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Dobsonian vs Cassegrain

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#1 Greyfox_MT

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Posted 09 December 2021 - 08:35 AM

I've currently got an ETX-90RA (90mm by 1250 focal length) that I really like but I know it is limited and I'm starting to get the itch for something better.

 

So, I'm at the fork in the road of what to go with next.  I've seen some reasonably priced Dobsonian and moderately priced Cassegrain scopes. 

 

I know size is the major difference with Dobs needing large tubes for the higher focal lengths.  Just looked at a Zhumell Z8 that only has a 1200 focal length.  Is that the equivalent of my ETX-90?  Should I look for a 10"?  I know a larger objective allows for higher magnification lenses and barlows.

 

Would it be worth the price to just step up to an ETX-125?  I have a good selection of camera accesories for the rear port of my ETX-90 that will work on the ETX-125.

 

Or should I look at a 6" Cassegrain?

 

Thanks in advance for any information to help guide me.



#2 Blacksmith

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Posted 09 December 2021 - 08:44 AM

If I were mainly going into visual astronomy Id go with the Dob. 8" is fine,  little difference between the 8" and 10" honestly.. The 8" would be a little easier to move and set up. Good DSO scope too. I really like my Mak but if I had to choose Id take my Dob over the Mak all day for visual astronomy.


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#3 Jon Isaacs

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Posted 09 December 2021 - 08:56 AM

I've currently got an ETX-90RA (90mm by 1250 focal length) that I really like but I know it is limited and I'm starting to get the itch for something better.

 

So, I'm at the fork in the road of what to go with next.  I've seen some reasonably priced Dobsonian and moderately priced Cassegrain scopes. 

 

I know size is the major difference with Dobs needing large tubes for the higher focal lengths.  Just looked at a Zhumell Z8 that only has a 1200 focal length.  Is that the equivalent of my ETX-90?  Should I look for a 10"?  I know a larger objective allows for higher magnification lenses and barlows.

 

Would it be worth the price to just step up to an ETX-125?  I have a good selection of camera accesories for the rear port of my ETX-90 that will work on the ETX-125.

 

Or should I look at a 6" Cassegrain?

 

Thanks in advance for any information to help guide me.

 

The 8 inch Dobsonian has a 1200 mm focal length, the same as your ETX-90. That basically means that the same eyepieces will provide the same magnification.

 

The important difference between the two is the aperture, 200mm versus 90 mm. The 8 inch has 2.25x better resolution and collects about 5 times as much light.

 

At the eyepiece, these differences are major/huge..

 

Jon


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#4 Dave Mitsky

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Posted 09 December 2021 - 09:02 AM

Choosing a Dob in terms of visual performance per dollar spent is a no-brainer.  A 10" Dob is a more capable deep-sky telescope but will require eyepieces with good correction.  Some coma may be visible with long focal-length wide-field eyepieces.


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#5 SeattleScott

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Posted 09 December 2021 - 09:03 AM

The 10” also has 1200mm FL. So does a 6” Dob.

I remember having little chuckle reading about the founding of the Harvard Observatory, because they only seemed to care about the focal length of their first scopes, not the aperture. FL is largely irrelevant (not completely). Aperture is what you should care about. 200mm versus 90 is a fairly huge difference. Once you get up to about 6”+ aperture, normally the atmosphere becomes the limiting factor in terms of maximum magnification, not the scope.

An 8” Dob will be a bit more portable, a bit lower maintenance, a bit cheaper and less concern about needing a coma corrector for ultrawide or hyperwide eyepieces. A 10” will give about 50% brighter views. That’s really what it comes down to. Consequently you can achieve higher magnification on faint objects with the 10”. The view dims as you increase magnification. Therefore the more light you are pulling in, the higher magnification you can use before you run out of light. For bright objects the atmosphere becomes the limiting factor (along with a few other issues you can mostly control for). So aperture (and the sky) is really what determines usable magnification, not focal length.

