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6" F/8

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#1 stargazer32864

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Posted 21 October 2020 - 04:09 PM

I'm planning on buying my nephew an Orion XT 6" f/8 reflector. He wants to see the moon and planets with it. He lives in Henderson, KY. Which Bortle it is, I have no idea. But I assume the skies are light polluted. My question(s) is...What will the planets look like through this type of scope? I have never had a reflector of less than 8". So, I don't know how the planets will look with what type of eyepiece. Will he need any filters? What type of eyepiece? How far can he push the magnification? Will he need a field flattener? Will he see the details of a planet? I don't know what he expects but the junk refractor he paid $40 for must be an achromat cause he was telling me that the planets we tiny and no detail and a lot of false color.  Any advice would be appreciated.

 

  Thanks~Robin


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#2 stargazer193857

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Posted 21 October 2020 - 04:38 PM

6" f8 will do 300x. Well, maybe that one was premium. But jupiter looked big, detailed, and bright enough.

No field flattener or filter needed for planets. A neutral filter is needed for the moon to avoid blinding light.
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#3 gwlee

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Posted 21 October 2020 - 04:43 PM

I have owned several XT6 and XT8 scopes and used them in the light polluted city and dark rural sights. The 41# XT8 has marginally higher optical performance, but the 34# XT6 isn’t far behind, and it’s much easier for me to handle. Both work very well for planets. The larger 8” is noticeably better for DSOs, but not dramatically better. 
 

With both, I like a 7mm EP (171x) for planets. Seeing conditions here usually limit the magnification to about 200x.  I have used 345x a few times when seeing is exceptionally good. Plossl EPs work OK for both scopes, especially the XT6. but eye relief gets too short to work well with eyeglasses in the shorter focal lengths used for planets. 

 

I have never felt the need use to filters for viewing planets with either scope. Don’t think field flatteners are used with small Dobs.


Edited by gwlee, 21 October 2020 - 04:48 PM.


#4 Alterf

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Posted 21 October 2020 - 05:20 PM

I use a 6" f/8 dob.  I routinely observe at 240x, and that is great for detail on Jupiter, Saturn, Mars.  I've been watching Mars lately and can see the ice cap in sharp relief; I can see many dark features; I can see cloud banks when they are large.  Jupiter's red spot is easy.  Earlier this year I caught a Jovian moon overlapping its own shadow on Jupiter's face, one of the coolest things I've ever seen.  I always see Saturn's Cassini division, and on the best nights I have seen six Saturnian moons from my suburb (but I usually see only five or sometimes four).  I find Neptune and Uranus at least once each year.

 

No need for a field flattener; the field is flat enough.  At f/8, there is no need for a coma corrector, either.  Just eyepieces will be fine.  I never use filters on anything, but looking at the moon destroys my night vision. The neutral density filter suggestion is a good one.  I sometimes view the moon at insane magnifications (up to 480x); the moon can take a lot of power sometimes.

 

I favor wide field eyepieces.   I have TV Plossls of 32mm and 25mm that are great.  At shorter focal lengths, wider fields are better.  At 20mm and below I have Pentax eyepieces, but I admit to using a smaller-field Vixen eyepiece for especially tight double stars (up to 480x).  The wider field eyepieces mean the user does not have to move the telescope as often.

 

Best,

 

Val


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#5 stargazer193857

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Posted 21 October 2020 - 05:44 PM

The most noticeable difference between 8" and 6" is the 6" resolves much fewer stars in M13.

That an my left elbow can't handle me lifting the 8" in one piece anymore. I've not tested the two at their limits on planets, but from what I saw a 6" do to jupiter, I think the atmosphere is the limit.

#6 stargazer32864

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Posted 21 October 2020 - 05:47 PM

I use a 6" f/8 dob.  I routinely observe at 240x, and that is great for detail on Jupiter, Saturn, Mars.  I've been watching Mars lately and can see the ice cap in sharp relief; I can see many dark features; I can see cloud banks when they are large.  Jupiter's red spot is easy.  Earlier this year I caught a Jovian moon overlapping its own shadow on Jupiter's face, one of the coolest things I've ever seen.  I always see Saturn's Cassini division, and on the best nights I have seen six Saturnian moons from my suburb (but I usually see only five or sometimes four).  I find Neptune and Uranus at least once each year.

 

No need for a field flattener; the field is flat enough.  At f/8, there is no need for a coma corrector, either.  Just eyepieces will be fine.  I never use filters on anything, but looking at the moon destroys my night vision. The neutral density filter suggestion is a good one.  I sometimes view the moon at insane magnifications (up to 480x); the moon can take a lot of power sometimes.

 

I favor wide field eyepieces.   I have TV Plossls of 32mm and 25mm that are great.  At shorter focal lengths, wider fields are better.  At 20mm and below I have Pentax eyepieces, but I admit to using a smaller-field Vixen eyepiece for especially tight double stars (up to 480x).  The wider field eyepieces mean the user does not have to move the telescope as often.

