1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

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MalVeauX
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1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

Hi all,

I'm looking at various options to convert 1.25" blocking filters (usually diagonal based) to straight through and 2 inch (or T2) for rigid imaging train use (imaging only, not looking at this for visual). I have already converted a few 1.25" diagonal blocking filters to straight through, but I have yet to take one fully to 2 inch (or T2) completely (ie, doesn't use 1.25" at all for any imaging train connection point).

Summary: How do you convert your 1.25" diagonal blocking filter to T2 or M48 (2") Straight Through?

I'm also curious, if an ITF is truly needed in a blocking filter assembly for imaging if the assembly is behind a DERF or even two DERF. I've seen others not use an ITF and simply use a narrow FWHM HA nighttime filter (7nm FWHM) for example instead. I'm still curious though if the assembly is after two separate DERF and after an etalon, what the point of the ITF really becomes relative to the actual blocking filter? Again, from imaging only standpoint with thermal loading already handled with two DERF prior to the imaging train already. I may be missing something here?

Is an ITF necessary in an imaging-only blocking filter assembly when there are 1 or 2 DERF already prior to the imaging train?

I currently have a working straight through 2 inch ready conversion, but it still has 1.25" parts. So it's not fully 2 inch. This is what I would like to change to get a more rigid and secure imaging train. These long rear mounted double stack assemblies get long and heavy so 1.25" is not a good choice there. I would be fine with T2 or M48 (2") options as they integrate fine into these imaging trains.

My current 1.25" to 1.25" & 2" straight through conversion is:

Harvested ITF and blocking filter in eyepiece cup from a Coronado diagonal blocking filter.

1.25" Nose with ITF inside it -> RAF M33x0.75 to M28,5x0,6M adapter -> 1.25" eyecup with 10mm blocking filter inside it -> 1.25" to 2 inch" adapter

I just insert this into any typical 2" to 1.25" coupler and it occupies about ~40mm of imaging train distance.

ITF_125inch_nose.jpg
ITF_125inch_nose.jpg (57.96 KiB) Viewed 3143 times
125inch_to_M48_2inch_Conversion_BF.jpg
125inch_to_M48_2inch_Conversion_BF.jpg (83.46 KiB) Viewed 3143 times
125inch_to_M48_inch_convert.jpg
125inch_to_M48_inch_convert.jpg (68.61 KiB) Viewed 3143 times
2inch_40mm_125inch_BF_converted.jpg
2inch_40mm_125inch_BF_converted.jpg (74.82 KiB) Viewed 3143 times

I would love to get the filters into either other holders and size up to T2 or M48 to allow them to be housed inside of a 2" assembly for more rigid and secure imaging train use.

Ideas?

Very best,


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by marktownley »

I too have thought the same in the past but have never come up with a solution.


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

marktownley wrote: Sat May 21, 2022 6:01 am I too have thought the same in the past but have never come up with a solution.
I know there are adapters that are 2" that have 1.25" rings that thread into them so that a 1.25" filter or assembly can be suspended inside a 2" chamber and threads down to whatever location in that assembly. I would do that but I need a good solution to remove the filters from the eyecup and nosepiece since they're not standard M28 or anything that can be readily adapted. And a bunch of little adapter rings is not a strong way to hold an imaging train. So I'd rather just take them out and re-mount them in something that can go 2". Just don't know the best way to mount them reliably and securely (and importantly keeping them centered).

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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by marktownley »

The only other way I thought was remove the filters from the housings and 3D print custom housings to fir 2" or T2.


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

marktownley wrote: Sat May 21, 2022 4:54 pm The only other way I thought was remove the filters from the housings and 3D print custom housings to fir 2" or T2.
This is honestly the easiest option. I just didn't want to go to this because the PLA material isn't as rigid as aluminum in 90 degree F ambient temperatures (Florida). The weight of my imaging train would bend the PLA material over time. I have some 3D printed things in my imaging train now and it works great, but none of it has the torque of a long telecentric type imaging train that tends to be 400mm long lol.

