Light pollution filters

[fortytwo]

Hi all,

Well, after a long time away from astronomy and with two months furlough stretching ahead I have decided to devote more time to it.

I have finally (phew) got to grips with polar aligning and star alignment and now find myself with questions around light pollution filters.

I have been using my full frame Canon SLR and have quickly realised I need to filter it, initially I was looking at one of these: Skytech

However I have read online that this will not work with full frame, now, I could use my APS-C Canon SLR but would like to take advantage of the higher resolution of the full frame, so what are my options?

I have seen 2" filters and talk of 'flatenners' whatever they are?

If someone could help explain the above or give any recommendations I would be grateful.

Finally, at the moment I have no interest in converting my SLRs to dedicated imaging cameras, just for clarity!!!!

Thankyou in advance for all help offered.

Cheers and clear skies to you all,

Jeff

 

Edited April 13, 2020 by fortytwo

[PeterCPC]

Lots of telescopes are not suitable for using with a full frame camera. It would help if you said what telescope you are going to be using.

Peter

[Peart]

I'm in the same position with light pollution filters and will be purchasing the one you have link. I'm still quite new but from what I have read it will work on the camera but I'm not sure if it will block out the LED streetlights that surround me as it only blocks sodium lines from the lights. But like I said I have read about them and that filter seems like a good start for me at least. 

[Wyvernp]

30 minutes ago, Peart said:

I'm in the same position with light pollution filters and will be purchasing the one you have link. I'm still quite new but from what I have read it will work on the camera but I'm not sure if it will block out the LED streetlights that surround me as it only blocks sodium lines from the lights. But like I said I have read about them and that filter seems like a good start for me at least. 

If you want to try and block different wavelengths of LP you may want to look at a tri or quad band filter, as I understand it will not block all of the light from LED lights but may give you more contrast depending on what you are photographing.

Edited April 13, 2020 by Wyvernp

[fortytwo]

47 minutes ago, PeterCPC said:

Lots of telescopes are not suitable for using with a full frame camera. It would help if you said what telescope you are going to be using.

Peter

Hi Peter, I have Skywatcher 200 refllector, Skywatcher 130 refractor HEQ5 Pro.

I have been using my 5Dmk4 on the 130 recently.

[PeterCPC]

Your scope will need to be able to cover a 44mm image circle so you need to check that.

Peter

[fortytwo]

33 minutes ago, PeterCPC said:

Your scope will need to be able to cover a 44mm image circle so you need to check that.

Peter

Looking online I cannot seem to find that information. It seems the consensus is that manufacturers will seldom give that information and there is no definitive way to calculate it.

I'm sure what you are saying is relevent, but I am struggling to understand why? Presumably it has to do with the size of the sensor in my camera, is this where the flattener comes in?

Can you explain what the relevence of that information is, and what impact it will have on the choices available to me with the current gear I own?

 

Thankyou,
Jeff

[CloudMagnet]

6 hours ago, fortytwo said:

Looking online I cannot seem to find that information. It seems the consensus is that manufacturers will seldom give that information and there is no definitive way to calculate it.

I'm sure what you are saying is relevent, but I am struggling to understand why? Presumably it has to do with the size of the sensor in my camera, is this where the flattener comes in?

Can you explain what the relevence of that information is, and what impact it will have on the choices available to me with the current gear I own?

 

Thankyou,
Jeff

The image circle is the size of the image that is actually projected out of the focuser tube into your camera. The issue arises when your camera sensor is larger than the area of light that comes out of the telescope.

This creates a vignetting effect (the edges are a lot darker than the center) and is a pain to process out and really means that you are wasting the large sensor size as no light reaches the outside pixels.

 

A flattener refers to an lens attached to your scope that corrects for coma, usually in smaller focal ratios. This is an effect that looks like the edges of the image is warping in a circle and makes an image look terrible. A flattener will help remove this effect.

[fortytwo]

15 hours ago, CloudMagnet said:

The image circle is the size of the image that is actually projected out of the focuser tube into your camera. The issue arises when your camera sensor is larger than the area of light that comes out of the telescope.

This creates a vignetting effect (the edges are a lot darker than the center) and is a pain to process out and really means that you are wasting the large sensor size as no light reaches the outside pixels.

 

A flattener refers to an lens attached to your scope that corrects for coma, usually in smaller focal ratios. This is an effect that looks like the edges of the image is warping in a circle and makes an image look terrible. A flattener will help remove this effect.

Hi CloudMagnet,

Thanks for the explanation, that mkaes sense as I have seen some vignetting in the very edge of the corners using the 5Dmk4, I've not seen any warping as yet.

I think then I might use my 550D APS-C Canon with the original filter in my OP although that camera doesn't have a built in intevalometer, but I can use the remote I have for it.

Do you think that the lower resolution of the 550D will cause much loss in the quality of the image result?

 

Thankyou again for the explanation,
Jeff

[CloudMagnet]

3 hours ago, fortytwo said:

Hi CloudMagnet,

Thanks for the explanation, that mkaes sense as I have seen some vignetting in the very edge of the corners using the 5Dmk4, I've not seen any warping as yet.

I think then I might use my 550D APS-C Canon with the original filter in my OP although that camera doesn't have a built in intevalometer, but I can use the remote I have for it.

Do you think that the lower resolution of the 550D will cause much loss in the quality of the image result?

 

Thankyou again for the explanation,
Jeff

The smaller size of the sensor will give a smaller FOV for a given telescope, something to bear in mind. A good calculation to use to work out the resolution/quaility is how "sampled" your image will be is using image scale. This website gives a good background on what an undersampled image and an oversampled one will look like: https://www.atik-cameras.com/news/rules-1-2-arc-second-per-pixel-and-when-to-break-them/

The calculation to use for your cameras: (Pixel Size (μm) of camera / Focal length of telescope) * 206.3 = angular resolution (arcseconds/pixel)

Aim for an image scale of 1-2 and you should be OK as this will avoid blocky or bloated stars.

[fortytwo]

On 14/04/2020 at 17:42, CloudMagnet said:

The smaller size of the sensor will give a smaller FOV for a given telescope, something to bear in mind. A good calculation to use to work out the resolution/quaility is how "sampled" your image will be is using image scale. This website gives a good background on what an undersampled image and an oversampled one will look like: https://www.atik-cameras.com/news/rules-1-2-arc-second-per-pixel-and-when-to-break-them/

The calculation to use for your cameras: (Pixel Size (μm) of camera / Focal length of telescope) * 206.3 = angular resolution (arcseconds/pixel)

Aim for an image scale of 1-2 and you should be OK as this will avoid blocky or bloated stars.

Thankyou for that Cloudmagnet. 
I’ve used that and I’ve found my cameras are 1.4 (550d) and 1.8 (5dmk4) so either will be good it would appear. 
I think I will try the 550 as the filters are designed for APS-C. 
I’ve been reading up on the various filters and learning how to interpret the graphs for each so just need to decide which filter to buy first. 
Thankyou again for the help. 
Jeff