Star Bloat

[SyedT]

Hi guys,

Looking for some collective wisdom from SGL regarding star bloat. I've recently acquired a lovely FSQ-106ED and the matched 0.6x reducer. I've noticed that the brightest stars are significantly bloated in the blue channel, the smaller stars seem to be OK so I think focus is fine.

Images attached below (unedited RGB, R, G, B respectively). I've done some reading and it seems that it's unavoidable even in some of the best scopes e.g. TEC 140. What I'm interested in are suggestions that a separate UV/IR cut filter can be installed in front of the imaging train to reduce bloat e.g. Astronomik L3. If anyone has tried that or there are any other suggestions to tackle this, would be much appreciated!

Equipment: QHY268M, Chroma RGB filters, Takahashi FSQ-106ED, Takahashi Focal reducer QE 0.6x

Acquisition: Gain 0 Offset 20. 50 x 120s exposures per channel.

Cheers

 

[GalaxyGael]

As far as I know, this QHY version of the ASI 2600 series does not have an inbuilt IR cut filter, which also acts as the UV cut. 

But, Chroma made recent changes to their blue and lum filters, moving the UV cutoff up to 420 nm, where it was lower in previous models. You probaby have the older models that passed much more UV that the curren gen.

Here are the old and new spectra:

Green = original Lum  bandpass

Red = February 2019 Blue bandpass

Blue and purple = April 2019 Blue and Lum bandpasses

[SyedT]

On 12/11/2021 at 19:30, GalaxyGael said:

As far as I know, this QHY version of the ASI 2600 series does not have an inbuilt IR cut filter, which also acts as the UV cut. 

But, Chroma made recent changes to their blue and lum filters, moving the UV cutoff up to 420 nm, where it was lower in previous models. You probaby have the older models that passed much more UV that the curren gen.

Here are the old and new spectra:

Green = original Lum  bandpass

Red = February 2019 Blue bandpass

Blue and purple = April 2019 Blue and Lum bandpasses

Thank you for the reply, that explains it. Much appreciated! Do you think adding a separate UV/IR cut filter will help?

[ollypenrice]

I'd give the UV cut filter a try.  I have both an FSQ106N (the old fluorite version) and a TEC 140 with TEC flattener. With the flattener (but not without it) the TEC produces tighter blue stars than the Tak and, indeed, tighter stars in general, but it has more aperture so it should.

Some specific processing is to be expected in the case of all bright stars, I think.

Olly

[GalaxyGael]

7 minutes ago, SyedT said:

Thank you for the reply, that explains it. Much appreciated! Do you think adding a separate UV/IR cut filter will help?

Not sure really, it is an extra layer of glass so will probably affect focus position, transmission etc. a little. It is what I would try if I was in your shoes. For example, the IR cut glass on the ASI2600 is none other than the luminance filter from zwo, identical spectrum and it works fine. A similar filter placed just before the sensor should help with the UV portion for your QHY - transmits between ~400-700 nm. Almost the same as the new chroma lum filter.

 

ASIDE: might be worth changing the thread title? Since the bloat is not from the FSQ106 scope, but from the filter.... 

[SyedT]

17 hours ago, ollypenrice said:

I'd give the UV cut filter a try.  I have both an FSQ106N (the old fluorite version) and a TEC 140 with TEC flattener. With the flattener (but not without it) the TEC produces tighter blue stars than the Tak and, indeed, tighter stars in general, but it has more aperture so it should.

Some specific processing is to be expected in the case of all bright stars, I think.

Olly

Thanks Olly. Do you have any tips for processing bright stars? I find the main issue is that while the core of the star gets reduced OK, the actual outer halo just displays "softening", with not much of an effect on the halo itself.

17 hours ago, GalaxyGael said:

Not sure really, it is an extra layer of glass so will probably affect focus position, transmission etc. a little. It is what I would try if I was in your shoes. For example, the IR cut glass on the ASI2600 is none other than the luminance filter from zwo, identical spectrum and it works fine. A similar filter placed just before the sensor should help with the UV portion for your QHY - transmits between ~400-700 nm. Almost the same as the new chroma lum filter.

