Solving Blue Bloat at source?

[DaveS]

My widefield imaging train is afflicted with Blue Bloat. I was using my 180mm f/3.4 Leitz Apo-Telyt-R on a SA with ASI 183 MM and Baader LRGB filters (An early version with quite a deep blue cut off). Last week I was at an astro meet and was imaging M31, just an hour in each channel (Not enough, I know)

The result is below.

This isn't the first version, I have been through all kinds of evolutions but none are any better than this unfortunately

Therefore I was thinking of solving the problem at source.

My options appear to be

1) A more modern apo 'scope. The closest is one of the Borg 55FL f/3.6 versions. I have also considered the Tak 60 with reducer, which comes in at a similar price when everything is added up. there are also a couple of Askar offerings of unknown quality.

2) Move to an OSC camera, whose colour response will be closest to the colour film the lens was computed for. A couple of options come to mind, the colour version of the 183, or the ASI 533, just a little concerned about the sampling with such large pixels relative to the FL of the lens. Keep the 183 mono for NB.

3) A different set of filters, with a blue cut-off that isn't so extreme. The Astronomik Deep-Sky set with the L3 IR / UV cut filter might be a possibility.

 

Does anyone have any thoughts on the above? Any other suggestions?

[irtuk]

from your post count I am guessing this is not a focus problem?

I have the same sort of symptom with green, but I am very new and I am fairly sure its a focus problem.

Ed.

 

[DaveS]

Well, partly a focus issue. As I was using a RDF for locating M31 I was reluctant to move to a bright star for focusing then try to relocate my target, so rather "fudged" the focus to a compromise between R,G, and B so I could run the whole sequence without interruption. However during the initial focusing I could see that the Bahtinov figure for the Blue filter was less well defined than the others, even at focus.

[Adam J]

14 hours ago, DaveS said:

My widefield imaging train is afflicted with Blue Bloat. I was using my 180mm f/3.4 Leitz Apo-Telyt-R on a SA with ASI 183 MM and Baader LRGB filters (An early version with quite a deep blue cut off). Last week I was at an astro meet and was imaging M31, just an hour in each channel (Not enough, I know)

The result is below.

This isn't the first version, I have been through all kinds of evolutions but none are any better than this unfortunately

Therefore I was thinking of solving the problem at source.

My options appear to be

1) A more modern apo 'scope. The closest is one of the Borg 55FL f/3.6 versions. I have also considered the Tak 60 with reducer, which comes in at a similar price when everything is added up. there are also a couple of Askar offerings of unknown quality.

2) Move to an OSC camera, whose colour response will be closest to the colour film the lens was computed for. A couple of options come to mind, the colour version of the 183, or the ASI 533, just a little concerned about the sampling with such large pixels relative to the FL of the lens. Keep the 183 mono for NB.

3) A different set of filters, with a blue cut-off that isn't so extreme. The Astronomik Deep-Sky set with the L3 IR / UV cut filter might be a possibility.

 

Does anyone have any thoughts on the above? Any other suggestions?

So a L3 in line with the B filter will clip it down to 430nm. I do this on my ASKAR FMA180 and it works really well, but the initial amount of blue bloat on the ASKAR FMA180 is quite a bit less than seen here on your lens. If you are thinking more about the new FMA230 then that is a unknown but it as a quad element reducer to the FMA180s tripled reducer, so my guess would be that the correction will be similar between the two. Personally if I had the cash I would be going with the Borg although even in this case and using fluorite at F3.6 i would say the correction still will not be perfect in the blue so a wide blue / lum filter would not be the best idea. 

 

look at page 2. 

Adam

 

 

Edited September 16, 2021 by Adam J

[DaveS]

Thanks Adam.

Still a lot to consider. I'm liking the ASI 533, just concerned about the pixel scale. The Askar FMA 230 is interesting, but unless they are using two different ED glasses it won't be corrected for 3 wavelengths.

I may initially go with the OSC camera option, perhaps with an Astronomik L3 to cut the blue. Then consider a new imaging telescope later.

[DaveS]

Found this online

Leitz Apo-Telyt-R

[vlaiv]

Changing camera will not help with optics aberration.

