Am I expecting too much from my DSLR

[jimjam11]

2 hours ago, ollypenrice said:

Worth looking into. Do you have a link?

Olly

Yes I saw it in this video:

It is not demonstrated but it is documented in the text. I seem to recall seeing a screenshot of the app where it is also visible.

There is a section within the app (shared with the sa mini). Hopefully this means you program your capture sequence and then upload it to the SA. However, I dont think the SA can trigger an astro camera which does limit its usefulness. 

Dithering is so important it should be default behaviour for all of these tracking style mounts.

Edited September 3, 2020 by jimjam11

[wimvb]

I’m not convinced that dithering with an RA only mount will do any good. Since the order in which you stack all subs isn’t important, you could lay them out as a sequence that shows a continuous drift. Unless you already have natural drift in declination. Ie, you need a polar misalignment in order for a RA dither to work.

@Stuf1978: very nice image, btw. Especially considering the conditions re moon and light pollution.

[Stuf1978]

1 hour ago, Hals said:

I foolishly tried to image the Wizard Nebula during the recent 94% full Moon, DSLR even with a Quad band filter the result lots and lots of noise, likely caused by over-processing too. Plus I was a tad out of focus...It's a learning curve for sure, but a fun one.

Fun and frustrating in equal measures 😂. Nearing full moon is a killer, plus I always struggle with targets with huge starfields ☹

5 minutes ago, wimvb said:

I’m not convinced that dithering with an RA only mount will do any good. Since the order in which you stack all subs isn’t important, you could lay them out as a sequence that shows a continuous drift. Unless you already have natural drift in declination. Ie, you need a polar misalignment in order for a RA dither to work.

@Stuf1978: very nice image, btw. Especially considering the conditions re moon and light pollution.

Thank you. Much happier with the second processing 😀

Ordered a EQ DIR cable from FLO today so I can get everything hooked up and get the HEQ5 dithering which will hopefully help with noise 🤞

[Pryce]

In theory RA dither should work if it only dithers in one direction and not back and forth.  Stacking software should in theory remove those hot pixels that would create thos stacking artifacts.

In theory that makes sense.  At least in my head

[wimvb]

1 hour ago, Pryce said:

In theory RA dither should work if it only dithers in one direction and not back and forth.  Stacking software should in theory remove those hot pixels that would create thos stacking artifacts.

In theory that makes sense.  At least in my head

In theory. But a drift between subs can cause so called walking noise, streaks in the drift direction. And RA dithering can be equivalent to such a drift. That’s why you need to be cautious with RA only dithering. It is possible to eliminate walking noise by careful calibration and stacking, but it requires expertise in stacking. Two dimensional dithering is more robust.

Edited September 3, 2020 by wimvb

[alacant]

I think the main requirement for dither to be effective is to move the camera in a random direction by a random amount BETWEEN frames, not constantly during them and drift or no drift, not always along the same axis. 

Cheers and clear skies.

[wimvb]

9 hours ago, alacant said:

effective is to move the camera in a random direction by a random amount BETWEEN frames,

Random in one direction can become the same as drift by reshuffling the frames. That’s why, as I believe you imply, two directions are needed.

Edited September 4, 2020 by wimvb

[ollypenrice]

10 hours ago, jimjam11 said:

 

Dithering is so important it should be default behaviour for all of these tracking style mounts.

Thanks for the link. Dithering isn't provided by the mount but by the guiding software which must, of course, communicate with the mount and the capture software in order to move between captures. I think most modern capture software programs will communicate with PHD and dither.  However, not everyone wants to dither: it's a complicated business to dither with a dual rig, for instance, and impossible if using an off axis guider on such a rig.

Olly

[wimvb]

1 hour ago, ollypenrice said:

Thanks for the link. Dithering isn't provided by the mount but by the guiding software

The star adventurer mini has an app which allows dithering without guiding. My guess is that the mount speeds up or slows down a random period. There is a setting in the app called ”dithering range”. The range is measured in arc minutes.

[jimjam11]

6 hours ago, wimvb said:

The star adventurer mini has an app which allows dithering without guiding. My guess is that the mount speeds up or slows down a random period. There is a setting in the app called ”dithering range”. The range is measured in arc minutes.

16 hours ago, wimvb said:

Random in one direction can become the same as drift by reshuffling the frames. That’s why, as I believe you imply, two directions are needed.