Ultimately you change magnification by changing eyepieces. I can achieve 300x magnification with my 800mm FL refractor or my 1800mm Mak. I just use different eyepieces (or a barlow) to get there.

Scott

Edited by SeattleScott, 09 December 2021 - 09:07 AM.

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#6 Echolight

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Posted 09 December 2021 - 09:44 AM

My C8 is running at about 1300mm focal length or a bit more with reducer and 2 inch eyepieces. The 20 year old scope itself was fairly inexpensive. About the same as a used 8 inch dob in the current market.

But after adding the used high capacity video mount, used reducer and new 2 inch diagonal, it is well above the cost of a new 8 inch dob. Approaching the cost of the more well equipped mass produced manually guided 10 inch dobs.

 

Although because I had other scopes to use, the cost of the setup as it is, has taken place over the course of a year and a half. So that has eased the pain a bit. It is certainly much smaller than a dob. The tripod and mount weigh about the same as the base of an XT8, but the tripod is much easier to carry around. And it gets much smaller for travel, if that’s your thing.

 

Sure I’d rather have the aperture advantage of a 10 inch dob. And some day I may. But the convenience factor, aside from the inconvenience of gathering pieces over a year and a half, the convenience factor of the C8 on a Bogen 3068 and modified 116 video head’s smaller form as disassembled, and the more convenient eyepiece orientation makes my C8 (in it’s current state) more easily manageable in and out of use, for me.

 

Off the shelf, for a new scope, an 8 or 10 inch manual dob would be the easier choice. 
 

Also, technically, as commonly stated by many, you’re not supposed to use a reducer combined with a 2 inch diagonal and eyepieces on a C8. And this certainly wouldn’t work on an SE mount where diagonal clearance at zenith is more limited. But it works for me.


Edited by Echolight, 09 December 2021 - 09:56 AM.


#7 Greyfox_MT

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Posted 09 December 2021 - 09:48 AM

Thanks everyone.  I think I'm going to keep an eye out for a 8" or 10" Dob then.  Where I live has little light pollution and I have even put a switch on our yard light.

 

I've got an idea for a transport mechanism using an old transmission jack with some easy roll wheels.  And it wouldn't take much to build a concrete pad out back for it to be setup on.



#8 Paul Sweeney

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Posted 09 December 2021 - 09:49 AM

There are major differences between the 2 scope types, so what you get depends on what your priorities are.

The dob is big and is an alt/az scope. The 8" dob is f6, so it is easy to collimate. These are good at low to medium powers. A good, solid general purpose scope.

Cassegrains tend to be tripod mounted, mainly on an EQ, but Alt/az is possible. The tube is much more compact than the dob's, but when the mount and tripod are considered, an 8" cass on an EQ mount is actually bulkier than the 8" dob. They are more for medium to high powers, mainly because of the longer focal lengths. Cassegrain type scopes need to be well collimated to perform well. Some cassegrain type scopes have trouble with dew collecting on the front glass surface.

Both dobs and casses can suffer from tube currents and have cooling issues.

Your maximum magnification depends mainly on your local seeing. My experience with an 8" dob and an 8" cass was that both maxed out at 200-250x. For the vast majority of the targets you observe, you will not be using anywhere near that much magnification.

Edited by Paul Sweeney, 09 December 2021 - 09:51 AM.

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#9 rhetfield

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Posted 09 December 2021 - 01:18 PM

If the OP tosses a 5mm eyepiece into a 1200mm focal length telescope on a perfect light and points it at Jupiter,  they will get 240x magnification.  In the 90mm Cat, Jupiter will be a white blob.  In the 200mm dob, it will be a crisp planet with many bands, storms, festoons, and moon crossings clearly visible.