Sure - you can change sensor for less sensitive one - but, read that sentence once again, here let me help you: Change the sensor for less sensitive one - does that sound like a solution to anything?

It is the lens that is the problem, and here is list of things that work:

1. Stop down the lens. This is not fast lens so stopping it down will make it even slower, but lens often have CA at max aperture.

2. Use aggressive filtering - use for example Astronomik L3 or maybe even Baader long pass 495 in front of your other fitlers

Using combination of above methods - I took following image with ST102 F/5 fast achromat that is otherwise quite "colorful character"

3. Change the lens. If you are happy imaging at F/3.5 then you will simply love imaging at say F/2.8? Get F/1.4 or F/2 lens and stop that down to F/2.8. In most cases that will clean up CA and you'll get good results.

Samyang 85mm F/1.4 (or T1.5 version to be precise) wide open produces this:

Stars have similar shape to those in your image, and there is a lot of CA, but here is test that I did with that lens and artificial star:

From left to right - F/1.4, F/2, F/2.8 and F/4 - no special filtering (I have several filters that I used with this lens to see effect - like Baader Contrast Booster and IDAS LPS P2). As you can see - stopping down to F/2.8 produces rather good results.

By the way - lens often are not designed for small pixels so it is best to bin your data with them (I used ASI178 that has same pixel size as ASI183 - 2.4µm and I binned that to 4.8µm as 2.4µm is too much for lens that is not diffraction limited).

How about Samyang 135mm F/2 lens? That lens is well known and regarded for AP applications?

4. I could give you further recommendations that involve small telescopes, but I'm not sure how much weight can SA carry. For example get F/5.5 doublet with small aperture, and x0.6 FF/FR from Long Perng. That should correct ok for 1" sensor size like ASI183 and you'll have F/3.3 with say around 250mm. If that is too long FL - then make 2x2 mosaics and bin x2 additionally to get scope which is effectively half of that - so 125mm (that will still be F/3.3 - slower than Samyang at F/2.8 but probably sharper).

I'm going to explore bringing my 80mm F/6 down to F/3 - but with small sensor like ASI178. Don't think that would work on larger sensors as corners would surely suffer from that much reduction.

[DaveS]

Thanks Vlaiv.

My thoughts on OSC was to get a roll off in the blue, like the colour film I guess the lens was computed for

(This is the ASI 533)

Combined with an Astronomik L3 ahead of it in the train to cut the deep blue and far red.

I have also considered the Astronomik Deep Sky filter set, again with the L3 as Luminance.

But I'm in no great hurry to spend my money so plenty of time to get things right.

[kirkster501]

Possible this is caused by UV/IR being let onto the sensor that should be filtered out by the filters, luminance and blue are the worse effected.    Lots of stuff online about this.  Try a UV/IR cut filter in the nosepiece of the filter wheel before the filters.

[DaveS]

There's no nosepiece, and nowhere to put a filter ahead of the wheel. The least expensive option would be a set of filters with less extreme blue response.

[vlaiv]

8 minutes ago, DaveS said:

Combined with an Astronomik L3 ahead of it in the train to cut the deep blue and far red.

I have also considered the Astronomik Deep Sky filter set, again with the L3 as Luminance.

But I'm in no great hurry to spend my money so plenty of time to get things right.

In that case - maybe star off by stopping the lens down a bit and see how you like it (both in terms of speed and in terms of blue bloat).

Get L3 as single filter and use L3RGB approach. Refocus on filter change of course.

Maybe you could try L3RG approach as well? That would require some fiddling with measurements and numbers - but could be worth while?

When doing LRGB we are actually doing more "work" than we have to. Color can be captured with only three components - R, G and B. We capture L because it brings in the best SNR - but in doing so - we add fourth component. That is not needed, three components are enough so we might as well go with LRG instead of LRGB.

I purposely omitted Blue from this - because of blue bloat, but also because it has the worst SNR out of three components for several reasons - atmosphere scatters the most light in blue part of the spectrum, blue is less common / intense than other wavelengths in astro sources - and when it is equally present (white and bluish stars) - it is the same energy and not the same photon count ( blue light is more energetic and for same energy that means less photons) and so on ...

