Manually dithering

[Mark1489]

I'm planning on starting to manually dither my frames. I'm using a star adventurer unguided, would I be right in saying that I could manually dither by pressing the arrow buttons to slightly slew the mount every few frames and slightly tweak the adjuster on the L bracket in the same way? I'm going to start using Backyard Nikon, can I use this to ensure my target stays in the field of view so I don't dither too far? ?

[alan potts]

A guiding program dithers by a very small margins in either a random way or circular but by very small amounts 1 pixel or believe 2 at most. The idea of guiding is to keep stars round I can't see how anyone no matter how good could do it with a mount comtrol, by all means have a go but I think your going to be disappointed. Backyard Nikon and Eos I think have dither programs.

Alan

[tooth_dr]

4 minutes ago, alan potts said:

A guiding program dithers by a very small margins in either a random way or circular but by very small amounts 1 pixel or believe 2 at most. The idea of guiding is to keep stars round I can't see how anyone no matter how good could do it with a mount comtrol, by all means have a go but I think your going to be disappointed. Backyard Nikon and Eos I think have dither programs.

Alan

I don’t think he means guiding manually but dithering manually. You are mixing two different concepts - guiding and dithering. For DSLR imaging the recommended dither is 15-20 pixels.

 

@Mark1489 I’m sure what you are suggesting could work with the SA, but dithering every frame is probably best.   

Edited April 24, 2019 by tooth_dr

[Mark1489]

2 minutes ago, tooth_dr said:

I don’t think he means guiding manually but dithering manually. You are mixing two different concepts - guiding and dithering. For DSLR imaging the recommended dither is 15-20 pixels.

 

@Mark1489 I’m sure what you are suggesting could work with the SA, but dithering every frame is probably best.   

Yes correct, unguided, manual dither between frames. Great I shall give it a go! I believe the SA doesn't have ascom and the only way to automatically dither would be via guiding which I want to refrain from if possible as I'm only using a 135mm lens

[alan potts]

10 minutes ago, tooth_dr said:

I don’t think he means guiding manually but dithering manually. You are mixing two different concepts - guiding and dithering. For DSLR imaging the recommended dither is 15-20 pixels.

 

@Mark1489 I’m sure what you are suggesting could work with the SA, but dithering every frame is probably best.   

I stand corrected sorry i just didn't feel it would be possible using a mount control, but does Backyard not have a program?

[ollypenrice]

There are two reasons for dithering (drizzle stacking and noise reduction) and I think you are talking about noise reduction. In that case you can certainly dither manually between subs. However, do you need to? How good is your polar alignment? If your PA is no more than moderately good then it may be that sufficient drift between subs is occurring naturally, especially unguided. Take one sub and stack it onto the next one based on star alignment, then look at the edges of the result image. Are the image edges pixel perfect in alignment? I'd be surprised but who knows? If you have a pixel or more's misalignment between subs I'm not sure what dithering manually will add.

For all that, it won't do any harm other than cost you a little bit of field of view at the end.

Olly

[david_taurus83]

Might not apply to the SA but technically you don't need a guiding system to dither. APT offers the ability to dither the mount without PHD for example. I was using it myself last night! Works really well!

[Mark1489]

16 hours ago, ollypenrice said:

There are two reasons for dithering (drizzle stacking and noise reduction) and I think you are talking about noise reduction. In that case you can certainly dither manually between subs. However, do you need to? How good is your polar alignment? If your PA is no more than moderately good then it may be that sufficient drift between subs is occurring naturally, especially unguided. Take one sub and stack it onto the next one based on star alignment, then look at the edges of the result image. Are the image edges pixel perfect in alignment? I'd be surprised but who knows? If you have a pixel or more's misalignment between subs I'm not sure what dithering manually will add.

For all that, it won't do any harm other than cost you a little bit of field of view at the end.

Olly

Hi Olly

Noise reduction mostly, I'm getting a little bit of colour mottle in processing - nothing major at all but thought if I'd be able to increase the quality slightly I wouldn't mind losing some field of view since I'm using a 135mm lens. I'm using a polemaster for PA, I've cycled through subs and noticed the stars drift back and forth along the RA between images, periodic error I'm guessing? Probably not much more than a star width or two though. While I was searching the forum I found posts stating around a 15 pixel dither while using a dslr for the best effect? I suppose it's one of those trial and error things and finding what works best for you and your equipment! Shame I couldn't do it automatically with the Star Adventurer but while I'm at dark skies I'll be with the mount anyway!

Mark

[Knight of Clear Skies]

16 hours ago, ollypenrice said:

There are two reasons for dithering (drizzle stacking and noise reduction) and I think you are talking about noise reduction. In that case you can certainly dither manually between subs. However, do you need to? How good is your polar alignment? If your PA is no more than moderately good then it may be that sufficient drift between subs is occurring naturally, especially unguided. Take one sub and stack it onto the next one based on star alignment, then look at the edges of the result image. Are the image edges pixel perfect in alignment? I'd be surprised but who knows? If you have a pixel or more's misalignment between subs I'm not sure what dithering manually will add.

For all that, it won't do any harm other than cost you a little bit of field of view at the end.

Olly

This is what I've found, I generally get 0.5-1 pixels drift when imaging unguided. It probably helps that I use a ball-head mount, so the camera is not aligned along RA and DEC lines. The only time I've had problems with banding is when I've used very short focal length lenses (35mm or less).

[ollypenrice]

32 minutes ago, Mark1489 said:

Hi Olly

Noise reduction mostly, I'm getting a little bit of colour mottle in processing - nothing major at all but thought if I'd be able to increase the quality slightly I wouldn't mind losing some field of view since I'm using a 135mm lens. I'm using a polemaster for PA, I've cycled through subs and noticed the stars drift back and forth along the RA between images, periodic error I'm guessing? Probably not much more than a star width or two though. While I was searching the forum I found posts stating around a 15 pixel dither while using a dslr for the best effect? I suppose it's one of those trial and error things and finding what works best for you and your equipment! Shame I couldn't do it automatically with the Star Adventurer but while I'm at dark skies I'll be with the mount anyway!

Mark

I don't image with a DSLR so I can't be sure about the dither needed to combat colour mottle but I seem to think Tony Hallas suggests at least 12 pixels, so that would be in broad agreement with your research. 

Olly