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#10 sevenofnine

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Posted 09 December 2021 - 03:20 PM

Welcome to C/N! welcome.gif

 

Just to give you an idea of the visual differences in aperture, M13 in my C-90 (1250 FL) looks like a dim view of the end of a Q-tip. In my Apex 127 Mak (1540 FL) it looks like a soft sparkling cotton ball. In my XT8+ (1200 FL) it looks like a tray of diamonds on a good night. With a 10" Dob, you will get the same look just more sparkle and more diamonds bugeyes.gif

 

The Dob gives you the most aperture per dollar spent. Your money goes mostly into the OTA. The rocker base is just particle board covered with melamine. AP is very limited. SC cost much more mostly due to the mount. Some more advanced AP is possible with additional equipment.  Best of luck to you and your choice! borg.gif


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#11 Dave Mitsky

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Posted 09 December 2021 - 06:06 PM

There's always the M13 aperture comparison at the Obsession Telescopes homepage to stoke aperture fever. wink.gif 

https://www.obsessiontelescopes.com/


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#12 vtornado

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Posted 10 December 2021 - 09:04 AM

A 127mm mak or 6 inch SCT would be a nice step up either would double your light gathering.  They can be put on a tracking mount.

 

A 6 or 8 inch dob would probably be cheaper, and an 8 inch dob will perform better.

 

The dobs can be made push to with an angle meter and degree circle.

 

As the aperture increases, cooling becomes a factor.  There are ways to mitigate this

but of course it takes more fuss.



#13 Barlowbill

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Posted 10 December 2021 - 12:01 PM

I have an ETX90-AT and an 8" Dob.  I love my "Moon Killer"  90mm.  Easy to move.  While I wish it was a 125mm, I would not be interested in replacing it with a 125mm for the same reason I would not be interested in replacing the 8" Dob with a 10".

Sure, I wish the 8" was a 10" but I wouldn't switch at this point.  I would want to make a bigger leap to a 12" Dob.  Same with the 90mm.  Needs to be a bigger jump than a 125mm.  Good luck



#14 robodan

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Posted 12 December 2021 - 07:32 PM

The best thing with an SCT over the 10" dob is far less maintenance. The 10" dob is open tube, so mirrors exposed to the elements so the coatings will deterate faster and need cleaning far more often.

The 10" dob is very bulky also and requires far more collimation and wont keep it as well as the dob.

I have both a 10" dob and SCT 9.25, and it's short tube making portability much easier, in fact maybe 5x easier to handle.

Never have had to collimate it since I have has it in years and do star tests often to check it.

#15 avongil

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Posted 12 December 2021 - 07:57 PM

I have a 10xt and an ET-90 RA. 

 

The 10XT produces very soft planetary views. The focus its maddeningly difficult on them.  The 90 cooks up the planets crispy and just about right every time.  A little small but crisp and with far far more detail.  Don't get rid of it until you get something else first.  I will always have some sort of Maksutov.  no fuss little guy.  I take it out, plot it on the ground and either use it alt az or rough align with polaris. 

 

If you really want a jack of all trades I feel like the 8" SCT is it.  I have a C8 on an advanced GT mount. I really hate the mount, but the scope is incredible.  Produces views just a hair less bright than the 10, but makes the planes look so good you swear they are fake.  Double stars are miles apart in the finder.  A 30mm 82 deg 2" eyepiece give you a fantastic space walk.   

 

The only issue is it does not get much use because of the mount.  I keep going back to the ETX-90 mount.  The 884 tripod is a dream. Converts to polar mode in 2 seconds. The fork mount is very easy to use and its only downside is the dreaded plastic all over it making it wobbly and weak.  

 

Perhaps the golden ticket to having only one scope is an F10 6" or 8" SCT on a fork mount.  This is of course personal preference, but Im keeping my eyes out for a CPC or LX90 or similar.  Maybe just an alt az mount for those heave and go kinda days. 



#16 Anony

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Posted 12 December 2021 - 10:08 PM

I have a 10xt and an ET-90 RA. 

