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New AT 6" and 8" Classical Cassegrain

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#1126 Bomber Bob

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Posted 29 September 2020 - 03:54 PM

Okay, looking just at the 2 x 8" CC versions...

 

AT  -->  https://www.astronom...n.html?___SID=U

 

GSO  -->  https://agenaastro.c...-ota-black.html

 

$900 each.  Not exactly cheap in my book, when a brand new GSO 8" F5 Newtonian is less than half that amount, but not excessive for what I consider a niche reflector.  I paid about $500 for a used Vixen VMC200L / F10 Field Mak-Cass.  I sold the VMC -- my vintage 1980s Meade 826 (8" F6 Newt) made it redundant -- but kept my antique Tinsley 6" F20 & vintage 1970s Edmund (3B) 4" F15 Casses.  I've been a fan of the design for 40 years -- so much capability in a compact / relatively lightweight reflector.

 

But... based on the content of this thread, I wouldn't recommend these CCs to a novice / beginner -- there are better-performing, easier to maintain reflector options at 6" & 8" apertures.  An experienced scope owner like myself... yeah, I'd be willing to try one, and improve as needed -- they're about 30% cheaper than a new VMC200L (and don't have that design's more complicated collimation requirements) -- if the mirrors are decent quality.  Because, the hardware on these CCs isn't expensive, I would expect the optics to justify the largest portion of the cost.  In that respect, rather like my $600 AT102ED, which is worth way more than I paid for it.

 

IMO, it would be fair to compare either of these CCs with a same-aperture mid-tier Newtonian for resolution, at least.  As for build quality:  Sounds like there are some issues, but nothing that I couldn't improve without harming the scope.  I mean, some folks swap the stock focusers on their $$$$ Takahashi refractors, so I don't consider that a show-stopper at this price point.

 

(I've been holding back my tax refund on the off-chance a vintage / antique 10" Cass pops up.  But... I may try one of these myself -- post a review of one.) 


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#1127 Thandal

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Posted 29 September 2020 - 06:06 PM

@Bomber Bob;  I agree it's definitely a niche instrument.  And based on my personal criteria, (of which cost was one) I went first for the 6" then, thanks to a fellow CNer who wanted to downsize, upgraded to the 8".

 

So far, given the extremely limited opportunities for using it that the weather has allowed, I've been pleased.  While I've been enjoying the views of solar system objects at higher magnifications, Jupiter and Saturn are now basically too low by dusk.  So Mars and Luna have been my targets recently.  (And Venus when I wake up early enough. lol.gif )

 

EDIT: Now, I'm on the hunt for a proper set of 2" eyepieces (new, or used) to make the best use of its (admittedly limited) FOV for the DSOs. cool.gif


Edited by Thandal, 29 September 2020 - 06:26 PM.


#1128 glend

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Posted 29 September 2020 - 06:25 PM

I would agree that the 8" CC is a niche scope, but importantly it gives you long focal length reach that your not going to get from an 8" Newt. As a planetary imaging scope, at f12, with high frame rate stacking, it would be pretty good.  As a Double Star splitting Hunter it is good. Don't expect to be able to compete with fast newts on nebula, or dim objects.

I am very happy with my 8" CC, for the uses mentioned, but I also have an 8" GSO f5 Newt for other things. The build quality of my CC is excellent, and importantly, the RC focuser alignment issues that plagued the GSO 8" RCs are not a problem in the CC, and i have owned a GSO RC previously. I actually bought a tilt corrector thinking i might need it, but it is not required in my experience. 


Edited by glend, 29 September 2020 - 06:30 PM.

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#1129 Bomber Bob

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Posted 29 September 2020 - 06:36 PM

At F15 & F20, I use my slow Casses the way I would a LONG refractor:  planetary, double stars, open & globular clusters, & planetary nebulae.  However, as I posted earlier, my 6" F20 gives remarkable views of the dust lanes in M31 @ 100x to 150x.  I didn't expect it, but I've enjoyed these views.  IF I could sketch worth a hoot...