Any thoughts on the ITF questions in the original post above?

Very best,


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by marktownley »

Ahh yes, not thought about the heat there...

I think I would just try it without the ITF and see how you get on, probably the best way.


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

marktownley wrote: Sat May 21, 2022 5:28 pm Ahh yes, not thought about the heat there...

I think I would just try it without the ITF and see how you get on, probably the best way.
I guess I'm just trying to understand what the ITF is actually for in an imaging setup.

It's in all blocking filter assemblies I've seen except old one where it was a coating on the objective (like old PST setups). But the newer ones all use one, Coronado, Lunt, etc, all use one.

When I look at the transmission plot of ITF's, they're just centered on 656nm with ok transmission (50%~70% or so) but are fairly wide, like 10+nm or more even in some cases. Blocks the rest out pretty far in IR and down into near UV. I guess I'm wondering.... why is it there? Is it there to accept thermal loading instead of the blocking filter? Or is it there to grind transmission down for some reason? I can see why a normal Ha imaging filter of 7nm FWHM could replace the ITF completely frankly. But it still makes me wonder .... why?

The blocking filter itself is far more narrow, at around 6A, so it's not about removing off-band frequencies. The blocking filter does that far better already. The ITF seems to be functioning as a pre-filter simply to offload any thermal loading that could occur on the actual far more expensive blocking filter and to simply protect the blocking filter. Other than blocking IR and some long IR it doesn't seem to serve much purpose. So I'm wondering if it's even needed, at all, in an imaging setup that already uses thermal handling before all the filters, like mine, with double-DERF prior to any filters.

Very best,


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by marktownley »

The ITF role is to block wavelengths >1500nm where 'normal' dielectric filters start to have a high transmission again. Something like KG3 is blocking all the way up to 3000nm.

Arguably these wavelengths won't affect imaging, but you wouldn't want all this long wavelength IR going in your eye.


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

marktownley wrote: Sun May 22, 2022 7:18 am The ITF role is to block wavelengths >1500nm where 'normal' dielectric filters start to have a high transmission again. Something like KG3 is blocking all the way up to 3000nm.

Arguably these wavelengths won't affect imaging, but you wouldn't want all this long wavelength IR going in your eye.
Ok, so largely its thermal cycle protection and long IR blocking thing. Makes sense in a system with no DERF and no long IR absorption in most of the commercial designs. I think in a full DERF setup, especially double, the heat is no longer an issue and so only the KG3 is needed to stop long IR potentially. Will have to look into this more.

Very best,


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by MalVeauX »

Update,

I was able to install this conversion blocking filter into an imaging train and it was sturdy enough. So maybe I don't need to go 2". I would like to use something better than a single thumb bolt, so I need to find a good way to figure out how to better attach where the eyecup piece is and remove that all together to a threaded adapter instead. Dunno how I will do this.

I was able to get it to focus in this configuration with minimal focuser tube out of the chamber with an IMX174 sensor:


Reduced_BlockingFilter_ImagingTrain.jpg
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Very best,


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by AndiesHandyHandies »

Hi

I use Blue Lightning filter 28.5m to pass through T2 fittings, and to 48mm, but for some reason they do not do pass through ones which is a shame.
Astrobin little 1.25" barlows take PST collimator lenses, one of the two styles.

Cheers. Andrew.


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Re: 1.25 to 2 Inch Straight-Through Blocking Filter Conversions | ITF needed? | What have you gotten to work?

Post by AndiesHandyHandies »

Hi

I use a UV-IR filter as a ERF in the Rumak 180 baffle tune for my PST mod 2. No plastic in the scope. Takes out 50%.

A 7nm Baader and then a 1.5A Omega Bobs filter as blocker before bino-viewers.

Cheers. Andrew.


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