 

ASIDE: might be worth changing the thread title? Since the bloat is not from the FSQ106 scope, but from the filter.... 

So would you say that the filter needs to be right next to the sensor? My issue there is that my backfocus is very tight so I'm using a direct connection from the camera to the filter wheel i.e. there are screws which go through the filter wheel directly into the body of the camera. The rest of my connections are threaded with no nosepiece interface. My current imaging train is: QHY268M --> QHYCFW3 --> M54-M54 male-male adapter --> Rotator --> M54 male-male adapter --> 4 mm M54 extension ring --> 0.6x reducer (which slots and then screws into the 0.6x reducer extension ring).

If the 0.6x reducer has female threads at the telescope end, could I attach the UV/IR cut filter to these threads? Or would that be too far away from the sensor?

I'm thinking of going for the Astronomik L3 filter: https://www.astronomik.com/en/uv-und-ir-block-filter/luminance-filter-l-1-l-2-l-3.html

Edited November 20, 2021 by SyedT

[GalaxyGael]

3 hours ago, SyedT said:

So would you say that the filter needs to be right next to the sensor? My issue there is that my backfocus is very tight so I'm using a direct connection from the camera to the filter wheel i.e. there are screws which go through the filter wheel directly into the body of the camera. The rest of my connections are threaded with no nosepiece interface. My current imaging train is: QHY268M --> QHYCFW3 --> M54-M54 male-male adapter --> Rotator --> M54 male-male adapter --> 4 mm M54 extension ring --> 0.6x reducer (which slots and then screws into the 0.6x reducer extension ring).

If the 0.6x reducer has female threads at the telescope end, could I attach the UV/IR cut filter to these threads? Or would that be too far away from the sensor?

I'm thinking of going for the Astronomik L3 filter: https://www.astronomik.com/en/uv-und-ir-block-filter/luminance-filter-l-1-l-2-l-3.html

I assumed there was space and that the lum filter would go somewhere in the train, but not necessarily close to the sensor for a particular reason.  I dont have experience to say whether it is better before or after the filters themselves, but hopefully wont matter much. The reducer likely has thread for a 2inch filter on the scope side, many of them do. When the filter in-situ on the ASI2600MC for example, is ~15-16 mm from the sensor, there are no reflection issues. If it is further towards the scope, relfection liklihood decreases as far as I know.  But its a Tak reducer, and I dont know what threads it has on the scope side.

[SyedT]

47 minutes ago, GalaxyGael said:

I assumed there was space and that the lum filter would go somewhere in the train, but not necessarily close to the sensor for a particular reason.  I dont have experience to say whether it is better before or after the filters themselves, but hopefully wont matter much. The reducer likely has thread for a 2inch filter on the scope side, many of them do. When the filter in-situ on the ASI2600MC for example, is ~15-16 mm from the sensor, there are no reflection issues. If it is further towards the scope, relfection liklihood decreases as far as I know.  But its a Tak reducer, and I dont know what threads it has on the scope side.

Thank you, this is really helpful! I'll have a look at the threads on the reducer today and put an order in.

Would the filter I suggested be OK? Or are there any other filters you've heard good things about?

[GalaxyGael]

Honestly, I'm an OSC only user and in direct RGB mode with the inbuilt lum filter. when I used to use a quad band filter on a quad apo, it had an adapter with M58 thread that had threads for a 2in filter on the scope side, which was very useful. That L3 filter looks good, but as narrow as I would go since it just about covers the visible red end of the spectrum. I would imaging it is OK for H_alpha and S(II) and all the reds, but it is cutting it close. While you are doing LRGB imaging, the 672 nm ionized sulfur emission gets cut off with the L3 filter (is seems, I cannot read precisely from the website moving images). It certainly helps on the blue end.