 

But the biggest surprise:  Seeing an albedo feature on Ganymede with a "small" 6" Cassegrain.  I didn't know that was possible until I saw it.  Seeing that night was near-perfect for planetary; otherwise, I doubt it would've happened.  I wouldn't expect that from these mid-tier CCs; but, I would expect the 8" model to show at least as much detail inside the GRS, or within Jupiter's EQ belts as my 8" F6 Newtonian at the same magnifications.

 

I am very happy with my 8" CC, for the uses mentioned, but I also have an 8" GSO f5 Newt for other things.

 

If you can afford multiple scopes, that's the way to deal with their strengths / weaknesses.  I wound up with 3 x 6" reflectors:  Tinsley F20 Cass, and Criterion RV-6 & Edmund F4 Newts.


Edited by Bomber Bob, 29 September 2020 - 06:41 PM.


#1130 jgraham

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Posted 29 September 2020 - 09:47 PM

Viewing deepsky objects with a long focal length scope may be something of an acquired taste. After fiddling with focal reducers I found that I prefer to leave my scopes at their native focal lengths and use long focal length, UWA eyepieces when the mood strikes. I really enjoy the 8" f/12 CC! One of these nights I'd like to compare it with my Mak 7, but to be honest I really don't care, they are both wonderful scopes.
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#1131 quilty

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Posted 30 September 2020 - 03:11 AM

that's why I mentioned the 152/740 Mak-Newton. It seems to bo very good at any object, DSO as well as at high power. And it seems to be in stock before Christmas again

And given the specs of 6 and 8 CC there was never a question  what they're disigned for and this is not DSO. Comparing them to fast f/7 refractors makes sense only at high power. In other words f/7 ed and apo refractors are only a (way more expensive) alternative to the CCs when they do as well at high power, and they seem to do or even to do better.

And here, where I'm living seeing conditions are moderate specially much light pollution, therefore my targets are bright but small, little chance of finding nebulae, so no DSO except star constellations. Therefore my liking of small but high performing scopes. And that's why I wonder if the cc6 is a keeper.


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#1132 Thandal

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Posted 30 September 2020 - 07:04 AM

<snip>

And here, where I'm living seeing conditions are moderate specially much light pollution, therefore my targets are bright but small, little chance of finding nebulae, so no DSO except star constellations. Therefore my liking of small but high performing scopes. And that's why I wonder if the cc6 is a keeper.

 

Only you can be the judge of whether a 6" CC "is a keeper".  wink.gif

 

I found it to be a great (small) instrument for what I wanted to do, but as it weighed more than my (small) mount could handle and I had to upgrade that, I decided I wanted the 8" model.  And I lucked into a great deal on one, so that's now what I have.  waytogo.gif

 

The weather has been miserable for observing almost all spring and summer, and so far autumn hasn't been much better.  Tonight may be the first time it will be worth setting-up in over two weeks.  If so, I'm going to try some "smartphone imaging".  Been reading that thread and have learnt a couple tricks that might make it possible for me to grab a decent pic.  I'll post something here if I'm successful.  Here's hoping!  grin.gif



#1133 quilty

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Posted 30 September 2020 - 12:09 PM

Mars is waiting. Last week I had some glances at 300x which was defenitely better than 225x, seeing a bit of landscape and icecap on top (should be north, shouldn't it?) and a slight idea of southern ice? which then is a bigger area. But still a dwarf at 300x like Jupiter at 225x :-). Thought Jupiter was a giant but it aint. A blue filter might do for mars, yellowgreen helps just a little.

@Terra: Beeing collimated once, there's no need to recollimate neither when changing eyepiece, rotating eyepiece, exchanging diagonal or inserting or removing extension ring. 

Removing or inserting another 1-inch extension might be necessary when changing from 1 1/4 to 2 inch diagonal but again, no problem at all. And why should it? Why should a square focusser or eyepiece be more an issue than at any other scope? Anyway, inserting extension rings might be necessary due to the long focal length, the very same you'll find at any other scope of the same focal length unless it isn't focussing the primary mirror or the focusser itself is extra long. The latter could be an upgrade point, but me, I'm fine with just one 1 inch extension. I think, if someone needs more than a 1 inch extension for just watching it might help to recollimate at maximum mirror distance or near that, for maybe a position at the very end is not advisable.



