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rorymultistorey

Is a 1/10th wave f6 mirror better than a faster synta f5 mirror for DSO photograhy?

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So I picked up a classic... a 1970's - 80's edmund optics f6 150mm newtonian...

edmund6_0.jpg.003e6bf28aef142952df30fbe2ab7b0c.jpg

... like this one but mine has a different mount. It was cheap. Very good condition. The focuser is pants. The mechanics of the secondary holder is (IMHO) brilliant and apparently the primary is 1/10th wave.

But its f6 and I trhink I'd rather swap it out for a faster synta f5 mirror. .  The thing is its a one shot job bc to make the f5 mirror work I will have to saw off a good few cm from the barrel of the scope.

So the question is: Is a high quality f6 mirror better than a faster synta f5 mirror for wide deep space astrophotograhy?

 

All comments gratefully received. 😉 

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3 hours ago, rorymultistorey said:

faster synta f5 mirror

Hi

Both mirrors would collect the same amount of light. To get faster optics I think you'd need a larger mirror,  so I don't think it would be worth it as you'd need a new tube/spider... Everything.

The main advantage of the f6 is that you -almost certainly-  wouldn't need a coma corrector, so no glass to introduce colour abberation:) The only  -not much of a- disadvantage is that you'd have a slightly narrower field of view when compared to the f5.

Just my €0.02 but HTH anyway.

 

Edited by alacant
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First, I'd be very, very skeptical of any old mirror that claims to be 1/10 wave.  Specious specmanship is the norm here ....1/10 wave on the mirror surface or in the focal plane? RMS or peak-to-peak? What wavelength was the measurement made at? ...It's a  complete minefield for the charlatan to exploit. If you really want 1/10 wave go to someone with the published means to test it,  or a very good personal reputation.  I've got a shed full of "1/10wave" mirrors, some from very respected names, which test out on a Zygo as more like 1/4 wave in reality. It's very hard to even test convincingly to that accuracy without very expensive test kit. 

Is it even relevant? I doubt it if your aim is prime focus photography with exposures in terms of long seconds or low minutes. Guiding errors, atmospheric distortion, poor mirror mounting, collimation errors, undersampling by the camera, will all add up making the use of a 1/10 wave mirror pretty much pointless. A GSO cheapie will do just fine.

A very good mirror might be relevant for planetery photography by lucky imaging but even here I have my doubts. 

This sort of high quality mirror does have its place for visual use.....once everything else in the optical chain is perfect. 

So what are my mirrors?....all 1/10 wave from OO, and the optical quality is indeed excellent on all of them. But Synta or GSO aren't far behind and on an average night it's hard to tell the difference. Total hypocrisy in a way after my previous comments, but just occasionally the difference is worth it visually. And you know that if the system is not working properly, it's not the mirror at fault. 

The faster speed of an f/5 mirror might well be an inprovement. But the collimation is more critical, not just the general scope collimation. The camera has to be exactly square to the focal plane which is harder to achieve.  The faster mirror has a wider convergence angle which makes the in-focus depth of focus smaller.

Is it really worth hacking the tube of an otherwise nice scope for the mirror swap?

Is the secondary large enough to illuminate your sensor without vignetting? This will get worse with a f/5 unless you change both mirrors together. 

I'd try it as is first. You might be quite happy with it as it is! The focuser will probably need a lot more attention than the mirror quality.

Just my 2p....from a guy who's trashed a few scopes "improving" them....

 

Edited by rl
additional info
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Many thx chaps... Most enlightening

BTW I used to go to a sailing club near Pershore when I was a kid.

So 1/10 wave is not necessarily going to help me for astrophotography. Squaring the camera is more important. I happen to have an orion optics OC1 (I think) 2 inch focuser which I'm going to fix onto the tube. I will be using a full frame canon 5d mark ii with a 0.9 x skywatcher coma corrector for imaging.

 

Having digested the above advice I think will use the brighter, newer, faster, synta f5 mirror AND controversially I will cut the end off the tube in order to reach focus. I mean how hard can it be 😉 (famous last words).

[Maybe I should see if someone is keen to buy it before getting the saw out. Does seem a shame to carve up such a well made scope.]

Also still curious to know whether high quality mirrors make any noticeable difference in  DSO astrophotograhy or whether the only real benefit is planetary.

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I've seen 6" f/5 OTAs change hands on ABS for £50....and they come with a 2" focuser. Worth the wait until one comes up? 

If you're going down this road I'd make sure the new mirror actually fits in the cell first, before cutting anything. Then, it might be possible just to drill a few extra holes in the tube 6" up and sit the mirror higher. Thus leaving you a way out withminimal damage if it turns out not to be a good idea.....

The sailing club is still there. But sadly the micro-brewery at the Brandy Cask pup is gone...

To your other point, the standard Airy criterion for "good enough" is 1/4 wavelength in the focal plane.  Here you see a double start as two slightly overlapping central discs with figure-of-eight diffraction rings around the outside. It's an arbitrary definition of resolving power but it's stood the test of time for a century or more. A better mirror tidies things up a bit..it's a bit clearer. 

From the AP point of view there is a couple of extra considerations going on:

 If the camera can't see all the details in the diffraction pattern because the whole pattern fits on a single pixel then there is not much point in worrying about the fine details of the  diffraction pattern ..the camera is the limitation on detail. This is the normal situation for prime focus AP where you might be struggling to keep exposures times down of fitting an object to the sensor size. . It's called undersampling. 

You can get round it by using smaller pixels..but then the other point comes in. The light is spread out over several pixels which means that the signal-to-noise ratio is degraded for each pixel. So the extra detail you hoped to see gets lost in increased noise....there is an optimum point for best detail which is what you aim for when doing planetery. 

Edited by rl
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Good suggestion above to avoid cutting the old scope up.

Do also consider the secondary size which may need to be increased to ensure good illumination due to the wider light cone of the faster scope.

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1 hour ago, rorymultistorey said:

The mechanics of the secondary holder is (IMHO) brilliant

Hi Rorymultistorey, got any pics? 

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2 hours ago, markse68 said:

Hi Rorymultistorey, got any pics? 

 

Pics of the secondary assembly ( i hope) 

Doing this from my phone so bear with...

Please note how thin the spider vanes are. They are  made from ribbon,  maybe rubber ribbon whoose tension can be tightened. 

How the the secondary only has an up down adjustment. 

And How the secondary can be rotated around a central shaft. 

Far better than the 3 screws which easily lead to miscollimation if your only collimating with a laser. 

IMAG3873.jpg

IMAG3874.jpg

IMAG3875.jpg

IMAG3876.jpg

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2 hours ago, Stu said:

Good suggestion above to avoid cutting the old scope up.

Do also consider the secondary size which may need to be increased to ensure good illumination due to the wider light cone of the faster scope.

Secondary is suprisingly big.  And the tube is made out of some kind of reinforced cardboard...  I think.  Looks eminently cuttable.  I think this lovely scope is getting the chop😈

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39 minutes ago, rorymultistorey said:

Pics of the secondary assembly ( i hope) 

Thanks very much Rory- what a quirky and interesting design! Looks great! I wonder what kind of ribbon it is- you'd think it'd stretch and loosen over time? Guess not- looks pretty taught. Does it hold the secondary solidly?

Agree on it being a much nicer mechanism than the simple 3 screws

Edited by markse68
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