But, it is also most identical to the ZWO lum filter spectrally and that is what I have inbuilt into the ASI2600Mc Pro, so it looks like the right option. Or you could get teh ZWO version which is likely a bit cheaper. The spectrum is here for comparison:

Teleskop-Express: ZWO 2" UV and IR blocking filter and luminance filter

 

[ollypenrice]

5 hours ago, SyedT said:

Thanks Olly. Do you have any tips for processing bright stars? I find the main issue is that while the core of the star gets reduced OK, the actual outer halo just displays "softening", with not much of an effect on the halo itself.

So would you say that the filter needs to be right next to the sensor? My issue there is that my backfocus is very tight so I'm using a direct connection from the camera to the filter wheel i.e. there are screws which go through the filter wheel directly into the body of the camera. The rest of my connections are threaded with no nosepiece interface. My current imaging train is: QHY268M --> QHYCFW3 --> M54-M54 male-male adapter --> Rotator --> M54 male-male adapter --> 4 mm M54 extension ring --> 0.6x reducer (which slots and then screws into the 0.6x reducer extension ring).

If the 0.6x reducer has female threads at the telescope end, could I attach the UV/IR cut filter to these threads? Or would that be too far away from the sensor?

I'm thinking of going for the Astronomik L3 filter: https://www.astronomik.com/en/uv-und-ir-block-filter/luminance-filter-l-1-l-2-l-3.html

I do big stars in Photoshop like this:

Make a copy layer.

Create a soft-edged eraser the size of the large halo and apply it at 100% over the halo and star on the top layer. This has no visible effect because the layers are identical.

Keep the top layer visible, make the bottom layer active and open Curves.

Put a marker on the curve just where the background sky or nebulosity is free from any of the bloat. Just alt-click on the background closest to the bloat.

Put a fixing point below the first marker.

Now grab the curve just above the background marker and pull it down. You can see the result through the 'hole' in the top layer. by playing with the curve and maybe adjusting the background marker you can reduce the offending star while looking where it's at in real time. I call this 'reverse processing' the star. Often this single Curve adjustment will work for more than one star in an image, in which case erase the top layer over the stars you want to reduce.

You can adjust the saturation in the bottom layer while you're at it.

Olly

Edited November 20, 2021 by ollypenrice
Typo

[SyedT]

6 hours ago, GalaxyGael said:

Honestly, I'm an OSC only user and in direct RGB mode with the inbuilt lum filter. when I used to use a quad band filter on a quad apo, it had an adapter with M58 thread that had threads for a 2in filter on the scope side, which was very useful. That L3 filter looks good, but as narrow as I would go since it just about covers the visible red end of the spectrum. I would imaging it is OK for H_alpha and S(II) and all the reds, but it is cutting it close. While you are doing LRGB imaging, the 672 nm ionized sulfur emission gets cut off with the L3 filter (is seems, I cannot read precisely from the website moving images). It certainly helps on the blue end.

But, it is also most identical to the ZWO lum filter spectrally and that is what I have inbuilt into the ASI2600Mc Pro, so it looks like the right option. Or you could get teh ZWO version which is likely a bit cheaper. The spectrum is here for comparison:

Teleskop-Express: ZWO 2" UV and IR blocking filter and luminance filter

 

Thank you, I'll go for the ZWO filter in that case! Will report back with results.

6 hours ago, ollypenrice said:

I do big stars in Photoshop like this:

Make a copy layer.

Create a soft-edged eraser the size of the large halo and apply it at 100% over the halo and star on the top layer. This has no visible effect because the layers are identical.

Keep the top layer visible, make the bottom layer active and open Curves.

Put a marker on the curve just where the background sky or nebulosity is free from any of the bloat. Just alt-click on the background closest to the bloat.

Put a fixing point below the first marker.

Now grab the curve just above the background marker and pull it down. You can see the result through the 'hole' in the top layer. by playing with the curve and maybe adjusting the background marker you can reduce the offending star while looking where it's at in real time. I call this 'reverse processing' the star. Often this single Curve adjustment will work for more than one star in an image, in which case erase the top layer over the stars you want to reduce.

You can adjust the saturation in the bottom layer while you're at it.

Olly

Cheers Olly, saving this to my workflow!