#1134 Peter Besenbruch

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Posted 30 September 2020 - 07:00 PM

Mars is waiting. Last week I had some glances at 300x which was defenitely better than 225x, seeing a bit of landscape and icecap on top (should be north, shouldn't it?) and a slight idea of southern ice? which then is a bigger area.

Currently, the South Polar Cap is visible. The North Polar cap isn't, but there are a lot of clouds to the north.

 

We get to see the southern hemisphere when Martian oppositions are favorable. When Mars and the earth are have a relatively distant opposition, we get to see the northern hemisphere. Thus, until we sent a bunch of orbiters over there, we knew significantly more about the southern hemisphere of Mars.


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#1135 luxo II

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Posted 30 September 2020 - 07:19 PM

 

And I don't think one should compare a 102mm (4") f/7 refractor to a 6" (or greater) f/12 instrument of any design.  And a Mak (or any other catadioptric 'scope) has it's own issues.

Of course you should compare scopes, occasionally.

 

I'll volunteer - my 20 year-old MK66 Deluxe (6" f/12) is up for any side-by-side bakeoff at the same location, same object, same seeing conditions and at the same magnification. What you get is what you see. And no, it doesn't have "issues", despite its age.

 

One interesting possibility would be a 6" f/12 old-school newtonian in a fully enclosed tube - I have seen and used one, and it was superb visually.


Edited by luxo II, 30 September 2020 - 09:29 PM.

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#1136 quilty

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Posted 01 October 2020 - 02:01 AM

Currently, the South Polar Cap is visible. The North Polar cap isn't, but there are a lot of clouds to the north.

 

We get to see the southern hemisphere when Martian oppositions are favorable. When Mars and the earth are have a relatively distant opposition, we get to see the northern hemisphere. Thus, until we sent a bunch of orbiters over there, we knew significantly more about the southern hemisphere of Mars.

Yes, Peter I heard so. So the south pole is on top of mars at moment and mars moves to the right as earth turns. Our north pole is on top, when I'm in the northern hemisphere, so considering mars and earth orbits like one plate with up (north) side and down side I think our north pole and the momentarly visible icecap on mars are the same side. But that might be thinking too simply, I'm afraid. 

And south pole now seems to be comlpetely hit by sunlight and north pole should be completely in the shadow and hidden unless its area is bigger. This suggests that martian rotation axe is tiltig like the earth's. But clouds could explain the white rim on north pole. 



#1137 quilty

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Posted 01 October 2020 - 02:32 AM

Of course you should compare scopes, occasionally.

 

I'll volunteer - my 20 year-old MK66 Deluxe (6" f/12) is up for any side-by-side bakeoff at the same location, same object, same seeing conditions and at the same magnification. What you get is what you see. And no, it doesn't have "issues", despite its age.

 

One interesting possibility would be a 6" f/12 old-school newtonian in a fully enclosed tube - I have seen and used one, and it was superb visually.

A Mak again. Seems to really be a smart design. This is about the GSO CCs, but the Bresser 5 inch Mak was my first scope and when comparing it's getting better and better. At 6 inch size my first choice was the Bresser Mak again, just for its fixed primary and Hexafoc-rack drive. Would you think the MK66 to perform significantly better in optical terms



#1138 luxo II

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Posted 01 October 2020 - 02:41 AM

Side by side with a new CC I don't think you'd see much difference, possibly none. If you want a fixed-mirror 6" mak, Intes MK67 was that. A 6" f/12 Newtonian would possibly be better baffled, but at the expense of a looooong tube.

 

But take a 20-year old CC or newtonian ... I doubt the mirror coatings will be in great shape.


Edited by luxo II, 01 October 2020 - 02:44 AM.

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#1139 quilty

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Posted 01 October 2020 - 04:59 AM

oh, if the MK66 performs simliar to my 5" Mak but on a 6/5 scale, it's supposed do do significantly better.



