M42x0.75 Filter Mount blanks

[Akarsh Simha]

Does anyone know where I may be able to procure an empty 2" filter housing? I searched wide on the internet and could not find one. I thought perhaps I could cut them on a lathe, but it is too fine a thread for my newbie level and also the lathe I have access to is an imperial lathe.

One alternative I thought of is to try to find a different standard mount (like Thorlabs' SM2) and an adapter from T-thread to that (Thorlabs sells a T-thread to SM2 for example). That gets kinda pricey with two components to buy.

Regards

Akarsh

[Akshay Subramaniam]

What are you planning on using the filter housing for? I needed a 2” filter drawer with a shorter physical thickness than what was available on the market so I designed and 3d printed it in pet-cf which has been working great so far as long as you print with a fine enough layer height.

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[Akarsh Simha]

You mean you 3D printed the M42x0.75 threads and it worked!??!

[Akshay Subramaniam]

Yeah, worked beautifully! I might only expect an issue if there’s too much load on the threads but for a filter holder, that’s unlikely to be too high.

On Mon, Sep 29, 2025 at 11:44 PM Akarsh Simha <akars...@gmail.com> wrote:

You mean you 3D printed the M42x0.75 threads and it worked!??!

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[Akarsh Simha]

I guess this also paves the way to a 3D printed filter slide!

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[Pawan Singh]

Akshay and Akarsh

I have also been looking at filter wheel/slide solutions for my dob. Where would you mount the structure shown by you guys on the focuser tube?

I used to use this specific one on my old Obsession - but on my new dob, there is not enough room to mount this.

https://astrocrumb.com/birch-cases

[Akarsh Simha]

Hi Pawan

Filters should technically be mounted in the light path where the light rays are parallel, which, in a telescope would be either at the objective or after the eyepiece. The objective is impractical of course, but I think we don’t mount it at the output of the eyepiece because it can make a mess with our eyes and the eye relief.

The compromise in amateur astronomy is to simply thread the filter close to the focus. This has the problem of different light rays that form the image having a different angles will see a different passband for the filter, especially more severe in fast telescopes. The other problem of placing the filters on the focus is that you will see the dust on the filter. So in fact, the filters should be placed as far from focus as possible

However, there is one more caveat. We use 2" filters at largest. The wide field eyepieces that have 2" barrels can start having vignetting if there is more than 2" of obstruction at the rear end of their barrels depending on how they're designed. This means we do want to place the filters as close to the back of your focuser barrel as possible, because otherwise you may introduce vignetting from the filter blocking some of the rays.

Therefore, most Dobsonian users have a filter slide mounted right behind the focuser barrel. Here is a good example: https://astrocrumb.com/

Regards

Akarsh

PS: Part of the reason I wrote out all of this is to check my understanding, if someone thinks something I said is wrong I'd love to learn

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[Pawan Singh]

Akarsh

Thanks for the explanation - now I understand why Astrocrumb is mounted at the back of the focuser barrel.

The problem I have is that on my new StarStructure, the way Astrocrumb filter slide is mounted on the barrel, when the focuser is at the outermost travel, the filter slide has nothing to grip on. In fact, it fell down on my mirror! Is there a way to mount filter slide at that location without attaching it to filter barrel?

[Akshay Subramaniam]

> different light rays that form the image having a different angles will see a different passband for the filter, especially more severe in fast telescopes

I did the math on this before buying 4.5nm filters for my f/3 scope and it lets through 88% of the incident light at the Ha spectral line. With lower wavelengths the band pass shift is smaller so unless you have a scope that is faster than f/3 or have filters with band passes narrower than 4.5nm, I suspect there won’t be a noticeable difference at visual wavelengths.

In terms of the filter swap solution, wouldn’t it be better to have a magnetic system where filters can just be slapped on behind the focuser barrel? The slide you linked is a nice system but seems like leaving all the filters out in the open like that is a recipe for very dusty filters. A mounting plate with magnets behind the focuser barrel and filter holders with the opposite magnet pattern will allow for fast swapping of filters without having all filters exposed to dust?

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