I dont have a huge amount of experience guiding/dithering with my SA, but if I take a sequence of subs with my SA and dont guide/dither I always get terrible walking noise. When guiding/dithering I have never noticed it but I always specify a large dither.

Using something like the SA it is impossible to get perfect polar alignment, so there will always be drift in dec:

If I randomly dither this in RA only I get:

At this point there is no correlation because of the drift, size of dither and the number of subs?

To make this work I think the dither amount needs to be large enough to eliminate/minimise the overlapping, this would need to be substantially larger than a random RA/Dec dither?

The longest FL I image with on my SA is 200mm which gets me 4"/px with an ASI1600. If I specified a 5' dither quantity I believe that would equate to 75 pixels which means it should be good for plenty of subs before the correlation becomes a problem? Without RA dithering walking noise would become apparent after a handful of subs.

At 50mm I think you would need to specify a dither amount of 20' to get a similar result.

[wimvb]

If you happen to aim just right, and get perfect polar alignment, then dithering in ra only wouldn't work. Reshuffling the subs can produce an apparent drift in ra. But if there is dec drift due to polar misalignment, ra dithering will break up the dec drift pattern, as your sample images show, and walking noise should be reduced. So there is a trade off between acceptable drift (elongated stars in worst case) and acceptable walking noise. My guess is that in most cases, ra dithering will improve image quality. But aiming for perfect polar alignment may be counterproductive. For dslr (and osc cmos cameras), a dither of at least 12-15 pixels is preferred, according to Tony Hallas. A larger dither will mean that you lose a larger area of your subs in stacking edges that need to be cropped. If you have a large sensor and/or small pixels, that may be acceptable.

[smr]

On 03/09/2020 at 10:32, Stuf1978 said:

Thanks for the link. I'll also have a look in your thread 👍

What isn't a headache in the world of astrophotography 😂

I don't know if my video will help but here it is ... how to control your mount with EQMOD and Stellarium... https://www.youtube.com/watch?v=mYTD7RuF3x8

 

[Stuf1978]

On 04/09/2020 at 20:41, smr said:

I don't know if my video will help but here it is ... how to control your mount with EQMOD and Stellarium... https://www.youtube.com/watch?v=mYTD7RuF3x8

 

Thanks I'll check it out 👍

Managed another session last night and reduced the ISO to 800 and the noise was more manageable 👍

 

[Pryce]

5 hours ago, Stuf1978 said:

Thanks I'll check it out 👍

Managed another session last night and reduced the ISO to 800 and the noise was more manageable 👍

 

Dude.  This is amazing!  Such an improvement!  Tiiiny tiny hints of curvature but hands down, this is great! 

I'm suuuper excited to see this!

 

Edit: Just looked at it again and even though I've had a few beers this evening I'm completely flabbergasted! Amazing result! The lower ISO did wonders on the noise!

 

Edited September 6, 2020 by Pryce

[wimvb]

Very nice result indeed. The stars look like tiny doughnuts, which suggests that focus can be improved. Perhaps focus shift?

[Stuf1978]

4 hours ago, Pryce said:

Dude.  This is amazing!  Such an improvement!  Tiiiny tiny hints of curvature but hands down, this is great! 

I'm suuuper excited to see this!

 

Edit: Just looked at it again and even though I've had a few beers this evening I'm completely flabbergasted! Amazing result! The lower ISO did wonders on the noise!

 

Thanks, there is still a small hint of banding on the lower section of the image but I reprocessed this numerous times with varying amounts of curve stretching and it all resulted in the banding 🤷‍♂️

1 hour ago, wimvb said:

Very nice result indeed. The stars look like tiny doughnuts, which suggests that focus can be improved. Perhaps focus shift?

Thanks, yeah I did notice that and tried to process it out. The odd thing is they didn't look like that in the stack straight out of DSS. I did mess about with some settings within DSS and used a different stacking algorithm with the addition of the hot pixel removal so I'm not sure if that has done something funky with the stars. 

[wimvb]

5 minutes ago, Stuf1978 said:

 

Thanks, yeah I did notice that and tried to process it out. The odd thing is they didn't look like that in the stack straight out of DSS. I did mess about with some settings within DSS and used a different stacking algorithm with the addition of the hot pixel removal so I'm not sure if that has done something funky with the stars. 

Just check a single sub. A focus issue will show up there.

[Stuf1978]

40 minutes ago, wimvb said:

Just check a single sub. A focus issue will show up there.