 

Best,

 

Val

Wow!! I think I'll have to get me one too. I have a 10" dob but I use that for DSO. I had an 8" dob but the images were poor. A year after getting it, I noticed that the primary mirror had two lines going halfway around it. I assumed that's why the images were poor. A flaw in the optics. The first thing I do now when I get a new scope is to check the optics for flaws.



#7 stargazer32864

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Posted 21 October 2020 - 05:49 PM

The most noticeable difference between 8" and 6" is the 6" resolves much fewer stars in M13.

That an my left elbow can't handle me lifting the 8" in one piece anymore. I've not tested the two at their limits on planets, but from what I saw a 6" do to jupiter, I think the atmosphere is the limit.

He will be using it for planets and moon only. Would the Televue plossls be better than the plossls that come with the scope?



#8 stargazer193857

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Posted 21 October 2020 - 06:31 PM

He will be using it for planets and moon only. Would the Televue plossls be better than the plossls that come with the scope?

Instead of TV Plossls, spend $129 on a 5.5mm Meade 82 deg. It will find the planets much easier, let them drift longer, and fit much more of the moon in view at high power.

Keep the 25mm Plossl. It is good.

Let your kid look through the 10mm Plossl just to understand what short eye relief is. Then donate it to the club.

If the Meade costs too much, get an Orion Expanse knock off, 6mm 66 deg. They are on eBay for $30 shipped and Agenaastro for $40.


Also get a barlow, 2x, celestron or GSO.

Edited by stargazer193857, 21 October 2020 - 06:34 PM.


#9 stargazer32864

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Posted 21 October 2020 - 08:56 PM

Instead of TV Plossls, spend $129 on a 5.5mm Meade 82 deg. It will find the planets much easier, let them drift longer, and fit much more of the moon in view at high power.

Keep the 25mm Plossl. It is good.

Let your kid look through the 10mm Plossl just to understand what short eye relief is. Then donate it to the club.

If the Meade costs too much, get an Orion Expanse knock off, 6mm 66 deg. They are on eBay for $30 shipped and Agenaastro for $40.


Also get a barlow, 2x, celestron or GSO.

GSO?? I haven't heard of that one.



#10 stargazer193857

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Posted 21 October 2020 - 09:02 PM

GSO?? I haven't heard of that one.


They are the company that makes optics for Orion, Sky-watcher, Meade, etc. Well, GSO and Synta.

GSO makes lower cost decent to good quality optics. $30 for an all metal barlow. But Televue is better and costs more. I've not compared the two side by side and don't know if the average person would notice the difference.

#11 stargazer32864

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Posted 21 October 2020 - 11:19 PM

They are the company that makes optics for Orion, Sky-watcher, Meade, etc. Well, GSO and Synta.

GSO makes lower cost decent to good quality optics. $30 for an all metal barlow. But Televue is better and costs more. I've not compared the two side by side and don't know if the average person would notice the difference.

Okay. I was planning on getting a Televue barlow for myself and a paracorr. What would be considered a premium Dob?



#12 stargazer193857

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Posted 22 October 2020 - 01:06 PM

Okay. I was planning on getting a Televue barlow for myself and a paraccord. What would be considered a premium Dob?


Televue likely has better polished glass. Probably also better coatings. I just don't know if the average person would notice the difference. I've not looked through one.

Premium often refers to several domestic scope makes such as Obsession and especially Newmoon. Figure $4000 for a 12.5", $2800 for an 8". They have premium domestic mirrors with a smoother polish, and also use Teflon to slew smoothly.

#13 SteveG

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Posted 22 October 2020 - 02:04 PM

Okay. I was planning on getting a Televue barlow for myself and a paraccord. What would be considered a premium Dob?

The Paracorr is for much faster systems than a 6" f8. You don't need it.

 

Does the 6" scope have a 2" focuser?


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#14 Pinbout

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Posted 22 October 2020 - 02:53 PM

6”f8 is great with good optics 

 

just stay close to street lights to keep eyes from being dark adapted.

 

4DA3B528-E6C4-4F56-B658-4E2689D052CF.jpeg


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#15 Bill Weir

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Posted 22 October 2020 - 03:51 PM

A few sketches all with my 6” f/8 

Mars. https://rascvic.zenf...b93f2#heecb93f2

 

https://rascvic.zenf...b93f2#heb12e8ad

https://rascvic.zenf...b93f2#h138e584c

https://rascvic.zenf...b93f2#h37ef1c87

https://rascvic.zenf...b93f2#h1db857fa

https://rascvic.zenf...b93f2#h3c01bf45

 

Nothing more to say.

 

Bill



#16 Ranger Tim

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Posted 22 October 2020 - 04:10 PM

The six inch f/8 is close to being useable without parabolization so optics are not difficult to make. 1-1/4 inch focuser is adequate for almost all viewing so weigh the investment in a larger focuser carefully. 32mm plossl might be all you need for wide views. Nothing longer. The six can be an excellent planetary scope in longer focal lengths, especially for the casual observer. Less awkward to manage and less affected by poor seeing. Less concern about mirror cool-down and no coma correction needed.

 

Poor atmospheric conditions will dictate your success with larger planets, especially Jupiter and Saturn. I really miss those humid, super stable nights back in Virginia! Don't miss the mosquitos though. f/8 should result in a nice focal length for solar system objects. Be sure to teach how to view solar safely.


