Catanonia

Fantastic colours there. Love to see this target done well and this is up there.

Wow, that 300 is really a beast.

Great detail there buddy and have some of these waiting for processing. Unfortunately they were from the Rasa8 and would probably only take up 2 pixels on your image

I have changed the thread name to more reflect this discussion as I think it contains some VERY useful information from @vlaiv and @ONIKKINEN

Others may like this thread and the title change will help to search it out.

10 hours ago, ONIKKINEN said:

This workflow is more or less what i do in Siril to my data. It looks scary but once you get used to it its just another step in the voodoo magic of astrophoto processing and its not that big of a hassle in the end. Splitting has some very small benefits to debayering and then binning, but everything in AP processing is done to get a small gain so why not do it i think.

I dont blink my data, or really visually inspect the subs in any way for that matter. I just use statistics measured from the subs to weed out the bad ones. I use measured FWHM, star roundness, background levels and SNR to make the decisions. In Siril i use the dynamic PSF function to create a plot of all the subs and its just a few clicks to determine what gets kept and what gets thrown out. I think in PI you could use the subframe selector tool to do the same, but with much better stats than what Siril can provide. Blinking can be fun to see asteroids and other anomalies in the subs, but other than that i dont give it much value for determining whether the sub is kept or not.

Sometimes after a night i see that almost all the subs are less than ideal and they get thrown out, but with long integrations i care less about losing a few hours of data since its nowhere near done anyway. I have 2 in progress projects that are at 15+ hours so far and i estimate i may need to double that to reach a result that i really like, so whats losing a few hours when im looking at 15+ hours of more in the end? For the most demanding objects i see that i not only need a long integration, but also will need all of the data to be better than average to get a sharp and deep image in the end.

I uses blink for Elon trails and obvious issues like out of focus

I will then start to use SubFrameSelector  before integration of channels to check and get the best ones. Played with it a few times so know how it works.

11 hours ago, vlaiv said:

Depends on what kind of subs you measured. To be sure - you can plate solve single sub to get exact sampling rate.

Without focal reducer, regularly debayered image will have sampling rate of 3.76 * 206.3 / 2000 = ~0.388"/px

Without focal reducer, splitCFA image (or rather color channel sub) will have sampling rate of twice above, so 0.776"/px

With focal reducer and 1430mm of FL, regularly debayered image - you will have 3.76 * 206.3 / 1430 = 0.542"/px

splitCFA + reducer will hence have 1.085"/px

To be honest, I'm not surprised with those FWHM results. Even slightly worse than excellent seeing will easily push resolution above 3" FWHM (or to around 2"/px equivalent).

 

Thanks, those are the magic numbers I needed. 

The rest I can work out from a couple of images dropped into PI analysis

32 minutes ago, vlaiv said:

Actually - maybe try it, you might be surprised.

Proper sampling rate can be deduced from data - look at average FWHM in arc seconds in your subs and you can get the sense of sampling rate required. Relation is rather straight forward FWHM / 1.6 will give you sampling rate.

If you have 3.52" FWHM or larger - 2.2"/px is actually proper sampling rate for that case. Even if your FWHM is smaller than 3.52" you might not loose much in terms of detail, yet you might get quite a bit in terms of SNR.

I did a quick check on unbinned (software or hardware) files.

I put in 0.79 as the arsecs/pixels = is this correct ??

If so, then I am hovering in that range and so may be worth it

BTW this is pretty poor data that was rejected as only 16 subs listed here worked out of 100

The process for those wanting to follow - Basically a summary

For my scope and camera combination I should be aiming at 1.5"/px

If I bin my data by 2x (software or hardware) on 2000mm @F8 then this will be almost twice as fast as using a 0.67 reducer @1400mm and F5.3 and no loss in resolution.

Therefore I will only use the reducer "if and when" I need a larger FOV for a target.

