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Need help deciding which 8" reflector to buy

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#1 Nanzer

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Posted 21 February 2020 - 03:39 PM

Ok, guys, sure this kind of question has been asked before, but here goes. I will be purchasing an 8" reflector. The two scopes I am looking at are almost clones. The GSO is a bit longer and heavier, but these details make no difference. The focuser looks nearly identical on both. The GSO comes with more goodies, but again, this will not be a deciding factor. I am only interested in choosing the best scope for viewing moon, planets and star clusters. These are my two choices:

 

https://optcorp.com/...g-ota-telescope

 

or

 

https://agenaastro.c...lector-ota.html

 

Both have the same aperture for primary, but the TPO has a focal length of 800mm and focal ratio of f4 compared to the GSO which has a focal length of 1000mm and a focal ratio of F5. I have no interest at all in astro photography. This will my portable reflector that will be used exclusively for viewing, not imaging.

 

Your opinions please?



#2 Augustus

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Posted 21 February 2020 - 03:41 PM

If you aren't doing AP I don't know why you'd want to buy an f/4 Newtonian that has likely inferior optics, de facto requires a coma corrector, costs more and will go on an awkward and heavy equatorial mount. Get a Dob.


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#3 Nanzer

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Posted 21 February 2020 - 03:48 PM

"If you aren't doing AP I don't know why you'd want to buy an f/4 Newtonian that has likely inferior optics, de facto requires a coma corrector, costs more and will go on an awkward and heavy equatorial mount. Get a Dob."

 

I will be getting a 12 inch dob. This 8" reflector will be my travel scope. Thank you for your comments, but I am not looking for why you would or would not choose this type of scope or a different scope, just which would be better for viewing of these two. It would be greatly appreciated if comments were limited to this answer. Thanks.

 

Also, I will using a Alt-Azimuth , not an equitorial for this scope.


Edited by Nanzer, 21 February 2020 - 03:50 PM.

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#4 vtornado

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Posted 21 February 2020 - 04:09 PM

Hello and welcome to cloudy nights.

 

f4 scope

Issues with coma when viewing wide field or off axis.

If you intend on using wide field eypeieces, they will suffer major off axis aberrations besides coma,

unless you buy top shelf stuff which is $$$

For moon and planets, in general you want the longer focal length, because you can user lower powered

eyepieces to reach the same mag.

For high power viewing collimation is critical at f/4

Is this scope optimized for visual or photography?  Usually a photo telescope has a larger diagonal.

(not optimal for planets)

and may not come to focus with some eyepieces without an extenstion.

 

f5 scope.

For high power visual this scope will just work better, because of longer focal length.

Easier to collimate, easier on eyepieces, less coma.

 

It will be harder to mount because it is 200mm longer.

 

What kind of mount do you intend on using? These scopes are hard to mount due to their size.

A CG5 is the lowest I would recommend.   There is also the issue of eyepiece position for a newt on an EQ

(solvable with wilcox ring)  There is no Alt-AZ at an affordable price that will hold these scopes


Edited by vtornado, 21 February 2020 - 04:10 PM.

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#5 Augustus

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Posted 21 February 2020 - 04:11 PM

"If you aren't doing AP I don't know why you'd want to buy an f/4 Newtonian that has likely inferior optics, de facto requires a coma corrector, costs more and will go on an awkward and heavy equatorial mount. Get a Dob."

 

I will be getting a 12 inch dob. This 8" reflector will be my travel scope. Thank you for your comments, but I am not looking for why you would or would not choose this type of scope or a different scope, just which would be better for viewing of these two. It would be greatly appreciated if comments were limited to this answer. Thanks.

 

Also, I will using a Alt-Azimuth , not an equitorial for this scope.

In that case I would still probably go with an 8" f/6 Dob. An 8" f/4 is going to suck and an 8" f/5 really has zero portability let alone price advantage over a Dob. Alternatively a 6" or 8" SCT or 6" f/6 Newtonian would probably serve your needs much better.


