Discussion on Software Binning / RC Reducer Review and final M81 Image

[Catanonia]

I am still working out all of the fun parts of the 10inch Open Truss RC from TS Optics. So far I have gotten the collimation down to a tee and now was the time to try out the TS Optics 2inch CCD Reducer 0.67x that is recommended by TS Optics to use with this scope.

https://www.teleskop-express.de/shop/product_info.php/info/p8932

This should have brought down the 10inch RC from 2000mm @F8 to 1340mm and @F5.3. Interestingly, after plate solving, the ZWO ASI Air Pro reports the FL as 1426mm, so I don't know what is going on here.

Here are my thoughts of the scope and reducer combination

• Getting the recommended 85mm back focus is not an issue and fairly easy to do with T2 adapters and there is plenty of room for filter wheels / drawers if you wish to use them. My back focus was bang on 85mm with good results.
• Getting focus was a completely different matter. Even with the smallest focuser adapter in place on the RC, I had to arrange the image train with the reducer right at the end of the train and push it all the way into the 2inch focuser. This I found strange that the only way to get focus was to push the reducer so far into the focuser, but at least it worked with about 15mm focus wiggle room.
• Getting good flats is essential - The flats are a reverse doughnut shape (See attached image) but do work.  The ASI Air Pro completely messes up auto stretching of the flats and is worrying at first until you realise and manually stretch the sub.
• I found the best way was to take flats was to use the white teashirt over the scope and point to morning sky.
• Light seep from the open truss was not an issue, but I did use an additional after market cloth scope shroud.
• Collimation was still bang on as expected and even out to the edges of the frame. So the reducer didn't mess with the good flat field the native 2000mm FL RC has.
• With the WO66 and ZWO 120mm guide camera on EQ8-R I can get 3min guided subs with no issues. Not too bad considering not a OAG setup. I did try 5 mins and got some slight eggy stars, so 3mins is my max.
• Images look great and well worth the relatively cheap price for the reducer.
• 1340mm is still a good FL to get good close in images with the advantage of the increased F ratio,

 

Image Details

• M81 in OSC 4.5 hours
• Scope - 10 inch Open Truss RC from TS Optics with TS Optics 0.67 Reducer
• Main Camera - ZWO 2600MC Pro and no filters
• ZWO EAF for focusing.
• Mount - EQ8-R Mount with WO66 / 120mm guiding (0.4 RMS PHD2 )
• Computer - ZWO ASI Air Pro
• 4.5 hours OSC data in 3min subs from Bortle 6 Skies

 

Image Attachments

• M81 - 4.5hrs
• Flat frame
• Single 3 min sub
• Setup

 

Edited March 30, 2022 by Catanonia

[vlaiv]

Given that ASI2600mc is APS-C sized sensor and that you are using x0.67 reducer - how much of the frame did you have to crop away / what is the useful surface of sensor in this combination?

[Catanonia]

8 minutes ago, vlaiv said:

Given that ASI2600mc is APS-C sized sensor and that you are using x0.67 reducer - how much of the frame did you have to crop away / what is the useful surface of sensor in this combination?

I don't have to crop any of the final images after calibration of flats / darks / bias (both native and 0.67 reducer)

• Well if I was being nit picking, perhaps 10% off the edges for perfectly round stars

I just decided to crop for framing and presentation of final image.

Edited March 28, 2022 by Catanonia

[vlaiv]

5 minutes ago, Catanonia said:

I don't have to crop any of the final images after calibration of flats / darks / bias (both native and 0.67 reducer)

• Well if I was being nit picking, perhaps 10% off the edges for perfectly round stars

I just decided to crop for framing and presentation of final image.

That is quite interesting.

1426 / 2040 = x0.7

Effective reduction factor was x0.7 and ASI2600 has diagonal of 28.3mm - that would mean that used field was 28.3 / 0.7 = 40.4mm

TS lists usable field of 30mm for 10" version.