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#17 stargazer32864

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Posted 22 October 2020 - 05:09 PM

The Paracorr is for much faster systems than a 6" f8. You don't need it.

 

Does the 6" scope have a 2" focuser?

It's the Orion XT6.


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#18 stargazer32864

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Posted 22 October 2020 - 05:13 PM

I'll have to get one for myself after I get one for Brandon. He's going to love it. Thanks!



#19 stargazer193857

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Posted 22 October 2020 - 07:26 PM

I must have missed the coma corrector mention. Definitely should not use one with a 6" f8.

The optics on the f8 are likely better than on an f5 just because the 6" f8 is usable even as a sphere, and I doubt 6" f5 are parabolised very well.

The 6" is $100 less than an 8". Many people buy them just for that reason.

#20 stargazer193857

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Posted 22 October 2020 - 07:28 PM

6”f8 is great with good optics

just stay close to street lights to keep eyes from being dark adapted.

4DA3B528-E6C4-4F56-B658-4E2689D052CF.jpeg


Just put the streetlight behind the aperture so it does not get in the tube and wash out the view. Even planets can be completely washed out by a street light if enough light goes down the tube.

#21 Zamboni

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Posted 22 October 2020 - 11:14 PM

My favorite telescope is a 6" f/8. I love this size and focal ratio because it's easy to collimate, doesn't need coma correction or field flattening, not particularly touchy in regards to the eyepieces used so even relatively cheap eyepieces work well, is easy to transport, and gives amazing views. I got my first job to afford the scope in 1998 and it's still my "daily driver" telescope. And it gives AMAZING views of the moon and planets. It's also just a no-brainer to use. Best beginner scope on the planet that still has utility to someone who's been in the hobby for nearly thirty years.

I do recommend a 13% transmission moon filter to get started, because the moon can definitely be uncomfortably bright in a scope this size.

Edited by Zamboni, 22 October 2020 - 11:18 PM.

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#22 MalVeauX

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Posted 23 October 2020 - 10:01 AM

Heya,

 

Don't bother with coma/flat field correction optics on an F8 scope, especially for visual. F8 has minimal to no coma in the visible field, so its not useful here. Save your coin.

 

Don't bother with a barlow, yet, unless it comes with the kit. Instead, look at a zoom eyepiece.

 

Use the basic eyepieces that come with it. When buying a new eyepiece(s) look at eye relief more than anything for comfort of viewing (lunar and planetary doesn't need wide field).

Really don't buy anything at all other than the basics, I would avoid all filters completely at this point. Anything too bright, just increase magnification and the brightness goes down. Just get out and view.

 

If you were to buy anything, I would argue for a zoom eyepiece, like a 7-21 or 8-24 flavor (1.25") for even more convenient viewing of planets and lunar surface (use the wide end to find your subject, then zoom in with a twist at will and don't fumble with swamping eyepieces, etc). Much more fun and convenient than swapping eyepieces and inserting barlows, etc, in the dark. I would argue for a Svbony 7-21, as this will get to 171x magnification which is plenty starting out with planets if seeing even allows, and as wide as 57x which is still low mag for a nice wide field still for hunting and cruising.

 

https://www.amazon.c...03465230&sr=8-1

 

6" F8 newtonians are super forgiving, excellent scopes; not just a starter, a lifer easy. Inexpensive and yet a real scope with good everything. Far more critical is to simply get some sky time. Keep it simple so that it's not a chore. Minimize down time and setup time. Just get it out there and cruise.

 

Enjoy!

 

Very best,


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#23 LDW47

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Posted 23 October 2020 - 01:50 PM

I'm planning on buying my nephew an Orion XT 6" f/8 reflector. He wants to see the moon and planets with it. He lives in Henderson, KY. Which Bortle it is, I have no idea. But I assume the skies are light polluted. My question(s) is...What will the planets look like through this type of scope? I have never had a reflector of less than 8". So, I don't know how the planets will look with what type of eyepiece. Will he need any filters? What type of eyepiece? How far can he push the magnification? Will he need a field flattener? Will he see the details of a planet? I don't know what he expects but the junk refractor he paid $40 for must be an achromat cause he was telling me that the planets we tiny and no detail and a lot of false color.  Any advice would be appreciated.

 

  Thanks~Robin

A perfect scope for a beginner, just make sure it has the 2” focuser, the 6” SW does for about the same price !



#24 stargazer193857

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Posted 23 October 2020 - 03:02 PM

A perfect scope for a beginner, just make sure it has the 2” focuser, the 6” SW does for about the same price !


I'm a much bigger fan of tension springs than I am of tension clutches.

I also wonder how well a 1.5" secondary would illuminate a 2" eyepiece.

True though about the price and equipment.
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#25 stargazer32864

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Posted 23 October 2020 - 03:09 PM

I do recommend a 13% transmission moon filter to get started, because the moon can definitely be uncomfortably bright in a scope this size.


 

Thanks! I was thinking about that. I didn't know which would be better, 25% or 13%.

~Robin


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