UV/IR cut filter or not with an OSC camera in a newtonian?

[ONIKKINEN]

I am thinking of rearranging my imaging train to incorporate a tiltplate to the camera, but to do this i would need to take out my filter holder and with it the UV/IR cut filter in it. I dont have the necessary adapters in hand to somehow be able to use both, but could if i ordered more (rather not).

So this made me think, how useful even is the UV/IR cut filter in a newtonian? The only source of star bloat common in refractors with the newtonian would be the lenses in the comacorrector. I bought the filter because i dont want the near infrared seeping into my images, possibly making true colorbalance impossible. But here are some observations on the matter. Below are pictures of the colour curve and mono QE curve of my camera in both the OSC and mono version (i have the OSC). If im judging the OSC curves correctly, it looks like they all merge to form a basically monochromatic signal (with less blue though) towards the near infrared parts. This i assume just means the pixels are a bit brighter but contain no real colour information anymore. The QE at this portion of the spectrum is also quite low and i dont think it would somehow delete colour information from the rest of the spectrum. Oh and by the way, the sensor window in the OSC version is the same as in the mono, just clear glass and quotes transmission all the way to 1050nm.

Of course i can just test this, but since clear skies are rare i would rather hear if someone has opinions on the matter before trying this out.

Also, i think i can just thread the filter into the comacorrector bottom threads on the telescope side. Would this cause any issues for some reason? The corrector i use is the Maxfield 0.95x so there is a small reducer effect as well.

[iantaylor2uk]

You don't really need a UV/IR filter for a Newtonian unless you are also using a coma corrector. The optics in the coma corrector will likely mean you need one.

[CCD-Freak]

If the window of your camera is not IR blocking use a UV-IR filter since IR leakage can mess with the color balance.  This image of the NGC7023 shows what can happen.  The star at the far left of the image should be red and not pink.  The microlens artifacts are much worse too.   The second image is the same setup but with the UV-IR filter.

Play it safe and use the UV-IR filter.

 

 

Edited December 12, 2021 by CCD-Freak

[ONIKKINEN]

4 minutes ago, CCD-Freak said:

If the window of your camera is not IR blocking use a UV-IR filter since IR leakage can mess with the color balance.  This image of the NGC7023 shows what can happen.  The star at the far left of the image should be red and not pink.  The microlens artifacts are much worse too.   The second image is the same setup but with the UV-IR filter.

Play it safe and use the UV-IR filter.

 

 

Yikes, thats a pretty significant change on the redder stars. How fast was the scope in your example, if i remember correctly you had one of the hypernewts/F2.8 or something of the sort? I would imagine the effect is lesser in my F4.2 but perhaps i wont risk it since this is what i feared when getting the filter.

[CCD-Freak]

Those images were made with a SharpStar 150mm F2.8 Hyper Newt.  The window of my ASI-533MCP is only AR coated so I use a 2" Baader UV-IR filter with a ZWO filter drawer with it all the time now after I wasted several nights thinking I didn't need the filter with a Newt.   

 

 

Edited December 12, 2021 by CCD-Freak

[neil phillips]

18 minutes ago, CCD-Freak said:

If the window of your camera is not IR blocking use a UV-IR filter since IR leakage can mess with the color balance.  This image of the NGC7023 shows what can happen.  The star at the far left of the image should be red and not pink.  The microlens artifacts are much worse too.   The second image is the same setup but with the UV-IR filter.

Play it safe and use the UV-IR filter.

 

 

Exactly of course UV IR Block is needed. Good advice 👌

[ONIKKINEN]

So the filter stays, one way or another.

Any opinions on the filter placement then? Does it matter whether its between the coma corrector and camera or before the coma corrector. My corrector has 2 inch filter threads on the bottom where i could also mount the filter to free up some space in the already narrow 55mm backfocus between the corrector and camera. My gut says it doesn't matter, or why would the thread be there in the first place, but i could be wrong.

[CCD-Freak]

You can put it on the front of the coma corrector and it might be the best place for it so that the UV and IR do not enter the optics of the coma corrector.   I often put it on the nose piece of my field falttener when I am using refractors.

 

 

[ONIKKINEN]

33 minutes ago, CCD-Freak said:

You can put it on the front of the coma corrector and it might be the best place for it so that the UV and IR do not enter the optics of the coma corrector.   I often put it on the nose piece of my field falttener when I am using refractors.

 

 

This would be the plan, if i can take the filter out of its holder. Thoroughly stuck this one, in the freezer it goes for a heat treatment 👍.

