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25585

Are larger secondary mirrors better for DSOs?

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Having a 59 inch focal length main mirror on my F5 Dobsonian 12 inch, would I see DSOs better with a 75mm rather than 63mm minor axis (& proportional major) secondary mirror?

I presume less good for stars & planets due to the larger central obstruction, but would the chances of some more photons reaching my eye (& easier collimation) be more likely with the larger elliptical?

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Going by the calculator & what manufacturers sizes are 63mm is OK. The latter I am always sceptical about, with regard to what will do. 

But real world use & handling away from arithmetic and millimeter makes me think the 75mm minor might be better, even if a bit over what I need. 

I found interesting the brightness fall away comment regarding F5 against F8. It's made me rethink length etc. 

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You would be better off not messing around with the secondary.

If DSO and more photons to your eye are your priority. Then there is one simple answer.

You need more aperture in your scope. Therefore a larger primary should be your concern. When it comes to DSO then aperture rules?

 

 

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Not so as you'd notice I should guess. Darker skies would have more dramatic impact than anything else.

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2 minutes ago, Moonshane said:

Not so as you'd notice I should guess. Darker skies would have more dramatic impact than anything else.

+1

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3 hours ago, 25585 said:

Going by the calculator & what manufacturers sizes are 63mm is OK. The latter I am always sceptical about, with regard to what will do. 

But real world use & handling away from arithmetic and millimeter makes me think the 75mm minor might be better, even if a bit over what I need. 

I found interesting the brightness fall away comment regarding F5 against F8. It's made me rethink length etc. 

My 200mm newt, obstructed 32% showed DSO very well last night, including faint galaxies. Mels calculator is very accurate and as Shane says dark skies mean more than anything.

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2 hours ago, Timebandit said:

 

You would be better off not messing around with the secondary.

If DSO and more photons to your eye are your priority. Then there is one simple answer.

You need more aperture in your scope. Therefore a larger primary should be your concern. When it comes to DSO then aperture rules?

 

 

12 is largest I can handle single-handedly. Need a new secondary, holder for it & stalk as the original has vanished. Thus I was considering a replacement size, not knowing it's original one's measurements. The scope would be for DSO spotting. 

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Astrosystems makes the best holder, hands down IMHO.You need to figure in offset at f5 if you are downsizing- for sure. If upsizing there is more flexibility- my 15" was not offset, by choice and we went to the next size up, but I'm still only 22% obstructed. Buy a top tier sec IMHO... Antares?

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Sorry. TLDR answer is no, the opposite is true. The greater CO will degrade precious contrast on DSOs, and your illumination field maxima would only apply to low power wide field EP field stops. Best compromise would be a middle choice size of 20-25% for all things including collimation, and full offset will help for illuminating your big EPs.

I use LocTite 2-sided tape--the 110 pound variety. Used it originally (lesser strength) and just remounted mine (5 years on). It sticks better with time as I could barely get it off. Also provides the spacing you need, but we're not clamping, just adhering to lightweight stalk. Which 2dary did you get? We're 12.5" and were encouraged to stick to 1.25" focuser w/ 2.14" 2dary (smallest), but we easily balked at both and are still only at 23% obstruction with 2.6". Used Mel Bartels calculator with the Stellafane  raytrace for 36mm72 EP and get excellent 75% illumination, even after going through a star diagonal. Re: real world use/handling, we don't often get to use our largest EPs so our 75% illuminated field shrinks in practice, and we're a more forgiving f/6 for collimation. Besides Bartels and Lockwood we also consulted Royce

http://www.rfroyce.com/diagsize.htm

Here's the tape

https://www.amazon.com/dp/B003U2B5EA/ref=asc_df_B003U2B5EA5364191/?tag=hyprod-20&creative=394997&creativeASIN=B003U2B5EA&linkCode=df0&hvadid=198062181806&hvpos=1o1&hvnetw=g&hvrand=4297119983086375083&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003932&hvtargid=pla-391176935453

Re: your Q about faint extended objects (DSOs) there's this

http://www.telescope-optics.net/telescope_central_obstruction.htm

In brief, you could go 30% CO w/ perfect optics and still retain a Strehl equivalent of (only) .80 wh/ is why most people try to stay around 20-25% (except for planetary). There are averages at play (esp'ly seeing) that will make real world vary from these formulations, but we wanna be ready for those great nights and dark skies, and w/ 12" you've got a DSO hunter so I think I'd err on the smaller side if possible and learn to love collimating. Moreover, your illuminated field will be smaller than you think b/c you won't be asking this using your lowest/widest EP, etc. You wanna be able to use your 4.7mm on a planetary nebula on a good night.

Decide whether you want convenience and want to fully illuminate (100% and 75%) your big EPs, or whether you wanna maximize for high power on faint fuzzies. Jetstream went the next size up (as we did) and was still only 22%, so if you have 3 sizes to choose from, no way I'd go biggest. I'd be middling, and a good compromise for f/5. And as he also said, go full (classical) offset to get all or most of the illuminated field for your big EPs.

Cheers

 

Edited by laowhoo
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10 hours ago, Stu said:

It is not necessarily a straightforward question.

:thumbsup: that is an understatement & a 1/2 :)

Ideally one would put the secondary outside the incoming light path, this however would require a tilted primary, resulting in horrid off axis distortions,,,,  unless :

one were to use an off-axis segment of a paraboloid primary.

Otherwise schiefspieglers rule (and Herchelian)  of large Fratio

ok, I know, hat&coat >>>

 

 

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Mels graphic calculator (his whole site) was down earlier but is up again. Try this

http://www.bbastrodesigns.com/diagonal.htm

I plugged your numbers in for a 63mm 2dary, 9 inches from it to focal plane of an EP w/ 36mm field stop, and your illumination numbers are better than ours (should be for our added light path--we barely squeaked in). Even if you're 10" from the 2dary your numbers are great. No way you should go bigger. (Hyperion 36mm 72 FS is listed as 32mm.)

Edited by laowhoo
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