That is quite a difference in field size.

[Catanonia]

@vlaiv - Attached is a calibrated sub

Light_M81_180.0s_Bin1_0001_c_d_r.xisf

[vlaiv]

2 minutes ago, Catanonia said:

@vlaiv - Attached is a calibrated sub

Light_M81_180.0s_Bin1_0001_c_d_r.xisf 298.61 MB · 0 downloads

Thanks, but I don't speak xisf ,

Maybe fits? (don't bother if it is too much hassle to upload it).

[Catanonia]

@vlaiv - Uncalibrated, but shouldn't matter. Will need debayering.

Looking closely, yes I can now see that there are elongations as you move out from the image and hence for a "perfect round star image" you would need to crop.

Light_M81_180.0s_Bin1_0001.fit

[Catanonia]

Here is one debayered and colour neutralised. @vlaiv

Stars are not "too" bad on the edges

 

Light_M81_180_0s_Bin1_0003_RGB_VNG.fit

Edited March 28, 2022 by Catanonia

[clarkpm4242]

Very nice.

Makes me realise how much more I can/need to do to refine my setup and images!!

Cheers 

Paul

[vlaiv]

32 minutes ago, Catanonia said:

Stars are not "too" bad on the edges

You are quite right! I was expecting much worse. Yes, there is some astigmatism, but this reducer does a good job of flattening image a bit (it's not flattener but it does slight flattening effect).

I'll need to revisit it with my RC8" and ASI1600 - it should work well in that combination also.

Just one more question - did you use CCDT67 or CCD47 (one is original AP reducer while other is TS/Chinese copy). I have CCD47 version, and my initial impression was not as good on RC8".

[Catanonia]

56 minutes ago, vlaiv said:

You are quite right! I was expecting much worse. Yes, there is some astigmatism, but this reducer does a good job of flattening image a bit (it's not flattener but it does slight flattening effect).

I'll need to revisit it with my RC8" and ASI1600 - it should work well in that combination also.

Just one more question - did you use CCDT67 or CCD47 (one is original AP reducer while other is TS/Chinese copy). I have CCD47 version, and my initial impression was not as good on RC8".

Yes, I was quite surprised at the result and was expecting to have to crop quite a bit.

I ordered and received the one in the original post and URL I listed.

I can only assume this is what TS Optics sent me.

[Same old newbie alert]

What method are you using for collimation, did I read you was using the ocal camera?

[vlaiv]

26 minutes ago, Catanonia said:

I ordered and received the one in the original post and URL I listed.

I can only assume this is what TS Optics sent me.

Ah, ok - that is the same one I have.

That is probably copy of following item:

https://www.teleskop-express.de/shop/product_info.php/info/p4955_Astro-Physics-0-67x-Focal-Reducer-for-astrophotography.html

same specs, but AstroPhysics reducer has been around for much longer. It was designed for slow refractors and other flat field telescopes (F/9 and above) and it turned out that it works well with RCs as well.

[Catanonia]

5 hours ago, vlaiv said:

Ah, ok - that is the same one I have.

That is probably copy of following item:

https://www.teleskop-express.de/shop/product_info.php/info/p4955_Astro-Physics-0-67x-Focal-Reducer-for-astrophotography.html

same specs, but AstroPhysics reducer has been around for much longer. It was designed for slow refractors and other flat field telescopes (F/9 and above) and it turned out that it works well with RCs as well.

Serious question mate

On one of my previous posts about the RC and reducer, you indicated that it might not be worth while due to crop / sensor / etc / etc

Is this still valid considering the results I have posted here ?

[Catanonia]

5 hours ago, Same old newbie alert said:

What method are you using for collimation, did I read you was using the ocal camera?

I use the Ocal for collimation and then star test / tweek

The Ocal this time got it very close (about 1/4 turn on one of the secondary screws)

[bob-c]

A lovely image well done.

 

Bob.

