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f/8 reflector question

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#1 Rigel_10

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Posted 20 February 2016 - 07:50 PM

Hello....I just ordered the Orion XT6 dob, I like that it's a f/8. I enjoy the views in my 70mm refractor (f/7) because it shows the stars as mostly round across the whole FOV. Can I expect similar results with a reflector with a longer focal length (f/8) compared to the usual f/5, f/4.5, etc?  

 

I'm planning on using it for mostly the moon, planets and such, so I don't need the wide FOV, Wanting less coma that comes with faster systems. Will the f/8 - 6 inch reflector give me a more pleasing view with less abbreviations found in the faster focal ratios?

 

Also...I'm wanting to flock the inside of the XT, does Astronomics sell flocking paper? if not, where?

 

Thanks for any feedback.



#2 Aleko

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Posted 20 February 2016 - 07:59 PM

My first scope was a 6-inch f8. I now own 8 and 10 inch reflectors of the same focal ratio, (along with too many others of shorter ratios).  In short, you are in for a treat with your f8.  Pinpoint star images, and even easy when it comes to collimation. 


Edited by Aleko, 20 February 2016 - 08:00 PM.

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#3 Ed D

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Posted 20 February 2016 - 08:35 PM

I bought an XT6 years ago and made another 6" f/8 a couple of years back.  Like Alex said, you're in for a treat.  The 6" f/8 is very manageable and light weight, cools quick, and has enough aperture to show you good planetary detail and a lot of deep space objects.  Don't worry about aberrations at f/8.  I still have and use my 6" Dobs in spite of having several other scopes, as many others do.  I think you'll find it's a keeper.

 

I definitely recommend flocking, especially if you live in bad light pollution with lots of ambient lights.  I have flocked scopes using several different approaches, from minimal to full.  I strongly recommend flocking only the front part of the tube directly under the focuser.  This gives you 99% of the benefit with minimal work.  You may also want to flock the tube just above the primary.  For the XT6 about 9" to 12" is sufficient.  ScopeStuff sells good quality flocking material.  The small piece will be more than you need.

 

Google is your friend - lots of good info on flocking Newts on the web.

 

Ed D


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#4 CHASLX200

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Posted 20 February 2016 - 08:37 PM

F8 is a dream. No Paracorr needed and easy to collimate.  


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#5 Rigel_10

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Posted 20 February 2016 - 08:39 PM

Thanks Alex & Soyuz!!...I'm very excited to hear the good news regarding the XT6 f/8, can't wait to begin that journey! I will visit some flocking sites, Thanks very much again for the help.

 

Clear skies

 

Edit...also wanted to add that, right now there's free shipping and no sales tax! so I lucky and saved a bundle, 

 

Cheers!


Edited by Rigel_10, 20 February 2016 - 08:42 PM.


#6 chrysalis

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Posted 21 February 2016 - 05:28 AM

Along with the other spot on comments, at f/8, eyepieces are much more forgiving and less expensive wider field ones will work nicely.



#7 Achernar

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Posted 21 February 2016 - 11:54 AM

Expect excellent views with any eyepiece, because coma is non-existent except at the very edge of the field of view. At F/8, Dob/Newts are forgiving of eyepieces and minor errors in collimation too. Also, it's much easier to make a F/8 mirror with an excellent figure, which will show when viewing the planets in particular. I have had a 6-inch F/8 Newtonian which I rebuilt into a Dob, and the primary mirror is essentially perfect. Under good skies, you will see a large number of galaxies, nebulae and star clusters, 12th magnitude galaxies can be seen through them.  For many years, a 6-inch F/8 Newtonian was the gold standard of amateur telescopes, for good reason.

 

Taras


Edited by Achernar, 21 February 2016 - 11:57 AM.

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#8 bartine

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Posted 21 February 2016 - 01:10 PM

Had a guy come by the house last night and we had out my 6" f8 (Criterion) and my 8" f6 (Discovery).

 

We went back and forth between the two scopes for over an hour, looking at multiple items.  The views were very similar through both scopes.  The 6" held its own!

 

I find the field of view is good at F8.  Only on the widest objects did it fall short of the 8" (but no coma).  And optically, F8 is a sweet spot.  Even at low powers, the 4 main stars in the Trapezium are clearly and easily split.

 

You are really going to enjoy this scope!


