Lucky imaging of DSOs using a ASI 178MM, C6 and AZ-GTi?

[Ags]

I have a C6 coming this week (calloo callay! o frabjous day!) intended primarily for planetary imaging and visual use, but DSO lucky imaging has caught my eye. It looks like meaningful images can be taken with 500ms exposures at F10, which I think my AZ-GTi can handle. The thought of imaging planetaries, galaxies and globs at 1500m focal length (or 945mm with a reducer) with my humble equipment has me excited. The ASI 178 MM is nice and sensitive and it it has little little pixels that should love the sharpness of lucky imaging. Something to keep me busy while we wait for planet season?

Time will tell if I can coax my setup to give results, but actual how-to info on DSO lucky imaging is hard to come by. Do I shoot video and stack like a planetary image in AS! or do I shoot individual frames and do a DSO pipeline in DSS? Does anyone know, or have any handy tips and tricks?

EDIT: Just adding one of my efforts so the picture displayed for the thread matches the subject.

Edited May 24, 2020 by Ags

[PeterCPC]

When I have tried "lucky imaging" I have stacked individual frames (lots).

Peter

[Owmuchonomy]

Not entirely sure but I do know AS! has a de-rotation function which will remove field rotation of your Az-Gti mount.

[Ags]

Surely a planetary pipeline would work on a planetary nebula? 😀

[vlaiv]

Point of lucky imaging is that you discard some of your subs, and only keep some of lucky subs that are sharp enough.

Otherwise you can consider lucky DSO imaging to be just a regular form of DSO imaging. Therefore you should treat it like regular dso images - calibrate them properly and so on. Issue is that you will have a lots of data and another issue is that you don't limit your self with sampling rate that is adequate for long exposure, but you rather aim for critical sampling rate of your telescope.

In case of C6 and ASI178 - you just use native pixel size, that is close critical sampling rate (f/ratio for critical sampling rate with 2.4um pixels is about F/11 - F/10 of C6 is close enough).

If you use DSS (which I would discourage in this case) - simply stack subs that have star FWHM below certain threshold. Otherwise use AS2/3! and do "scientific" stack to produce 32bit image. In the end, continue with planetary type workflow and apply wavelet sharpening in your processing. This will give you best sharpness.

Don't expect that you will capture much, but what you capture - should be sharp / much sharper than you can get with regular long exposure imaging (if you do lucky imaging properly). In order for this type of imaging to work good and provide "deep" images with good SNR, you really need to have very large telescope that gathers a lot of light - like 16" or above. 6" is just going to give you a "taste" of this.

 

[Ags]

Yes, I have seen amazing results with large dobs. But also some impressive stuff with a C8, so worth a go, especially for the brightest bright targets.

[vlaiv]

Depending on your budget - there might be something else that is very interesting / tempting to try out with C6.

EEVA with C6 and ASI178 and this:

https://www.teleskop-express.de/shop/product_info.php/info/p11425_Starizona-Night-Owl-2--0-4x-Focal-Reducer---Corrector-for-SC-Telescopes.html

This will make your C6 F/4 scope. It works with only smaller sensors, but ASI178 is rather small so should not have any problems.

C6 with that reducer will give you 600mm of focal length and with ASI178, if you bin x2 you will get 1.65"/px. That is very fine resolution to be working at for EEVA.

[michael.h.f.wilkinson]

I have used the ASI178MM for DSO imaging with decent results, using my APM 80mm F/6 on an EQ3-2 mount, and likewise with the bigger ASI183MC. Using even a chea EQ mount you do not have to limit yourself to sub second exposure times, and that makes life a lot easier

ASI178MM result (3890s total integration, only L band, 60s subs, as I recall)

ASI178MM result, very short integration, LHOO filter combination (a bit weird, but nice to catch in less than an hour)

ASI183MC result, 162 60s subs

[vlaiv]

34 minutes ago, michael.h.f.wilkinson said:

I have used the ASI178MM for DSO imaging with decent results, using my APM 80mm F/6 on an EQ3-2 mount, and likewise with the bigger ASI183MC. Using even a chea EQ mount you do not have to limit yourself to sub second exposure times, and that makes life a lot easier

That is just regular DSO imaging with shorter exposures. You won't achieve any additional sharpness over regular DSO imaging, except for some issues with mount. Seeing related blur will be the same.

Lucky imaging differs in its goal - to remove as much as possible seeing induced blur.

[Ags]

Well, Ias a first step I am going to try to image a glob exactly like I image a planet. We'll see!

[michael.h.f.wilkinson]

2 hours ago, vlaiv said:

That is just regular DSO imaging with shorter exposures. You won't achieve any additional sharpness over regular DSO imaging, except for some issues with mount. Seeing related blur will be the same.

Lucky imaging differs in its goal - to remove as much as possible seeing induced blur.

I know, I just wanted to illustrate how a planetary camera can do DSO work

 

[Ags]

Very nice M51!

[michael.h.f.wilkinson]

Just now, Ags said:

Well, Ias a first step I am going to try to image a glob exactly like I image a planet. We'll see!

Globs work well, they have good surface brightness 

[Ags]

Just a thought - If I am shooting exposures between 0.5 and 5 seconds in length, can I guide using the images taken? Obviously the guiding wouldn't help individual subs, but would prevent incremental drift and would enable dithering?

[Ags]

Here is my first attempt - 50 2.7 second exposures (150 discarded). At 2.7 seconds I am throwing away bad tracking and wind gusts not bad seeing, but I never thought I would be able to image a galaxy at 1500mm focal length with an AZ GTi. Obviously I need at least 10 times as many exposures. Also need to figure out how to apply darks in AS3!. It takes a bit of patience as you have to keep the galaxy on the chip manually.

