10 replies to this topic

[dswtan]

I am puzzled by seeing two main "sizes" of Newton's rings posted in photos here -- one type is about a dozen large shadow bands equally spaced (by eye) across an image. That is the type I get on my LS60T/ASI174MM. I need massive amounts of tilt to get rid of them -- the full range of my ZWO tilter. The other sort is hundreds of narrow bands, again seemingly regularly spaced. I am wondering if those need much less tilt than I have to use. These two types don't seem to vary between different image scales, though I have not done a detailed study. 

 

I am familiar with the physics (https://en.wikipedia.../Newton's_rings) showing how the bands of the rings change in size from the origin (center), but at the outer areas, I presume it approximates to the regular pattern (dozen or hundreds) I'm seeing in photos.

 

Questions:
1. Is that true, that what we are seeing here is *apparently* regular spacing, but that's just because the distance from the origin (wherever that is in this scenario?) is relatively great?

 

2. Are the two size types I'm referring to just different manifestations of the same thing and actually there's a continuum of sizes -- some of us get the big "dozen" fatter ones, others get the 100s, and still others get everything in between?

 

3. In night astrophotography, I'm aware of how you can calculate what the cause of an optical aberration is by its size -- for example, you can guess what might be causing a donut around a star such as a reflection from a filter or whether a dust mote is on the sensor glass or some other part of the system. Do the different scales of Newton's rings tell us anything about what the underlying cause might be in our setups, and whether there is a better adjustment we can make to address them?

 

Meanwhile, I will be working on flats, which I'm familiar with from night astrophotography, and trying to apply that to my daytime repertoire...

[futuneral]

Not sure if you intended to attach photos, but if so - they are not there.

 

I've also seen something similar to what (I think) you are describing. My two ideas were: a) they are different modes of the same interference. like they are due to the same "gap", but one is at the frequency, and the other one is at like 1/8th of it or something. b) the two are caused by different surfaces. maybe one by the gap between the elements in the lens and the other one is from the coating. Or any other similar combination.

 

What I concluded however is that proper flats remove both, so I stopped thinking about it

 

Tip: if you're doing larger than full disc imaging, point the scope at the center of the sun, and set it sufficiently out of focus. This works great as a flat. I'm not sure what I'd do if my image scale was smaller though..

[dswtan]

Thanks futuneral. No photos attached, but I'll post some examples from a search. Not my photos -- image credits referenced where known. 

 

Different modes makes sense. Flats really do seem to be the answer in my case, since I'm not happy with the degree of tilt I need (and the ZWO tilter adjustment is awkward too). More experiments to do...

 

Dozen:

Ref: https://www.cloudyni...ngs/?p=10233381

Hundreds:

Ref: https://www.cloudyni...gs/?p=10255567 
 

 

This one was interesting -- seems to show the two "modes" simultaneously:

 

Ref: https://www.cloudyni...ts/?p=10541157 

[rigel123]

Your first example definitely shows Newton Rings, the second looks like a fixed pattern of noise or an issue with either a cable or the camera, and the third shows both that same fixed pattern and Newton Rings.

[chemman]

So there could be another interference in these screen captures, Moiré pattern.  I was evaluating a new imager today and had to zoom in and out to convince myself what I was seeing.  After looking at the resultant I am now positive the patterns I was observing had more to do with interference created by pixels, camera vs. display.   I was using an Sony IMX253.

Edited by chemman, 29 November 2020 - 10:16 PM.

[dswtan]

First experiments with in-capture flats in Sharpcap today -- wow, clingfilm seemed to work quite well for my purposes. This seems to be the way to go for me... 

 

Thanks for the comments above, everyone!

 

 

[chemman]

First experiments with in-capture flats in Sharpcap today -- wow, clingfilm seemed to work quite well for my purposes. This seems to be the way to go for me... 

 

Thanks for the comments above, everyone!

 

What is this clingfilm you speak of?

[dswtan]

What is this clingfilm you speak of?

Ah, sorry, my UK origins showing - Saran/Glad/Plastic Wrap would be US terms.

https://en.m.wikiped...ki/Plastic_wrap

 

I will also try the proven breakfast cereal bag material when I finish my Frosties (“Frosted Flakes”!!).

 

I was surprised how opaque to Ha other translucent materials I had handy were, BTW.

[ASTROTRUCK]

cling wrap - did you use just a single layer or did you layer  it into layers & if so how many ?

 

also

 

there are 2 types of tilters

a cheapier & a pricier

 

http://www.icstars.c...eliminator.html

 

http://www.davidcort.../20130513sa.php

 

you have to replace the tilt screws to allow for more tilt

 

another project !!

 

my power mate has more rings than saturn

Edited by ASTROTRUCK, 30 November 2020 - 08:57 PM.

[dswtan]

Re: ASTROTRUCK

 

- I tried both 1 and 2 layers. 2 seemed noticeably more effective. I didn't have time to try more, but I suspect more would be even better as I could still see the admittedly-gigantic AR2786 through 2 layers, and framing to avoid it was tricky for various reasons at the time.

 

- Yes, I have the cheap ZWO tilter and I find those quite difficult to adjust (it probably goes with my generic problems with any 3-pivot adjustments in astro unfortunately, i.e. SCT collimation, etc.!). Thanks for reminding me about the Daystar one of a different design. I used to think it was pricey, but solar is so expensive anyway, maybe one day...

[ASTROTRUCK]

you should check out which of the 2 ring eliminators is the one that can be modified for more angle - just dont remember which one - the idea is to increase the tilt with new  longer  screws   just dont remember which of the 2 - if you have one check out the 2 sites i sent above then decide which is yours  to possibly modify - i do remember the modification does work BUT  you then need to seal the spacing of the tilt with electrical black tape top prevent light seepage.