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#9 Stephen Kennedy

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Posted 21 February 2016 - 01:39 PM

For astronomical telescopes the coma aberration is an inverse function of the square of the focal ratio.  An f/5 will have a coma free image circle only (5^2)/(8^2) = 25/64 = 0.39 the size of the coma free circle of an F/8.  Spherical aberration and astigmatism are also inversely proportional to focal ratio.  While not an issue for reflectors, CA is also an inverse function of focal ratio.  Image scale is a direct function of focal length.  The only disadvantage for a long focal length is that when imaging, exposure time is proportional to the square of the focal ratio.  



#10 CHASLX200

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Posted 21 February 2016 - 03:16 PM

Every 8" F/8 Newt i have owned would kill a 8" SCT.  I now have a Cave 8" F/8.5 that is drop dead good.  I think i have had around ten of these F8 Newts over the years from many makers and none were bad.  No Paracorr or extra fussy collimation.


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#11 Astrojensen

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Posted 21 February 2016 - 03:53 PM

 

Every 8" F/8 Newt i have owned would kill a 8" SCT.

Most SCTs don't put up much of a fight either, to be brutally honest.  :grin:

 

 

Clear skies!

Thomas, Denmark


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#12 CHASLX200

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Posted 21 February 2016 - 04:03 PM

 

 

Every 8" F/8 Newt i have owned would kill a 8" SCT.

Most SCTs don't put up much of a fight either, to be brutally honest.  :grin:

 

 

Clear skies!

Thomas, Denmark

 

I had one 1984 Black C8 that would come pretty close.  That was a one off stand out for sure.  Would take 500x and ask for more. the other 26 or so Celestron SCT's i have owned would never come close to that C8.  



#13 Astrojensen

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Posted 21 February 2016 - 04:43 PM

All joking aside, let's get back to the 6" f/8's. I am currently playing around with one, a Sky-Watcher 150mm f/8, more or less the same as the Orion 6" f/8. I have mine mounted on an EQ-6 (same as Orion Atlas) with the tripod legs shortened to 12". This works really well and is a very nice combo. 

 

I've found that a few modifications/tricks is increasing the performance a LOT on lunar/planetary objects:

 

1: The tube should be insulated or replaced with one of non-conducting material, such as cardboard, wood or fiberglass. 

 

2. It should extend at least 12" in front of the focuser, to prevent the observer's breath from drifting through the light path.  

 

3. The plastic disk behind the primary mirror must be removed to allow it to cool better. 

 

These simple tricks go a long way. I haven't tried flocking the tube, but my next project will probably be changing the tube from metal to a Kr├╝pax (phenolic paper) tube, which will be 10" or so longer than the original. When I'm at it, I suppose I could flock it as well. I'll also install a 2" focuser. I already have one ready, I just need an adapter plate for the new tube. 

 

 

Clear skies!
Thomas, Denmark


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#14 KerryR

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Posted 21 February 2016 - 06:31 PM

Protostar's flocked tube liners look like a great addition to a commercial Chinese dob-- flocking difficulties and thermal issues both solved at once...


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#15 Rigel_10

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Posted 21 February 2016 - 08:39 PM

I've already ordered some flocking material at, scopestuff.com , With any luck it should arrive next Friday around the same time the XT telescope hits my door *fingers & toes crossed*

 

Sounds like a 6" f/8 is the way to go...the long range weather forecast is even looking clear *-* I'm sure I'll give a first light report next weekend if everything works out.

 

:)


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#16 KerryR

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Posted 22 February 2016 - 09:02 AM

6" f8 import dobs are great scopes- They're engineered nicely, they often have decent optics, they're easy to collimate, they cool fast, they can be carried, mounted, in one hand (if they have the tension springs or other suitable alt. tensioner), and the eyepiece and finder sit at a comfortable seated-height.


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#17 dmgriff

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Posted 25 February 2016 - 12:06 PM

I have a Hardin (GSO) 6in f8 dob. Nice optics. Great lunar/planetary/genral use scope, imo.

 

As long as you can move the dob around, a scope that can last a lifetime. 

 

As I age, it is easier to mount the ota on a manual cg5 GEM, that I leave outside (covered), than try to carry the dob.

 

Good viewing,

 

Dave



#18 daquad

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Posted 25 February 2016 - 02:46 PM

Had a guy come by the house last night and we had out my 6" f8 (Criterion) and my 8" f6 (Discovery).

 

We went back and forth between the two scopes for over an hour, looking at multiple items.  The views were very similar through both scopes.  The 6" held its own!