[alan potts]

That's as good as any image I have seen of this galaxy, superb shot. I use to stay at the Old Hall many time when visiting Middlewich Ideal Plant. I liked Sandbach.

Alan

[vlaiv]

9 hours ago, Catanonia said:

Serious question mate

On one of my previous posts about the RC and reducer, you indicated that it might not be worth while due to crop / sensor / etc / etc

Is this still valid considering the results I have posted here ?

Not sure what you are asking.

I'll give my view on the subject of reducer and if I fail to answer your question, then let me know so I can be more specific.

There are two things that I can think of at the moment, that are important for discussion on focal reducers and RC.

1. Usability of focal reducer.

This is the question of usable / corrected field with given scope. Because of data published by TS, I was under impression that focal reducer won't do much in widening the FOV on large sensors. TS states that RC10 for example has corrected field of about 30mm.

When using focal reducer, those 30mm get shrunken to 30 * 0.67 = 21mm, so only central 21mm of 28mm APS-C sensor (or about 3/4 of it) should have good stars. Reasoning comes from the fact that this is simple reducer and not focal reducer / field flattener that corrects for curvature (which reduces other aberrations as well - or makes them less visible). I've read somewhere that although this is not FF/FR - it does flatten field a bit.

In any case - above would make it not quite usable on APS-C sized sensor. RC8" users report that they had to use it at x0.72-0.75 with 4/3 sized sensors to get good results (changing reducer-sensor distance changes reduction factor) - which is in line with this since RC8" also has ~ 30mm of usable field and 30 *0.72 = 21.6mm (ASI1600 has diagonal of 22.2mm).

This is the reason I was surprised to see much larger corrected field in your image with RC10, as TS claims 30mm for it as well

2. Imaging speed

Many people equate F/ratio to speed of image capture. This is only true if one keeps pixel size constant (does not bin or use different camera). Using focal reducer there fore "increases" speed of the system as it reduces F/ratio.

Given that we over sample with long focal length scopes - this is not correct as we need to bin our data. For this reason - it is better to look at final sampling rate and decide on focal reducer based on that and not F/ratio alone.

It can happen that you have faster system without reducer than with if you can match sampling rate better with binning.

This can particularly be true in cases where you need to crop away part of the image due to outer field aberrations (like in above case) or framing. If you can frame target without use of reducer - then why use reducer in the case where you can better match resolution of the image without it.

For example, you have 2000mm of focal length and 3.76um color camera. Let's say your target resolution is 1.5"/px. Should you use focal reducer or not (if the target can be framed without it).

Actual sampling rate of OSC camera is half of that of mono, so we have 2 * 3.76 * 206.3 / 2000 = 0.776"/px as base sampling rate. Bin that x2 and you get 1.55"/px - excellent match for target resolution.

What about case with reducer? You'll have ~1400mm FL (1340 or if you have slightly different reduction - like in your case 1400 and something - so let's round that to 14000) and sampling without binning will be:

2 * 3.76 * 206.3 / 1400 = ~1.11"/px

Now that is too high if you don't bin and too low if you bin x2 (2.22"/px). Most people won't bin and the will in fact have slower system.

Things change if you can achieve 1.1"/px (very hard to do) - then it makes sense to use focal reducer as you can better hit your target resolution at 0.776"/px will be over sampling and slower.

Now, from what I can measure on your image - you left it at 1.1"/px - and it can be seen, image is slightly over sampled.

One of these two images was reduced to 66% of original size and then upscaled back to original size (which would make it loose detail if detail is there to begin with), other is original without any change. Can you tell which one is which?

This means that all the detail in the image could have been captured at 66% resolution - or 1.66"/px.

1.55"/px would be better imaging resolution and in this case - it would be faster and better to image without reducer and bin x2 after you debayer without interpolation.

[Catanonia]

1 hour ago, vlaiv said:

Loads of good stuff.

Thanks buddy. 

This is going to take some time to read and digest, but yes this was the topic we touched on before this thread.