I will do software binning in PI

The workflow is as follows for me on OSC ZWO 2600 MC Pro

• Calibrate all images with darks, flats and bias
• Blink images and remove bad ones
• Perform a SplitCFA process on the calibrations to split them into CFA0 (R), CFA1(G), CFA2(G), CFA3(B) channel directories
• Combine the 2 greens directories into 1
• Align all the images to a reference one with ImageRegistration
• ImageIntegration to produce Red, Green and Blue channels
• Perform an IntegerResampling 2x bin on each of the RGB channel integrations
• Use LRGBCombination to bring them back to a RGB image
• Process as normal

2 hours ago, vlaiv said:

Exactly.

If you think about it - color sensor / bayer matrix is nothing more then RGB filters on top of pixels. It is just the matter of grouping each color into a image - and that is what splitCFA does.

You'll also notice that each of those 4 images is in fact twice as smaller in height and width than sensor size. This is the reason why color sensors have effectively smaller resolution and not the one indicated by pixel size (every pixel of each color is spaced "two pixels apart" really as they are interleaved depending on color filter on them).

Tried out the process on some old data and worked out how to do it.

Will remember this now for the next imaging session and go with 2000mm F8 @bin 1 and use software binning. I will only use the reducer when I "need" the wider field

Shame I have a couple more images with the reducer to process and binning them would not be wise at 2.2"/px so will not re-bin them

Thanks @vlaivfor taking an extraordinary amount of time to explain this.

14 minutes ago, vlaiv said:

That does not seem right. Here is what you should do:

....

 

So bypass Debayer and do it yourself with the SplitCFA and of course allows you to IntegerResample at will

Just did a SplitCFA on a non Debayered sub and it did split to down to 4 components which is nice

So if I do this to all the calibrated images, move the greens, I can then process like a RGB image as I used to in mono camera's

A bit more complicated, but not too much hassle.

Thanks for your time @vlaivfor this discussion. Much learnt

Update.

The 2 tools in PI are SplitCFA and MergeCFA. 

SplitCFA will take a load of images and split them into the 4 channels in a nice directory tree structure, BUT and here is the kicker, the MergeCFA can only take 1 set of files, ie cannot process a whole directory like the Split does. This means realistically it is limited to being done on a stacked linear image otherwise I would have to manually merge 100's of files at a time, very long and laborious clicking on 100's of 4x images and getting them in the right order.

So routine in PI is

• Calibrate
• Debayer
• Align
• Combine - linear stack
• SplitCFA
• IntegerRe-sample to 2x or 3x on each channel
• MergeCFA to give back the final 2x or 3x binned stack

It seems to work

17 minutes ago, ONIKKINEN said:

Actually i have not used Pixinsight either, the script i use is for Siril.

The command in question for Siril is seqsplit_cfa "your sequencename".

I would be surprised if PI doesnt have the same feature though.

Thanks, trying to work this out in PI and keep coming up with Drizzle. Not sure if I am going down a dead end here.

Edit - In PixInsight it is called SplitCFA and MergeCFA - Reading up on it now

19 minutes ago, vlaiv said:

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.

Ah this is key and not what I was doing. I was Integer binning (in PI) after all the processing   doh !!! and that is probably why I did not see any differences or reasons to go down the software binning route.

My process will now be as follows

• Take the images at 1x bin
• Calibrate them
• Debayer them
• Integer bin them
• Stack them
• Process as normal

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.

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 ?

@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

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.

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)

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 ?

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.

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

@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

@vlaiv - Attached is a calibrated sub

Light_M81_180.0s_Bin1_0001_c_d_r.xisf

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.

On 26/03/2022 at 18:53, powerlord said:

ver nice detail you got there. I need to have a go at this again - when I tried I didn't get anything like the detail you have. well done sir. 👏

Thanks buddy. 

I am at 2000mm FL here and not sure what FL you tried at.

I was quite surprised at the detail I got considering my Bortle 6 skies

On 27/03/2022 at 13:34, MarsG76 said:

Excellent image... one of the galaxies I wish I could try and image from Australia.

 

You are missing out, Bodes and Cigar are great "big(ish)" galaxies for us northerners

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