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Filter use in EAA

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#1 DeepSkyDiver

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Posted 27 July 2021 - 03:54 PM

Just wondering what filters everyone is using out there, if any? I’m in bortle 4 here in central PA, but have some nasty LED streetlights cascading pollution into my backyard and into the sky above(they are non-hooded, so it goes in a nearly 360 sphere). Plus there is always the times when the moon phase is less than great for any kind of observing. I’m currently running a mono camera, the ASI178MM to be exact. Are there any 1.25” filters you guys/gals use to coax more contrast/detail out of DSO’s? I have some optolong UHC and CLS filters from my visual kit I can try. worth a shot or any other recommendations?

Also side note and question, I have the 1.25” nose piece on my 178mm, and a .5x focal reducer screwed into the end, it’s not spaced perfectly so I’m guessing my focal ratio is somewhere around f5.5-6. My question is, when I add a filter onto the end of the reducer, will it change my focal ratio at all? I popped one in shortly last night and definitely noticed my focus was completely different, and not wanting to get the mask out and re-focus I took it off and kept on using it with no filter.

Thanks all! Once I figure out how to add pictures I would love to share and get some tips and critique on my EAA. All the pictures from my 178 are almost 4K native and they’re way too big to upload. This is the first forum I’ve ever joined so I am very much a greenhorn haha!

#2 Alien Observatory

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Posted 27 July 2021 - 08:48 PM

Even though you are using a mono cam, I could see a worthwhile  effort to use your existing filters and see if it improves the images.  With any filter your exposure times will get longer and with many mono cams in EAA use a Ha filter 5 to 50 nm band width (narrow band to wide band) is the goto Filter under most sever Light Pollution locations.   Pat Utah smile.gif


Edited by Alien Observatory, 27 July 2021 - 08:48 PM.

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#3 JKowtko

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Posted 29 July 2021 - 07:18 AM

Re: your side question --

 

My understanding is that the addition of filter glass will only push back the focusing point (around 1mm due to the refractive index of the glass) ... I wouldn't expect the focal ratio to change at all -- unless you put the filter in between the reducer and the camera.

 

So as long as you have a decent amount of focuser travel it should just be a matter of adjusting your focus a bit.  However if you have something like a RASA where the focuser travel is very limited then you have to make sure you have the camera back focus adjusted correctly.

 

I haven't paid attention to this specifically when using filters with my Dob and 224mc, so someone please correct me if I'm wrong.

 

-- John



#4 alphatripleplus

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Posted 29 July 2021 - 07:44 AM

 My question is, when I add a filter onto the end of the reducer, will it change my focal ratio at all? I popped one in shortly last night and definitely noticed my focus was completely different, and not wanting to get the mask out and re-focus I took it off and kept on using it with no filter.
 

I don't know what kind of scope you are using. If it is an achromat, you may well notice some change in focus with different filters as an achromat will have some variation in focal length with focal length, resulting in chromatic aberration. For example, if you have a blue filter as opposed to a narrowband H-alpha filter, I would expect an achromat to show a difference in focus. If you have a reflector or SCT, this would not be the case. A triplet APO would also show a minimal change in focus with wavelength, at least in the visual spectrum.



#5 DeepSkyDiver

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Posted 29 July 2021 - 03:36 PM

Thank you for the replies all, I will try out the UHC filter and see if it makes a difference. I would really love to try a narrowband H-alpha filter as well.  I am using a Celestron 6se alpha, standard Alt/az configuration for now. In the mean time, I have figured out the focus issue. The Celestron 1.25” visual back has a step down inside of it(not sure why) and adding the focal reducer and filter do not let the camera sit fully in the visual back. It just barely fit with the 1.25” nosepiece on my 178mm + focal reducer. But, Adding the filter turned out to be too long and hit the shoulder inside the visual back. It pushed the whole image train back a 1/4” or so. So that greatly changed the focus since the camera sensor shifted further back. I have a Celestron 2” sct visual back and a 1.25-2” adaptor coming so I don’t run into this issue again when swapping filters. 


Edited by DeepSkyDiver, 29 July 2021 - 03:43 PM.



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