I have had more impressive looking results doing live stacking with an ST80, but this is at 4 times my usual focal length and at F10 instead of F5 - I am keen to have another go but shooting a proper series of subs - at least 1000.

Edited March 18, 2020 by Ags

[Ags]

Just documenting my progress. Here is M3 - 21 x 2 minutes SER sequences of 333ms frames at 360 gain. I kept the best 75% in Autostakkert, then stacked the 21 resulting TIFFs in Deep Sky Stacker (after cropping all the TIFFs to the same dimensions). I also shot 6 minutes of darks and lights at the same gain and 333 millis. I shot in 2 minute sequences because Autostakkert doesn't handle field rotation. After a night of cropping in Gimp, I may pay for Registar...

I think my focus could be a little better, maybe. If we get more clear weather these coming weeks I will try to add color.

Edited March 26, 2020 by Ags

[Ags]

I think this one is better, just a 2 minute sequence of 200ms frames, keeping 75%. I still have to stack it with the other 20 I captured. I tried to shoot color tonight to add to this but couldn't get enough signal - Autostakkert doesn't seem to be able to sharpen stuff if it is too faint. I tried 4x4 binning and 800ms frames, but it wasn't enough for the blue which came out as a mush. 

[Ags]

Had another go at M82. 15 3-minute sequences of 500 ms frames, 75% selected and stacked in Autostakkert. I did a bit of stretching and sharpening in Gimp.

[Ags]

@vlaiv any idea why Autostakkert is over-correcting for my flats? I shoot the flats with the same settings as the lights, but the flats are 50 times brighter (based on histogram peak).

[vlaiv]

1 minute ago, Ags said:

@vlaiv any idea why Autostakkert is over-correcting for my flats? I shoot the flats with the same settings as the lights, but the flats are 50 times brighter (based on histogram peak).

Do you use flat darks?

I think that it would be a good idea to do calibration yourself. That way you have more control over the process. Here is the workflow that I would recommend.

Shoot lights as .ser sequence. Of course shoot as raw 16bit data. Shoot darks, flats and flat darks the same way.

Use PIPP to convert .ser sequence into bunch of .fits files. Make sure no other processing is done by PIPP - we just want fits files at this stage. In fact, you can skip this step if you use SharpCap and tell it to record sequence as .fits files in the first place.

Drawback to this method is that you are going to have bunch of files instead of one. Upside is that you will control calibration process and work in 32bit mode.

Then use ImageJ to create master calibration files and to calibrate your lights. If you don't know how to do it - I'll walk you thru it step by step (although I already did a topic on that and small tutorial - I think it can be found on SGL somewhere).

In the end you load bunch of .fits in AS!3 and tell it to create 32bit result (last time I checked it was a feature for "scientific use" - and worked only on mono subs - so you are fine there).

Then process in Gimp...

 

[Ags]

Yes I use flat darks - these are same same as my ordinary darks because the darks, flats and lights are all shot with the same settings.

[vlaiv]

26 minutes ago, Ags said:

Yes I use flat darks - these are same same as my ordinary darks because the darks, flats and lights are all shot with the same settings.

Exposure length?

Darks need to match lights in exposure length, and flat darks need to match flats in exposure length.

Flats must not clip.

If you want and if it is not too much trouble for you - maybe post single light, dark, flat, flat dark for inspection and calibration? Post them in fits format if possible (or even whole .ser files if they are not too large, or maybe use file transfer service of some kind).

I'd be happy to take look and see if I can find any issues with files for you.

[celestron8g8]

On 25/03/2020 at 17:15, Ags said:

Just documenting my progress. Here is M3 - 21 x 2 minutes SER sequences of 333ms frames at 360 gain. I kept the best 75% in Autostakkert, then stacked the 21 resulting TIFFs in Deep Sky Stacker (after cropping all the TIFFs to the same dimensions). I also shot 6 minutes of darks and lights at the same gain and 333 millis. I shot in 2 minute sequences because Autostakkert doesn't handle field rotation. After a night of cropping in Gimp, I may pay for Registar...

I think my focus could be a little better, maybe. If we get more clear weather these coming weeks I will try to add color.

I know your second image was better but this one surprised me cause my first scope of decency was a Bushnell 5-1/2” newt and M3 was the first DSO i ever saw and it was just slightly dimmer than this but i was so excited that made my decision to get my Celestron C8 . After that it took awhile to find it again but i bought Nightskies by Terence Dickinson and that’s how i started learning the night skies ! That was in 97’ . I still have both those scopes but i retired from astro work last year . Need to sell them but just can’t get the energy though to do that yet  . 

[Ags]

Thanks @celestron8g8, I am glad my humble image brought back good memories!

I am certainly enjoying my new C6. Can't wait to see Jupiter through it!

[Ags]

1 hour ago, vlaiv said:

Darks need to match lights in exposure length, and flat darks need to match flats in exposure length.

Yes, darks, lights and flats are all shot with the same Gain and Exposure Length. I'll try and get some FITS files for you (they are currently buried in SER files).

Temperature should be the same. Flats are shot indoors immediately after the session, so I don't think the camera chip warms up in 7 minutes. Flats are shot by putting a sheet of paper over the front of the scope and pointing it at a room light. The histogram peaks at 50% with no clipping according to Sharpcap. In contrast my lights are so underexposed I could use them as darks. Could the big exposure difference between lights and flats be an issue in itself?

Edited April 2, 2020 by Ags