[Catanonia]

@vlaiv

I think I have my head around this and it seems to be a question between

• a. No reducer -  1x hardware bin and 2x bin in software and cropping the image

versus

• b. With reducer - 1x hardware bin on the camera and not cropping the image

(assuming you can frame your target)

Edit

After looking and playing with CCD Suitability calculator it seems I have the choice between A and B above. A would be more optimal if I can frame, B would be more optimal if I need more widefield view.

But the differences are close, 1.5 "/px compared to 1.1 "/px and fall within the green band.

I still (am sure others as well) need to get head around that 1x bin does not always give you more detail than 2x bin

 

 

Edited March 29, 2022 by Catanonia

[vlaiv]

40 minutes ago, Catanonia said:

After looking and playing with CCD Suitability calculator it seems I have the choice between A and B above. A would be more optimal if I can frame, B would be more optimal if I need more widefield view.

But the differences are close, 1.5 "/px compared to 1.1 "/px and fall within the green band.

Ok, so CCD suitability calculator is wrong in couple of ways (although people seem still reluctant to accept that).

41 minutes ago, Catanonia said:

I still (am sure others as well) need to get head around that 1x bin does not always give you more detail than 2x bin

Not really sure how to best explain this one.

To me it is obvious, but that is simply because of the way I think about things (given my background and previous work in graphics, math, signal processing ....).

I guess simplest analogy is measurement and measuring accuracy.

You can measure tall building with a yard stick to a good precision, right? You can't measure height of your desk with a yard stick very accurately, right?

In first instance - if you round to a length of a stick - your error will be few percent. In second case - you can miss up to 50% - which is large error.

In order to measure something accurately - you need to have enough precision and that precision depends on size of thing you are measuring.

Same goes for image. How dense pixels you need (it is actually "distance" between two pixels that counts and not the size of single pixel) depends on how big detail in the image you want to measure / record.

It turns out that detail that you get in astro images depends on seeing, mount performance and aperture size. It is often not as high as we would like (and think it is). If there is no detail to be measured - using high resolution will produce same results as using low resolution (measuring desk height is equally efficient in millimeters and microns within given error of measurement).

Since images are impacted by noise - if measurement error is lower than noise - you won't even see measurement error and for all intents and purposes - both measurements will yield same results.

(in reality there is additional component that there really is no detail beyond certain scale - but that has to do with wave nature of light, and frequencies and Fourier transforms and Nyquist sampling theorem and so on).

For most amateurs, top limit seems to be around 1.5"/px. Sometimes, people with excellent mounts, great skies and large apertures can go down to 1"/px - but I don't think I've ever seen image that is effectively below 1"/px - unless it comes from professional telescopes that are often more than 1m in diameter and are situated at high altitude sites with great seeing.

Ok, so why bin or match pixel size to what we can capture? - even if we there is no detail to be recorded - why does it hurt to use smaller pixels?

Well it hurts on two different levels - first is real and second is aesthetic.

1. We often base our decisions on "speed" - we don't want to expose for 20h to get good image so we want fast setup. People value small F/ratios and use focal reducers for that same reason - but what they should be looking instead is to avoid over sampling.

If you image at 1.1"/px instead of 1.5"/px - it will take you 1.5 x 1.5 / 1.1 x 1.1 = ~1.85  or 85% longer to reach the same SNR. This is simply because you spread light over larger surface and lower recorded signal - less signal, lower SNR, simple as that.

So instead of looking at F/ratio and focal reducers and all that - people should really look at their effective pixel size and optimize that.

Speed of the system is best described as "aperture at resolution". Fix resolution to what you can realistically achieve given your sky and mount (and potential aperture size) and then throw as much aperture at that resolution as you possibly can. Just make sure that you can also vary pixel size (one part of resolution equation) - either by changing camera or by binning.