 

I find the field of view is good at F8.  Only on the widest objects did it fall short of the 8" (but no coma).  And optically, F8 is a sweet spot.  Even at low powers, the 4 main stars in the Trapezium are clearly and easily split.

 

You are really going to enjoy this scope!

the 6" f/8 and the 8" f/6 have the same focal length, so a given eyepiece should give the same field of view with either scope.  I assume the 6" field was smaller because the focuser is only 1-1/4", whereas the focuser on the Discovery is 2"?



#19 CCD-Freak

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Posted 25 February 2016 - 09:14 PM

Years ago I bought an Edmund 8" f8 mirror in a cell at the TSP swap meet for $40.  As it turns out is was the best $40 I ever spent.  I built a simple Dob around the mirror and it gives awesome views with every eyepiece I ever stuck in it.  Back in the old days f8 was the norm....maybe there was a reason.   On the other hand a 40" f8 Dob would be a bit overwhelming. 8^)

 

8inF8-Dob-JL-xsm.jpg

 

John

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#20 Rigel_10

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Posted 27 February 2016 - 01:23 PM

I took the new XT6 out for a spin last night (so to speak), the inside & outside of focus star test showed very close images, the best I've seen!! I still need to do some tweaking with the secondary mirror. So last night I ordered a  1.25"/2" laser collimator, from Astronomic's store. Astronomic's is only about 150 miles from where I live, so should receive the tool in a couple days.

 

But yes!..Orion nebula looked fantastic in the XT...as did Sirius, shinning like a welders arc, much more impressive than my 70mm of course. Still waiting on my flocking paper, but this scope should serve me very well in the coming years. It's the f/8 system that makes this scope preform better then my other f/4.5 system scopes, I don't see myself using the faster systems anymore, but that's just my opinion.

 

Clear skies tonight! 


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#21 Astrojensen

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Posted 27 February 2016 - 03:28 PM

 

It's the f/8 system that makes this scope preform better then my other f/4.5 system scopes,

Wrong, it's the better optics overall. 

 

 

I don't see myself using the faster systems anymore, but that's just my opinion.

So you don't want to try a 12" f/5 dob? (as an example) A 12" f/8 is mighty uncomfortable to use.

 

 

Clear skies!
Thomas, Denmark



#22 Rigel_10

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Posted 27 February 2016 - 07:18 PM

 

 

It's the f/8 system that makes this scope preform better then my other f/4.5 system scopes,

Wrong, it's the better optics overall. 

 

 

I don't see myself using the faster systems anymore, but that's just my opinion.

So you don't want to try a 12" f/5 dob? (as an example) A 12" f/8 is mighty uncomfortable to use.

 

 

Clear skies!
Thomas, Denmark

 

I'd rather climb a few steps to look through the f/8 12", the view would be worth the extra effort...ain't scared :)


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#23 ed_turco

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Posted 28 February 2016 - 10:25 AM

For astronomical telescopes the coma aberration is an inverse function of the square of the focal ratio.  An f/5 will have a coma free image circle only (5^2)/(8^2) = 25/64 = 0.39 the size of the coma free circle of an F/8.  Spherical aberration and astigmatism are also inversely proportional to focal ratio.  While not an issue for reflectors, CA is also an inverse function of focal ratio.  Image scale is a direct function of focal length.  The only disadvantage for a long focal length is that when imaging, exposure time is proportional to the square of the focal ratio.  

Only for nebulae, the exposure for stars is the same, given any f/ratio under consideration in this forum.

 

 

Ed


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#24 CHASLX200

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Posted 28 February 2016 - 04:08 PM

 

 

 

It's the f/8 system that makes this scope preform better then my other f/4.5 system scopes,

Wrong, it's the better optics overall. 

 

 

I don't see myself using the faster systems anymore, but that's just my opinion.

So you don't want to try a 12" f/5 dob? (as an example) A 12" f/8 is mighty uncomfortable to use.

 

 

Clear skies!
Thomas, Denmark

 

I'd rather climb a few steps to look through the f/8 12", the view would be worth the extra effort...ain't scared :)

 

And no Coma corrector needed. I would much rather have a F/8 over a F/5.


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#25 Astrojensen

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Posted 28 February 2016 - 05:38 PM

My knees strongly prefer f/5 over f/8 in those sizes. 

 

 

Clear skies!
Thomas, Denmark




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