#1140 Thandal

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Posted 01 October 2020 - 08:24 AM

And south pole now seems to be comlpetely hit by sunlight and north pole should be completely in the shadow and hidden unless its area is bigger. This suggests that martian rotation axe is tiltig like the earth's. But clouds could explain the white rim on north pole. 

 

The view in (almost) every astronomic telescope is inverted, flipped top-to-bottom.  In certain designs it is also flipped left-to-right.

 

As all such directions are arbitrary, (What defines "up" or "west" for the universe?) most astronomers, amateur and professional, don't care and once used it, ignore it.  It only becomes relevant when describing where something is (e.g. a polar cap) in relation to something else (e.g. the rest of Mars.)  Then, it matters and it's important to know the "compass" directions as seen through the instrument or on the image in question!

 

Oh, and Mars' axis of rotation isn't "tilting" on any timescale we can observe directly.  But it is is tilted 25° with respect to its own orbital plane, which is 1.85° with respect to the ecliptic, (Earth's.)  That's enough that we see different amounts of the northern or southern hemispheres during different times in our respective orbits (and during different orbits!) around the Sun.  Sometimes more of one, sometimes more of the other, depending...  cool.gif


Edited by Thandal, 01 October 2020 - 08:42 AM.


#1141 quilty

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Posted 01 October 2020 - 08:55 AM

inverted yes but not always bottom-down. I think the icecap is on top of mars when we look at it standing. And it is completely sunlit, that's why I thought it must be sometimes in the shadow otherwise it would melt down as our north pole does in spite of long polar night.

With 25° permanent tilt to its own orbital plane there should be just one icecap, the one that's permanently hidden from the sun even more so considering the climate changelol.gif. But this doesn't seem to be the case.



#1142 Thandal

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Posted 01 October 2020 - 08:57 AM

<snip>

A 6" f/12 Newtonian would possibly be better baffled, but at the expense of a looooong tube.

Good luck finding one!

 

Even the classifieds don't have anything in reflectors slower than f/8.  At least, not in the first half-dozen pages I checked. lol.gif


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#1143 Thandal

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Posted 01 October 2020 - 09:07 AM

<snip>

With 25° permanent tilt to its own orbital plane there should be just one icecap, the one that's permanently hidden from the sun even more so considering the climate changelol.gif.  But this doesn't seem to be the case.

 

It's because of the tilt that Mars has seasons, just like Earth.  Australia isn't "permanently hidden from the sun" and neither is Mars' "North" Pole.

 

It just so happens that as we look at Mars today without any optical aid, during this particular opposition the SOUTHERN ("bottom", in the conventional sense) hemisphere is tilted towards us.  With optical aid, features become visible.  And the brightest one is the SOUTHERN Polar Cap.

 

Depending on an instrument's design, and the observer's orientation to the eyepiece (or image) that "south" pole might be "up", "down", or "sideways".  waytogo.gif


Edited by Thandal, 01 October 2020 - 09:10 AM.


#1144 quilty

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Posted 01 October 2020 - 10:23 AM

It's because of the tilt that Mars has seasons, just like Earth.  Australia isn't "permanently hidden from the sun" and neither is Mars' "North" Pole.

 

It just so happens that as we look at Mars today without any optical aid, during this particular opposition the SOUTHERN ("bottom", in the conventional sense) hemisphere is tilted towards us.  With optical aid, features become visible.  And the brightest one is the SOUTHERN Polar Cap.

 

Depending on an instrument's design, and the observer's orientation to the eyepiece (or image) that "south" pole might be "up", "down", or "sideways".  waytogo.gif

I believe that at moment we're seeing the southern pole but not yet get it. Standing there and looking at mars without any device Mars is moving to the right due do earth roataion. And the icecap is on top isn't it? But this is off topic I need to research how mars works.



#1145 quilty

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Posted 01 October 2020 - 10:35 AM

Got it, I think. rotation axis is stable while revolutioning around sun thus yielding summer and winter. And wiki tells martian polaris is Deneb, which means Deneb is standing right above martian north pole. I will check that next possiblility.