Edited by Augustus, 21 February 2020 - 04:11 PM.


#6 Nanzer

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Posted 21 February 2020 - 04:25 PM

Thank you vtornado.  Portability is key here. And there are tripods this tube can be securely mounted too, not cheap, but they exist.

 

Thank You again Augustus, but again, there are reasons I am choosing this set up over a dob. Weight for a dob would be about the same after adding tripod to kit, but a dob will not be as portable due to base. I have had rotator cuff surgeries and can stow this scope and a tripod for it on backpack more easily than a dob. While an 8" dob may be better for viewing, it would not only be awkward in terms of position of viewer in relation to scope, but far more awkward for stowage.


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#7 BillP

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Posted 21 February 2020 - 04:25 PM

If you want a smaller car transportable 8" Newt then I would not go with either.  I would instead go for the Sky-Watcher 8" Collapsible Dob.  It is only 36" long collapsed on the mount and it is f/6 so easier on eyepieces and to collimate.

 

https://www.amazon.c...676456047&psc=1

 

If I had to go with one of your two choices would take the f/5 model as an f/4 is going to be very tough on eyepieces so you would need best corrected ones possible and a Paracorr would be a must IMO.

 

PS - Just seeing that you might want to put this in a backpack!?!  If so then would still get this SW Dob and just use the collapsible OTA which makes it a lot smaller then mate it with whatever tripod mount you were thinking to handle something like this.  But frankly, the idea of backpacking with a Dob this big sounds a little crazy.


Edited by BillP, 21 February 2020 - 08:53 PM.

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#8 Augustus

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Posted 21 February 2020 - 04:39 PM

Thank you vtornado.  Portability is key here. And there are tripods this tube can be securely mounted too, not cheap, but they exist.

 

Thank You again Augustus, but again, there are reasons I am choosing this set up over a dob. Weight for a dob would be about the same after adding tripod to kit, but a dob will not be as portable due to base. I have had rotator cuff surgeries and can stow this scope and a tripod for it on backpack more easily than a dob. While an 8" dob may be better for viewing, it would not only be awkward in terms of position of viewer in relation to scope, but far more awkward for stowage.

I think you are underestimating the size of an 8" or a mount for it drastically. A tripod for almost any of the mounts that hold an 8" Newt is going to be a 2" steel-legged 15-pound unit and the tube will not fit most backpacks.


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#9 aeajr

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Posted 21 February 2020 - 05:07 PM

Based on your observing goal, I would go for the F5 GSO.   F5 will have less coma.

 

So, I answered your question.  Now I have a few if you care to share this information.

 

 

I am curious what mount you will use.

 

And I would ask why an 8" reflector as opposed to a more compact 8" SCT or a  6" refractor.

 

You don't have a signature so I don't know if this is your first scope or what other scopes you own.

 

When you say travel, do you mean by car, train or air?


Edited by aeajr, 21 February 2020 - 05:11 PM.

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#10 thomasr

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Posted 21 February 2020 - 05:40 PM



a dob will not be as portable due to base. I have had rotator cuff surgeries and can stow this scope and a tripod for it on backpack more easily than a dob. While an 8" dob may be better for viewing, it would not only be awkward in terms of position of viewer in relation to scope, but far more awkward for stowage.

It sounds as if a SCT ticks a lot of the boxes you're looking for.



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#11 GoFish

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Posted 21 February 2020 - 05:47 PM

I, too, think I might choose a different kind of OTA for a travel scope.

 

But, given the two choices posed in the OP, take the f/5 every time.  I have both an 8" f/5 and an 8" f/4.  I am listing the f/4 in the classifieds at this very moment, and keeping the f/5. 

 

It is hard to explain to someone what a fiddly thing an f/4 reflector is if they haven't owned one.  Never is a long time, but right now I think I will never own an f/4 Newt again.

 


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#12 Nanzer

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Posted 22 February 2020 - 12:25 AM

Ok, I have some crow to eat. I give. I had not considered the tripod/mounting issue carefully enough. Going with a 6" optical tube by GSO with switched out focuser. The Explorer Scientific Twilight I Mount should work well with smaller optical tube.