 

The 10XT produces very soft planetary views. The focus its maddeningly difficult on them.  The 90 cooks up the planets crispy and just about right every time.  A little small but crisp and with far far more detail.  Don't get rid of it until you get something else first.  I will always have some sort of Maksutov.  no fuss little guy.  I take it out, plot it on the ground and either use it alt az or rough align with polaris. 

 

You are getting better views (albeit smaller) of the planets with your little 90mm mak vs a 10" dob? Hmm...

 

The planetary views I get through my 8" dob look sharp to me, and I get that 'fake' look when the planets + atmosphere play nice together -- meaning it looks almost like someone pasted a photo of saturn from an astronomy book at the end of my scope.

 

I don't have a mak, but I have a C5 + slow 80mm refractor to compare to. I'd think the refractor is sort of similar to the 90mm Mak view (at least light gathering wise).. and I can get surprisingly decent views of the planets with it. Just that they are nowhere near what I can get in my larger dob.


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#17 Ulmer Spatz

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Posted 13 December 2021 - 05:30 AM

I've got an idea for a transport mechanism using an old transmission jack with some easy roll wheels.  And it wouldn't take much to build a concrete pad out back for it to be setup on.

Keep in mind that with most Dobsonians, you can separate and re-attach the optical tube from and to the base in less than a minute. With an 8", you'd wind up with two pieces at about 20 lbs. each. A 10" would separate into two pieces around 25 lbs. each. Of course, doing this would require at least two trips to get you set up. Some people do prefer to keep the scope all in one piece on a dolly.


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#18 Jon Isaacs

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Posted 13 December 2021 - 06:42 AM

The 10XT produces very soft planetary views. The focus its maddeningly difficult on them.  The 90 cooks up the planets crispy and just about right every time.  A little small but crisp and with far far more detail.

 

 

Something isn't right.  Even a perfect 4 or 5 inch refractor will not show more detail than a decent 10 inch Dob.  The usual suspects are collimation and thermal equilibrium.  With a larger scope, getting it cooled down to near ambient takes some time.

 

Jon


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#19 Anony

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Posted 13 December 2021 - 04:39 PM

Keep in mind that with most Dobsonians, you can separate and re-attach the optical tube from and to the base in less than a minute. With an 8", you'd wind up with two pieces at about 20 lbs. each. A 10" would separate into two pieces around 25 lbs. each. Of course, doing this would require at least two trips to get you set up. Some people do prefer to keep the scope all in one piece on a dolly.

Something that made my life much easier was simply buying a cheap strap-a-handle. Connect it to the scope, move it in two pieces. Super easy and my back thanks me.

 

Dolly would work too, unless one has stairs, then that's sort of a pain.



#20 spaceoddity

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Posted 13 December 2021 - 04:45 PM

Something isn't right.  Even a perfect 4 or 5 inch refractor will not show more detail than a decent 10 inch Dob.  The usual suspects are collimation and thermal equilibrium.  With a larger scope, getting it cooled down to near ambient takes some time.

 

Jon

Agreed, my XT10 beats my 8SE on planets hands down. Of course the seeing in PA being what it usually is, means that it's a rare night to get crisp planetary views with any scope.

 

I have a barlowed laser(glatter tublug) to collimate my newtonians and double check with a cheshire so I know my collimation is pretty close to perfect, much harder to determine with an SCT. I think my 8SE is OK but I'm no expert on star tests so I'm not certain. 


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#21 Dave Mitsky

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Posted 13 December 2021 - 05:32 PM

Resolution or the ability of a telescope to reveal detail is a function of aperture.  The larger that aperture, the greater the resolution.

 

https://www.chegg.co...on-of-telescope

 

The images may look crisper in a small aperture because the magnification being used is likely to be lower and the telescope is more likely to produce a more "aesthetic" image if the seeing is poor.

 

https://skyandtelesc...ing-the-seeing/


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