#1146 Spacedude4040

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Posted 01 October 2020 - 11:57 AM

Hmmmmmm ........ I was just wondering if any other manufacturer was reading these 46 pages and willing to build a true Classical Cassegrain. I believe there is a market even at twice the price of these GSO  Cassegrain what ever they are?

The 8” Gso CC can’t keep up with a 8” f6 newt due to the laws of optics but a real CC would be right there.

I always love the measurements of “good enough for me” “works for me”  “should be the same etc.

These are great beginner scopes with a lot of bang for the buck but will fall short next to a real Classical Cassegrain.

Mike



#1147 Bill Barlow

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Posted 01 October 2020 - 12:02 PM

Side by side with a new CC I don't think you'd see much difference, possibly none. If you want a fixed-mirror 6" mak, Intes MK67 was that. A 6" f/12 Newtonian would possibly be better baffled, but at the expense of a looooong tube.

 

But take a 20-year old CC or newtonian ... I doubt the mirror coatings will be in great shape.

I owned an Intes MK67 MAK and it has a moving primary mirror.  I believe the MK66 has the fixed primary and uses a focuser mounted onto the rear cell.

 

Bill



#1148 Thandal

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Posted 01 October 2020 - 12:21 PM

<snip>

... but will fall short next to a real Classical Cassegrain.

 

Hmmm... Why do you say these (GSO-built) CCs aren't "real" ones?

 

A fixed parabolic primary and a hyperbolic secondary as the only optical elements ahead of the focuser/eyepiece/imager.  IFAIK, that's it for criteria for a CC. 

 

1678793-1601573505.png

 

Although I'm not sure if adding a tertiary mirror to divert the light path out the side means it's not still a true CC, I have had the privilege of using the Link 36" CC telescope (before COVID-19 closed it down.)  smile.gif 
 

 

1678793-1601572718.png

 

 

If I'm mistaken, what would make the GSO instruments "real" CCs?


Edited by Thandal, 01 October 2020 - 12:32 PM.


#1149 Spacedude4040

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Posted 01 October 2020 - 01:22 PM

Hmmm... Why do you say these (GSO-built) CCs aren't "real" ones?

 

A fixed parabolic primary and a hyperbolic secondary as the only optical elements ahead of the focuser/eyepiece/imager.  IFAIK, that's it for criteria for a CC. 

 

1678793-1601573505.png

 

Although I'm not sure if adding a tertiary mirror to divert the light path out the side means it's not still a true CC, I have had the privilege of using the Link 36" CC telescope (before COVID-19 closed it down.)  smile.gif 
 

 

1678793-1601572718.png

 

 

If I'm mistaken, what would make the GSO instruments "real" CCs?

 

Well from my figuring and not really wanting to go down that worm hole again.

I would say they are not using a 6” Parabolic mirror. My belief is they are using their 6” RC primary and trying to use what they think is the parabolic area of that primary as a RC primary basically has two focal points blended together from my understanding. So like inside 2/3 is parabolic and outside 1/3 is tipping away from parabolic. That’s why RC telescopes have large secondaries to catch the angle change that tips away because this helps to reduce comma and flatten the field.

So doing the flash light test I didn’t see a very sharp edge at the 138mm mark so the reduction of the primary may be even more then we think because of the fanning outwardness of the RC primary. 

Oh no it’s pulling me in.

Mike



#1150 Spacedude4040

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Posted 01 October 2020 - 01:33 PM

The reason why it will fall short is it’s not using the full resolution or light gathering ability of a 6 inch parabolic mirror. The next problem is the obstruction of the mirror retaining ring which gives you a 44% obstruction! I don’t know if it relates to the 8” thou, so it might be one reason why the 8” is better then the 6” if they use the same ring.

Somebody needs to measure it.

In my eyes a 8” f6 will have more Resolution, light gather ability and a smaller obstruction maybe around 20%

Mike


Edited by Spacedude4040, 01 October 2020 - 01:36 PM.

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