 

New travel scope:  

 

GSO 6" f/6 Newtonian Reflector Telescope - Steel OTA (White) # GS530

https://agenaastro.c...lector-ota.html

 

New focuser:

GSO 2" Linear Bearing Crayford Focuser for Reflectors - Dual Speed

https://agenaastro.c...dual-speed.html

 

Explorer Mount:

https://explorescien...wilight-1-mount

 

Thanks to all who commented.



#13 areyoukiddingme

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Posted 22 February 2020 - 02:15 AM

I'd be surprised if the mount will handle that well. I'd look for comments from people with that particular mount, and how far they take it.



#14 Nanzer

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Posted 22 February 2020 - 02:38 AM

Reviews say it's rock solid at 20lb capacity.

Based on Youtube vids, It appears the diameter will easily fit the mount with a little room to spare.


Edited by Nanzer, 22 February 2020 - 02:40 AM.


#15 B l a k S t a r

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Posted 22 February 2020 - 10:01 AM

This is what I want, now please confirm my most excellent choices!  No offense intended, honestly!  These folks have a lot of knowledge and experience and are eager to help out.

 

Please pardon my blunt.



#16 airbleeder

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Posted 22 February 2020 - 10:14 AM

   I have a Twilight  I mount. IMO, it will not handle a 6"f6 reflector. I've used mine with a 4"f6.5 achro (9lbs)for a couple of years. I placed the head on a beefier tripod with 1.75" SS legs. I recently bought a 4"f7 ED triplet which weighs 9.5lbs. The vibration was so bad that the combo was basically un-usable until I really beefed up the altitude arm by cutting aluminum blocks to insert into the cavities. I also cut gaskets from milk jugs to tighten up between some of the bolted pieces. It's now solid enough for the ED102, but I wouldn't recommend it for a 6"f6 reflector although I've  never even seen one. I don't think it would handle the length or the load. Just my opinion.


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#17 Jon Isaacs

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Posted 22 February 2020 - 10:16 AM

Reviews say it's rock solid at 20lb capacity.

Based on Youtube vids, It appears the diameter will easily fit the mount with a little room to spare.

 

I own a Twilight 1.

 

It is unusable with a 12 pound refractor, 26 inches long. It's adequate for an 80 mm F/7 app.

 

Jon


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#18 GoFish

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Posted 22 February 2020 - 10:19 AM

Ok, I have some crow to eat. I give. I had not considered the tripod/mounting issue carefully enough. Going with a 6" optical tube by GSO with switched out focuser. The Explorer Scientific Twilight I Mount should work well with smaller optical tube.

 

New travel scope:  

 

GSO 6" f/6 Newtonian Reflector Telescope - Steel OTA (White) # GS530

https://agenaastro.c...lector-ota.html

 

New focuser:

GSO 2" Linear Bearing Crayford Focuser for Reflectors - Dual Speed

https://agenaastro.c...dual-speed.html

 

Explorer Mount:

https://explorescien...wilight-1-mount

 

Thanks to all who commented.

I have some concerns about that mount with that scope, too.

 

For comparison, I have a Vixen Porta II mount (20 lbs rating by Vixen) on a Berlebach Report tripod.  I believe that is comparable to the Twilight I (18 lbs rated by ES) and its metal tripod. 

 

My 6" f/5 Newtonian is too much scope for my mount. My scope has 750mm focal length, the one you are considering has 900mm focal length, has a longer tube, and is heavier.

 

Until I had actually tried it in the field, I would not assume that combo will work.


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#19 csa/montana

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Posted 22 February 2020 - 10:32 AM

Moved to Reflectors, as General Observing does not allow equipment discussions.



#20 Nanzer

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Posted 22 February 2020 - 11:01 AM

The tube I'm looking at is only 15 lbs with accessories. Throw a heavy eye piece on there and it should round up nicely to just under 20 lbs. I looked at all the possible Vixen mount/tripod combos and I don't think any of them would have worked for a "7 diameter tube. I'll let you know how the combo I've chosen works out. It's what I'm going with.