You can try LRG approach with the data you already have (it won't reduce bloat as bloat is also present in L as it covers whole spectrum) - just to try it out. make sure your exposure lengths are same, compute synthetic B as L - G - R and compose LRGB image that way and see how close it is to true LRGB.

[vlaiv]

2 minutes ago, DaveS said:

There's no nosepiece, and nowhere to put a filter ahead of the wheel. The least expensive option would be a set of filters with less extreme blue response.

Here is an idea - Since you have 180mm F/3.5 lens - it will have ~52mm of aperture so probably threaded for 52mm filters. Get 2" filter (which is M48) and appropriate adapter. Mount L3 filter in front of the aperture. This will create small aperture stop - say you'll work at 46mm of clear aperture (46-47mm is the most clear aperture for M48 filter) and this will give you 180 / 46 = 3.9.

You'll be effectively imaging at F/4 that should clean up stars a bit and you'll have benefit of stacked L3 filter?

[DaveS]

Thanks Vlaiv, some very good suggestions. Will have to print a holder for the filter as the lens has an odd 60mm Leica filter thread.

A bit busy ATM (Getting ready for a trip to the dentist, urgh.

[ollypenrice]

Unless it has been radically redesigned, I wouldn't go near a Tak FS60 for imaging. I've processed data from two examples in the past and both had truly abominable blue bloat.

Olly

[DaveS]

Many thanks for the alert Olly.

Back from the dentist. He's found a molar that needs to come out. Got decay in the root under a crown.

[A very bad removed word]

[DaveS]

Seems like I'm going to be assimilated by the Borg. "Resistance is futile".

[vlaiv]

3 minutes ago, DaveS said:

Seems like I'm going to be assimilated by the Borg. "Resistance is futile".

Should consult @alan potts on that one. I think he has Borg and has done some imaging with it at rather fast F/3.6 or something - and I think there is also CA present with that setup.

[DaveS]

Ah ha. Didn't know of anyone else who has one.

[vlaiv]

4 minutes ago, DaveS said:

Ah ha. Didn't know of anyone else who has one.

Not sure which version he has, but here is a thread featuring Borg image in it:

 

[vlaiv]

That is the focal length you are aiming at again?

How about something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p10095_TS-Optics-PhotoLine-60-mm-f-6-FPL53-Apo---2--R-P-Focuser---RED-Line.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p7943_Long-Perng-2--0-6x-Reducer-and-Corrector-for-APO-Refractor-Telescopes.html

That will give you ~200mm FL at F/3.6.

I've seen accounts that x0.6 reducer by Long Perng can happily illuminate and correct ASI183mm sized sensor.

Above combination won't break a bank like Borg

[DaveS]

Many thanks Vlaiv. Will have a closer look when I'm on the office computer. On my phone ATM.

[DaveS]

Had a look at those links, very interesting, and in synch with what I had been thinking as another possibility.

[alan potts]

8 hours ago, vlaiv said:

Should consult @alan potts on that one. I think he has Borg and has done some imaging with it at rather fast F/3.6 or something - and I think there is also CA present with that setup.

Mine is F 4.3 Vlaiv, and whilst I have never been totally happy with the scope I have got a bit better. I feel results have improved since I bought the stronger Astonomik's L 3 IR/UV filter, I seem to recall you among other recommended.

Alan

[DaveS]

A tentative plan of campaign is to buy a set of Astronomik Deep Sky RGB filters, and add a L3 Luminance plus a H-alpha.

Then consider  new telescopes or cameras for next year.

[mgutierrez]

On 17/09/2021 at 14:31, DaveS said:

A tentative plan of campaign is to buy a set of Astronomik Deep Sky RGB filters, and add a L3 Luminance plus a H-alpha.

Then consider  new telescopes or cameras for next year.

Hi,

I'm quite interested in this thread.

What did you do finally?

I have a TS102ED f7 refractor. I've just purchased a mono cam and now I'm still deciding which filters to buy, taking special attention to improve and reduce bloated stars (my doublet shows it noticeably). Astronomik deep sky + L3 may suit my needs, but a bit expensive for now. I wonder how a "standard" LRGB baader filters would behave.

cheers