[iantaylor2uk]

My advice would be to try it with and without the filter. 

The photos below were all taken by me last year with an f/4 12" Newtonian, and I did NOT use a UV/IR filter (and the camera was a ZWO 071 MC Pro which does not have a UV/IR filter in front of the sensor). 

These photos are not perfect as I did not use a Coma Corrector (I didn't have one at the time) and so have been cropped a bit. 

Hope this helps

https://photos.app.goo.gl/zXxym5fbJ6JW6oKP6

https://photos.app.goo.gl/Jjpiu73ym3Wme7J57

https://photos.app.goo.gl/eDNKesm8ZSNoLCPX9 (there's some walking noise in this one unfortunately - although these were only 10 sec exposures)

[iantaylor2uk]

Also worth adding you may need to increase your distance to the camera sensor slightly if you are using a filter. Typically, if the distance should be 55 mm, and the filter thickness is 1.8 mm, you should increase the distance to the sensor to 55.6 mm. A good explanation of why this is so can be found in this useful YouTube video: (the relevant discussion is at around 6 mins)

 

[ONIKKINEN]

@iantaylor2uk I thought pure mirror systems fold light of all wavelengths pretty much the same. Not exactly the same, some reflection graphs i have seen have a drop in ultraviolet and infrared, but that shouldn't effect the focal point. So the chromatic aberration in the other example is from the coma corrector lenses, both from the fast focal ratio lightcone entering at an angle and from the lenses being curved themselves.

I cant really shoot without a coma corrector with the F4.5 primary, its just too messy already quite close to the center. But just out of curiosity i might try without the filter to compare results.

The Maxfield 0.95x corrector isn't all that picky about the distance it seems, i broke one adapter and had to swap it for a few others i had lying around and i ended up being 1mm short. Couldn't spot the difference, so the filter thickness is probably also not that important to take into account. Also, its difficult to tell how much spacers are needed when the corrector doesn't correct all the way to the edges. Spot sizes in APS-C size fields are noticeably elongated with mine.

[ONIKKINEN]

Looks like there might be a clear night in the next few days so i could test the effects of filter and no filter at F4.2 just for curiosity's sake, IF i were able to free the filter from its prison.

I have it sitting in one of these: https://www.teleskop-express.de/shop/product_info.php/info/p10857_TS-Optics-M48-Filter-Holder-for-mounted-2--Filters---Length-15-mm.html

Dont really know why i got one to be honest, i could have just screwed the filter to the coma corrector to begin with. I thought this looks sturdier than filter drawers so probably better and since i didn't plan on changing it or ever using other filters i didn't think more of it. But now the filter prison works against me pretty bad. There is nothing to grip in the inner holder ring so i really cant come up with a solution to non destructively take it out. Perhaps i could drill 2 holes to the inner ring and fit a tool of some kind in them to get a better grip, but i would risk destroying the filter in the process. Well, ill throw it in the freezer once more. Maybe it changes its mind and wants to unstuck itself.

[iantaylor2uk]

Good luck with the weather.

In my previous link to the YouTube video (higher up in the thread), the part of the video about the flattener obviously only applied to refractors, but the interesting part of the video (at around 6 mins) that discussed the effect of filters on back focus applies equally well to reflectors or refractors. If you insert a thin piece of flat glass into the optical path (of either a reflector or a refractor) it will push out the focus point slightly (due to refraction of light through the filter).

[Pitch Black Skies]

I believe that while chromatic aberration isn't present with a reflector, the colour balance is disrupted if a UV/IR filter isn't used.

Add a refractive coma corrector to the mix, and you have a recipe for disrupted colour balance and chromatic aberration.

Best to use the UV/IR I think (that is if you can release it). But even better to conduct an experiment and compare the results with and without. That way others will learn, plus you'll have ☮️ of mind.

Try to give it a bit of integration time for a more accurate result.

[Ags]

Light leakage in the IR spectrum can really mess colors. Here is a worst case scenario - with the IMX 462, if you do not use an IR cut filter you are really imaging in IR with a little bit of color data added. Your "blue" channel would be mostly IR signal.

Edited December 16, 2021 by Ags

[vlaiv]

Coma corrector is too close to focal plane to be able to impart significant chromatic aberration. It is also not bending light significantly to separate wavelengths much.

You can check spot diagram of various models of CC - and you'll see that there is no significant separation of colors.

 

[vlaiv]

As has been pointed out - UV/IR cut filter is needed to achieve true color image (or rather - to have a chance to achieve it).