#21 aeajr

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Posted 22 February 2020 - 11:40 AM

Nanzer,

 

A 6" Newtonian will ride that mount better than an 8".  However, the key point that people are trying to share is that maximum load is the maximum load, not the optimum load.  If the mount is rated for 20 pounds, that might work with a 20 pound SCT or Mak set-up, though it would still be somewhat shaky.   The mount may be able to  handle 20 pounds, but how well will the tripod handle it?   And, 20 pounds of long Newtonian is a different matter.

 

Carry a 14 pound bowling ball.   Now carry a 14 pound wide screen TV set or a long 14 pound tube.   The carry is quite different.  The longer loads exert a greater torque on the mount.  And if you have any sort of breeze, the long Newtonian is going to act like a sail and shake the mount.

 

That optical tube is 14 pounds with no eyepiece. Put a large 2" in there and and you are up around 16 pounds.   Now add a telrad, about 3/4 pound.  And each of those added loads will be out on the end of the tube, not over the mount.  So you might choose to add 8 ounces to a full pound of balance weight at the other end.  

 

A 6" F6 Newtonian is a LONG tube with the weight, focuser, finder, mirror, distributed well away from the mount.   This gets worse as you add large eyepieces.  And, as you have seen, weight is not the sole factor.

 

A 6" SCT is about 9 pounds and is a short, compact load that is close over the center of the mount.  A 4" F7 refractor has is about 9 pounds, and has a much smaller cross section but comparable light gathering as it has no central obstruction.

 

 

Buy whatever you like.   Won't impact any of us.  But you opened a discussion and people are trying to help you and share experiences.

 

We still don't know why you want a Newtonian.   In your first post you said you were only "interested in choosing the best scope for viewing moon, planets and star clusters".  So, why a Newtonian?   

 

Those are easy targets that can be handled by any type of scope.  And some might suggest that, for the Moon and Planets, a refractor, Mak or SCT might be a better choice for a travel scope.

 

Star clusters, as a category, is all over the map so some better understanding of those would help.   The Pleiades is almost 2 degrees wide.  Globular clusters will often need fairly high power and you will definitely want high power for the Moon and planets. 

 

Personally, for travel, I would prefer a 4" F7 ED refractor or 6" SCT over a 6" Newtonian .  You can even take it on the plane as carry on or put it, padded, in your luggage. These can be put in a back pack for hiking.  They  cost more to buy, but it would travel better and have similar light gathering. 


Edited by aeajr, 22 February 2020 - 11:58 AM.

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#22 airbleeder

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Posted 22 February 2020 - 12:39 PM

The tube I'm looking at is only 15 lbs with accessories. Throw a heavy eye piece on there and it should round up nicely to just under 20 lbs. I looked at all the possible Vixen mount/tripod combos and I don't think any of them would have worked for a "7 diameter tube. I'll let you know how the combo I've chosen works out. It's what I'm going with.

  That's exactly what you should go with and after you've had some experience with the combo maybe you can share you're experience with someone who is about to make that same mistake. 

 

Edited to soften it up. 


Edited by airbleeder, 22 February 2020 - 06:14 PM.

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#23 N3p

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Posted 22 February 2020 - 01:10 PM

Ok, guys, sure this kind of question has been asked before, but here goes. I will be purchasing an 8" reflector. The two scopes I am looking at are almost clones. The GSO is a bit longer and heavier, but these details make no difference. The focuser looks nearly identical on both. The GSO comes with more goodies, but again, this will not be a deciding factor. I am only interested in choosing the best scope for viewing moon, planets and star clusters. These are my two choices:

 

https://optcorp.com/...g-ota-telescope

 

or

 

https://agenaastro.c...lector-ota.html

 

Both have the same aperture for primary, but the TPO has a focal length of 800mm and focal ratio of f4 compared to the GSO which has a focal length of 1000mm and a focal ratio of F5. I have no interest at all in astro photography. This will my portable reflector that will be used exclusively for viewing, not imaging.

