for all your filters. Many brands use the same thickness for all their glass filters. If that isn’t an option, then don’t worry about it. If you don’t notice a big difference and/or it doesn’t bother you, then I would just keep doing what you’re doing..
Completely off topic. How many people have said you look a lot like Captain America? 😅 Trust me! It's there!
Too many. 😭