2. Aesthetic reason being - max zoom.

Many people are unaware of this because they like to look at whole image. Your image above is rendered by SGL on my desktop computer at 67% of its original size - and it looks good because at that rate it is properly sampled. I like to look at small detail in the image next to main object - I like to see background galaxies or small features of the image and I often look at image at 100% - to be able to see all of that.

If image is oversampled - it just looks blurry and bad at 100%, so question is - if you did not want people to look at image at full size - why did you bother to make it that size in the first place?

Alternative view - if you are going to make image - then make it viewable / looking nice for all zoom levels up to 100% (most people know that zooming past 100% simply yields poor results as image can't record detail past 100% level).

In the way - over sampling is the same as zooming in past 100% - you get the same result - things get bigger but there is no detail in the image - everything looks bigger and blurry.

[vlaiv]

1 hour ago, Catanonia said:

I think I have my head around this and it seems to be a question between

• a. No reducer -  1x hardware bin and 2x bin in software and cropping the image

versus

• b. With reducer - 1x hardware bin on the camera and not cropping the image

I think you have cropping part reversed - cropping is needed if you have reducer for two reasons - first is edge of the field aberrations (if exist in a setup) will be worse with reducer (not true for matched reducer / flattener as those correct for aberrations) and second is framing - reducer often give more space around the objects and if you want to emphasize them - you want to crop away that excess space.

[Catanonia]

36 minutes ago, vlaiv said:

I think you have cropping part reversed - cropping is needed if you have reducer for two reasons - first is edge of the field aberrations (if exist in a setup) will be worse with reducer (not true for matched reducer / flattener as those correct for aberrations) and second is framing - reducer often give more space around the objects and if you want to emphasize them - you want to crop away that excess space.

I meant with reducer I would get wider field of view compare to without reducer where I would need to crop the image to get the same framing on non reducer compared to a reduced one

(of course there is still cropping for star shapes in potentially both scenarios.

 

Your long explanation above makes sense now you have described it that way. 

My take home from all this is that I really should be imaging at 2000mm F8 and then binning that data to give me 1.5"/px UNLESS I cannot get the framing I want, ie wider field of view and hence adopt the reducer with the known loss in SNR

The question is, should I bin in hardware or software. Hardware would save disk space, load times etc etc, but then I would loose raw data and the ability to change my mind.

I have been in the past processing the 1xbin data and at the very end Integer reducing it down to size. I think this might be the wrong way

 

What would be your recommendation for this setup in terms of hardware / software binning and if software binning, when should it be applied ?

 

Edited March 29, 2022 by Catanonia

[Catanonia]

5 hours ago, alan potts said:

That's as good as any image I have seen of this galaxy, superb shot. I use to stay at the Old Hall many time when visiting Middlewich Ideal Plant. I liked Sandbach.

Alan

Thanks buddy. The Old Hall was done up about 6 years ago into a new fancy pub / restaurant. Quite nice now.

Sandbach is a nice quiet / quaint little market town. One of the many reasons I bought a house here.

[vlaiv]

5 minutes ago, Catanonia said:

The question is, should I bin in hardware or software. Hardware would save disk space, load times etc etc, but then I would loose raw data and the ability to change my mind.

I have been in the past processing the 1xbin data and at the very end Integer reducing it down to size. I think this might be the wrong way

It depends if you use CMOS or CCD sensor.

With CCD sensor it makes sense to use hardware binning. With CMOS sensor - it is really the same.

I prefer doing like you did up until now - download regular subs and then decide afterwards what sort of bin factor to apply. I even apply different bin factors depending on what I'm trying to do with data. I might use one bin factor for image as I like ability to zoom in to see detail - and different, higher bin factor to produce some sort of data diagram.

For example - if you want to produce surface brightness chart - you want higher SNR data and you don't really care about small details in the image - then you can use higher bin factor.

There are also several different ways to bin data - and you can choose among them in processing.

You can even bin after stacking (although this can have different results depending on what sort of alignment interpolation you used) - just make sure you do it while data is still linear prior to any processing.