 

Your opinions please?

I would not pick the F4 Newtonian because the F5 will already induce more then enough coma in the views for the visual observation. With F5, I often see people complaining about Coma, personally I find it tolerable and don't want to buy or use a coma corrector.

 

I'll be the only person saying this but but it's true, I use a 200x1000 on an equatorial mount EQ5 and I totally love the setup. It's hard to use but once the target is inside the FOV, the tracking is marvellous, very precise.  Strangely, the telescope and accessories exceeds the total capacity of the EQ5 but it's working anyway and views are stable.

 

Both were sold together by Skywatcher with conflicting specification between the telescope and the mount.

 

All discontinued today.

 

Good Luck!

 

 

 

 

 

 

 

 


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#24 GoFish

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Posted 22 February 2020 - 11:13 PM

I think I saw the horse twitch!  Better go beat it again ...

 

If the Twilight mount fails to perform, the cause will be rotational vibrations. The kind caused by little nudges to the tube when trying to focus. The little nudges act at a distance away from the center of support for the tube and tend to start the system a-rotating and a-vibrating. 

 

For a given nudge, how long the vibrations last is a function of two main things:

  • the stiffness of the mount (head/tripod system)
  • the mass moment of inertia of the scope

Once you’ve settled on the mount, then the only way you can reduce the effects of the vibrations is by reducing the mass moment of inertia of the scope. 

 

What’s the mass moment of inertia (MI)?  The sum of the masses of each piece of scope times the distance from that piece of scope to the center of the mount times distance from that piece of scope to the center of the mount. See how all the mass gets multiplied by the distance two times, i.e. distance squared?

 

Ed’s 20 lb bowling ball has a way, way smaller MI (effectively zero) compared to a 32” long, 20 lb OTA with the largest concentrations of the mass being at the ends.  That’s the simple physics behind why the longer OTA is so much more demanding, mount wise, than shorter OTA’s, even if the weights are the same. 

 

An undermounted scope is not fun to use and really, really hard to focus because of the vibrations. A 6” SCT with f/6.3 reducer is roughly equivalent optically and has a lot better chance to work on the Twilight because it’s MI is so much smaller. 

 

That GSO scope sounds like a very nice scope and will probably be a joy to use on the right mount. Unfortunately, portable alt az mounts that can handle an OTA that size are scarce and expensive. Maybe the Stellarvue M2C with one of ash tripods could handle it?  I’ve been considering the M2C as a replacement for my Porta II. 

 



#25 Nanzer

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Posted 23 February 2020 - 02:13 AM

Well, again, thanks for the input. If modding is required, I'll mod it. I have to say, once upon a time I went to a star party where a certain amateur astronomer lacking in what could be called social or communication skills thought much too highly of himself and was very off putting. Not just about the hobby, but of other's beliefs, secular or religious. He thought himself clever, really he was just an **** who drove people away.

 

I don't discount the experience of any one posting, but I'm not entirely new to this hobby. I think perhaps some of the comments here reflect the arrogance of hobbyists that can be so alienating to those new to the hobby. Luckily, I'm not new to this hobby. No need for anyone to soften comments that were created and posted, not for my benefit, but because some people like the sound of their own voice more than actually coming up with helpful alternatives or solutions. Maybe a different mount will be chosen or maybe I'll go back to to the 8" scope and build my own mount and tripod. I am not without build skills and might surprise some here. I have more conversations to have with sales reps, so if I end up going with different equipment, then I end up going with  different equipment. Whatever I decide on, I'll be sure to report back .

 

Ultimately, it's been a while and and I just couldn't find the old pages I used to go to for determining best focal speed for viewing based on a scope size I'm unfamiliar with. That question was answered with the first response to my post and confirmed with the second. So again, Thank you all for your comments; most were genuinely kind hearted and helpful, the few that weren't, were entertaining. Wishing all my fellow star gazers clear, dark skies-


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