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Cause of wavy lines using 3x and 5x Barlows

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#1 Don Marcotte

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Posted 18 May 2019 - 12:21 AM

 A few years back I tried a 5x Powermate with a Lunt LS60-THa scope. Last summer I tried a 3x Explore Scientific extender with an RC6 scope. I was using a CCD camera with the Lunt and a DSLR with the RC6. In both cases I saw wavy lines. A few months ago I saw a comment in a CN thread that the problem was caused by .... I can't remember.

 

I am considering buying a 2.5x PM or 2x ES for lunar imaging with a 100ED and RC6. I've had success in the past with a 2" 2X PM and my Lunt LS60Tha but worry that it might be a problem with the 100ED and RC6.

 

My goal is close-ups of the moon with optics that can be handled by my iOptron CEM25 mount. Close-up is a relative term when my main OTA has to be under 12lbs. 

 

Don



#2 Christian-UAE

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Posted 18 May 2019 - 12:47 AM

Those are Newton Rings.
You can use a tilt adapter to cure this.
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#3 james7ca

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Posted 18 May 2019 - 01:06 AM

Probably Newton rings for the solar (H-alpha) images/scopes. However, that shouldn't be a problem for lunar, planetary, white light solar, and DSO images (meaning that you should NOT see that problem when using barlows on your other scopes).



#4 CharlesW

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Posted 18 May 2019 - 10:29 AM

You might try this formula for choosing a barlow: (5 X pixel size of camera) / focal length of scope. 



#5 RedLionNJ

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Posted 18 May 2019 - 10:55 AM

Newton's Rings (NOT "wavy lines", but sharply-defined arcs) are only going to be an issue with the Lunt H-alpha scope. They are a by-product of reflections and are only an issue at discrete, narrow-band wavelengths where you have a ton of light.

 

These will not be an issue with the RC6 or any other situation where you do not use an extremely narrow-band (sub-Angstrom) pass filter with a whole lot of light to spare.

 

Tilting to remove them (even in solar imaging applications) is not a good idea at high resolutions, as the tilt detracts from the crispness of the image. You can, however, get away with this at full-disk or even partial-disk image scales.



#6 Don Marcotte

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Posted 18 May 2019 - 11:02 AM

Please forgive my ignorance, but aren't Newton's rings circular? The incident with the 3x ES and RC6 was a terrestrial target. The 5x PM and LS60THa was solar.

 

Thanks for the formula. For greater clarity, the resulting number would be the max magnification? For my ASI1600 and 100ED, 3.8x5/900 equals .021. So a 2x barlow?

 

I saw a reference to a tilt adapter. I assume that means there must be some sag in the light components of the light path. Is that correct? Due to the long back focus of the RC6, it's a possibility but the the extensions are very rigid and screw on very tightly.

 

Thanks guys for the information.



#7 CharlesW

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Posted 18 May 2019 - 03:25 PM

Arghhh, sorry, it isn’t focal length, it’s focal ratio. A typical RC6 is an f9, so the result would be 2.1, the barlow would be a 2X. You need 5 or 6 micron pixels before you even get to a 3X. 



#8 james7ca

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Posted 18 May 2019 - 05:49 PM

Interference effects like Newton rings don’t have to be perfectly circular. Have you ever seen the multicolored rings (sometimes distorted) that happen on oily surfaces that are mixed with water (like on asphalt after a rain)? The reason why they appear more or less circular or straight on solar telescopes is because the thin spacing that cause them are fairly uniform and plane parallel (but, you’ll probably see some variation in the symmetry).

 

Anyone who has ever done darkroom (photography) work has probably experienced the wavy lines that can appear in a print because of a close separation between the film and the condenser optics in the enlarger. This can also happen when making contact prints. In fact, these are also known as Newton rings.

 

That said, you shouldn’t experience Newton rings when using a properly made/functioning barlow except when observing with an extreme narrowband instrument. However, interference effects are possible whenever you have thin films or thin separations between two pieces of glass-like materials. Thus, a two element lens could develop a thin separation that would cause “wavy lines,” or you might just have a normally cemented doublet that is beginning to separate (or maybe water between the elements). That kind of problem (in “white” light) should be rare, so it’s not something to fear in terms of using or buying a barlow.



#9 Don Marcotte

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Posted 20 May 2019 - 09:33 PM

Since both my RC6 and 100ED are f/9, a 2x magnifier seems reasonable. I've owned and sold two Powermates, a 2x and 5x. I'm wishing I had held onto the 2" 2x. I only need the 1.25" version now. Televue sells a 1.25" 2.5x PM. Explore Scientific has a 1.25" 2x barlow. I could go with a TV 2x 1.25" Barlow. It was a 3x 1.25" ES that showed the Newton's Rings on a terrestrial target with a Sony A7R and RC6.

 

Don




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