Just checked and yeah it's there but only very very slightly, had to look at 200% to confirm. So looks like focus was out a little 👍

[carastro]

As regards banding, firstly I can't see any in your image, but DSLRs are known for this I used to own a 450D and had a lot of problems with it. 

Can I ask if you are downloading to the laptop or SD card?

One thing I found that helped with the banding was to remove the SD card when capturing direct to the laptop, there was some theory about the card being in the camera causing some electrical interference when downloading to the laptop.  I found this made a good improvement.  

Lastly, the amount of noise with a DSLR is always going to be a problem as it is not cooled.  

Carole 

[Stuf1978]

18 minutes ago, carastro said:

As regards banding, firstly I can't see any in your image, but DSLRs are known for this I used to own a 450D and had a lot of problems with it. 

Can I ask if you are downloading to the laptop or SD card?

One thing I found that helped with the banding was to remove the SD card when capturing direct to the laptop, there was some theory about the card being in the camera causing some electrical interference when downloading to the laptop.  I found this made a good improvement.  

Lastly, the amount of noise with a DSLR is always going to be a problem as it is not cooled.  

Carole 

Thank you. I'm capturing images direct to the SD card, however I'm looking into getting everything hooked up to the laptop and capturing data in APT so that may help

Yeah, the DSLR is always going to be a compromise but it's all I have to work with at the minute. I will be going to down the cooled dedicated astro camera route at some point though so it's just something I'll have to live with and try to minimise for the time being. 

[smr]

Banding is a problem in Canons. In my Canon 80D I've seen it sometimes, but in exposures which are 5 minutes or longer, anything less and I don't really notice it, and even in some 5 minute exposures it's not apparent - probably an air temp thing.

I noticed banding heavily in my newly acquired Canon 600D though, so I'm going to have to dither in APT more than the default dithering values provide.

There's also a debanding script in Noel Carboni's action tool set which also does a really good job of removing banding, it's impressive and doesn't seem to lose any IQ fidelity when examined 1:1.

[Rustang]

18 hours ago, Stuf1978 said:

Thanks I'll check it out 👍

Managed another session last night and reduced the ISO to 800 and the noise was more manageable 👍

 

Stunning! What's the target? 

[Stuf1978]

1 hour ago, smr said:

Banding is a problem in Canons. In my Canon 80D I've seen it sometimes, but in exposures which are 5 minutes or longer, anything less and I don't really notice it, and even in some 5 minute exposures it's not apparent - probably an air temp thing.

I noticed banding heavily in my newly acquired Canon 600D though, so I'm going to have to dither in APT more than the default dithering values provide.

There's also a debanding script in Noel Carboni's action tool set which also does a really good job of removing banding, it's impressive and doesn't seem to lose any IQ fidelity when examined 1:1.

I have a Canon 80D as well but haven't really used it for any astro, I do have plans to use it for more broadband targets but haven't had the chance yet. 

Cool, I have the Astronomy Tools Action set so I'll have a look for it as I can't say that I've noticed it

1 hour ago, Rustang said:

Stunning! What's the target? 

Thanks, it's the North America Nebula in Cygnus (NGC7000)

 

Edited September 6, 2020 by Stuf1978

[Rustang]

18 minutes ago, Stuf1978 said:

I have a Canon 80D as well but haven't really used it for any astro, I do have plans to use it for more broadband targets but haven't had the chance yet. 

Cool, I have the Astronomy Tools Action set so I'll have a look for it as I can't say that I've noticed it

Thanks, it's the North America Nebula in Cygnus (NGC7000)

 

I didn't recognise the crop, I like it tight on that area how you have it, I would be more than happy to capture something like that. What are the details if its ok to ask, exposure times etc!? 

[Stuf1978]

1 hour ago, Rustang said:

I didn't recognise the crop, I like it tight on that area how you have it, I would be more than happy to capture something like that. What are the details if its ok to ask, exposure times etc!? 

Yeah it's nice to get it tightly cropped in as there are some interesting shapes in the dust lanes and brighter areas of nebulosity. No problem the details are:

39 x 6minute subs with darks, bias and flat frames
Skwatcher Evostar ED72
Canon 450D self modded 
Sywatcher HEQ5 Pro
Skywatcher 50mm guidescope
ZWO ASI 120mm guide camera 
PHD2 guiding 
Optolong L-eNhance filter 

OVL field flattener 

Polar aligned in sharcap

Stacked in DSS and processed in photoshop. I'm in a bortle 8 zone and there was a ~97% moon. 

 

Edited September 6, 2020 by Stuf1978