[Pitch Black Skies]

17 minutes ago, vlaiv said:

Coma corrector is too close to focal plane to be able to impart significant chromatic aberration. It is also not bending light significantly to separate wavelengths much.

You can check spot diagram of various models of CC - and you'll see that there is no significant separation of colors.

 

I think I'm confusing chromatic aberration with star bloating and halo's, as seen in John's image.

[ONIKKINEN]

8 minutes ago, Pitch Black Skies said:

I think I'm confusing chromatic aberration with star bloating and halo's, as seen in John's image.

The pink starbloat is due to chromatic aberration, at least thats what it looks like. You can see the diffracted spider being out of focus and dark in the middle, not unlike a very out of focus shot. In that example a reducer of i believe around 0.7x is used with the Hypernewtonians and so the effect is easily noticeable. With my 0.95x? Remains to be seen (if the filter ever comes out 🙄). That particular star is also an M-type star (cold surface temperature, deep red) so it radiates strongly in infrared also, so this is probably the worst case scenario.

[ONIKKINEN]

39 minutes ago, Ags said:

Light leakage in the IR spectrum can really mess colors. Here is a worst case scenario - with the IMX 462, if you do not use an IR cut filter you are really imaging in IR with a little bit of color data added. Your "blue" channel would be mostly IR signal.

If im not mistaken this particular camera has been designed around IR imaging? Mine is not nearly as efficient in near IR. Would make a superb Lunar camera though. I wonder if guiding would also be better with IR pass?

[ONIKKINEN]

36 minutes ago, vlaiv said:

As has been pointed out - UV/IR cut filter is needed to achieve true color image (or rather - to have a chance to achieve it).

I am planning to shoot the Coma cluster of galaxies whenever Coma Berenices rises a bit higher and was hoping i could squeeze in the extra signal from without the filter. Its not a particularly colourful target so might not be an issue. Ill test it first and see if i could get away with not using the filter as i could really use the extra QE. But if true color suffers too much, i wont risk it.

[Pitch Black Skies]

10 minutes ago, ONIKKINEN said:

The pink starbloat is due to chromatic aberration, at least thats what it looks like. You can see the diffracted spider being out of focus and dark in the middle, not unlike a very out of focus shot. In that example a reducer of i believe around 0.7x is used with the Hypernewtonians and so the effect is easily noticeable. With my 0.95x? Remains to be seen (if the filter ever comes out 🙄). That particular star is also an M-type star (cold surface temperature, deep red) so it radiates strongly in infrared also, so this is probably the worst case scenario.

Do you think it will make much of a difference in placing the UV/IR filter just in front of the camera sensor versus at the end of the coma corrector?

I'm thinking if the coma corrector isn't significantly bending the light, it shouldn't make a huge difference?

[ONIKKINEN]

4 minutes ago, Pitch Black Skies said:

Do you think it will make much of a difference in placing the UV/IR filter just in front of the camera sensor versus at the end of the coma corrector?

I'm thinking if the coma corrector isn't significantly bending the light, it shouldn't make a huge difference?

I suppose the camera doesn't care whether the IR is in the coma corrector or not, as long as its not getting all the way to the sensor so not important how far the filter is. Assuming no vignetting of course, but it is a 2 inch filter so not an issue.

[vlaiv]

29 minutes ago, Pitch Black Skies said:

I think I'm confusing chromatic aberration with star bloating and halo's, as seen in John's image.

Not sure what image are you taking about.

Star bloat can be due to many things - one of those is chromatic aberration.

Chromatic aberration is due to refractor lens not bringing all wavelengths of light to same focus - lens has shorter focal length for some wavelengths then for other. Effect is very recognizable as it presents most times as blue (sometimes red) halo around stars (especially white/bluish ones).

Take for example this image of M45:

There is no mistake - stars have blue (and a bit red closer to core) halo.

This does not happen with mirror telescopes.

It does not happen with good APO scopes neither unless you let in IR and UV part of the spectrum. ED doublet and Triplet scopes are optimized to bring to common focus two or three wavelengths in visible part of spectrum. Further away from visible part of spectrum - bigger deviation of focus.

If you don't remove these (essentially not important wavelengths as far as color and visual representation of target are concerned) - they can cause star bloat. Depending on camera used and its QE - star bloat can have color or be white (if camera is equally sensitive for each color in far IR).

This does not happen with mirror telescopes. Even using corrective refractive elements in optical path don't usually lead to this. Most corrective elements don't disperse light very much in order to introduce significant amount of chromatic aberration.