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Old 29-03-2020, 04:49 PM
glend (Glen)
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New 10" GSO Cassegrains have arrived, pondering

So i see the new 10" f12 GSO Truss Cassegrains have finally arrived, and I have been pondering how this scope would be useful. At 3m focal length it seems a good platform for planetary, double/triple star, and planetary nebula observation, and high frame rate video stacking ( which I am not set up for yet), and honestly may never get around to at my age.
I have watched the Orion video of their version of the 8" tube model, and read the Cloudy Night thread on collimation of the 8" model. Given that these scopes seem to be built out of the GSO RC parts bin, except for the mirrors of course, I am leery of stepping into more drama with those GSO focusers (which require the optional tilt ring to overcome their natural misalignment). But maybe better the devil you known eh? I owned a GSO RC8 for a couple of years.
So the question to the few owners out there, and I know of one at Mt Tambourine, is how are you finding the collimation. Both out of the box setup, and dealing with the stock focuser wonkiness? Any general feedback or comments welcome.
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Old 29-03-2020, 07:17 PM
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Merlin66 (Ken)
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Could be an idea instrument for spectroscopy.......
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Old 30-03-2020, 11:50 AM
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What are the advantages of a Cassegrain over an RC? Longer focal length for the same aperture?

Greg.
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Old 30-03-2020, 12:51 PM
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mental4astro (Alexander)
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Greg,

This GSO is a "classical" Cassergrain (CC of the purposes of this post). RC's are also Cassegrains... (dah...)

The differences are a few, for which one needs to then make their own judgement as to which optical system best suits their requirements.

* A CC will have a smaller central obstruction & smaller diameter baffle on the primary.

* A CC is typically slower in focal ratio, anywhere from f/10 to f/20

* An RC will often have focal reducers and field flatteners made for them. CC's can have field curvature issues depending on the design of the primary and secondary mirrors. This field curvature has been rectified with other designs, such as the RC, Dall Kirkham, Klevtsov, and even the Schmidt and Maksutov.

* RC's were designed as a photo instrument from the first instance. Visual use is a secondary concern.

There is no one perfect instrument. And every person needs to then evaluate each design on its merits for what their own purposes are. I have tried to give an unbiased summation on the properties of each, and NOT give a pro and con for the very same reason above that each person needs to make their own call on the suitability of any given design. Some people only want photo, others visual and other a bit of each...

What does not change is $$$ will often influence the final quality, or average quality of mass production instruments, an this includes mechanical as much as optical.

Alex.

Last edited by mental4astro; 30-03-2020 at 03:04 PM. Reason: typo
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Old 30-03-2020, 01:31 PM
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I'm using a meade f /6.3 focal reducer with my RC6 for EAA and it's great for my needs. this is the maximum reduction I can use as there is no more in focus. Here is an Image taken with my ASI385 mc camera with the focal reducer.

Martin
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  #6  
Old 30-03-2020, 02:28 PM
N1 (Mirko)
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Like the motor focuser Martin
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Old 30-03-2020, 02:50 PM
glend (Glen)
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I am posting a link to the Orion salesman review, of the 8" tube based version

https://youtu.be/6_25qjndvm0

I make the point that this is a salesman talking. If you listen at about 1:20 into the video he mentions flat field correction in passing. I have no idea if he is correct about correction.
Chances are the 10" design is the same, at least in terms of the mirrors, focuser (as far as I have been able to deternine). The 10" truss design will not have the baffling that the 8" tube version does, although there is likely a baffle tube below the secondary as in the RCs. Central obstruction is 34% which is less than a RC.

And Greg, yes longer focal length than the RC08, as it's an f12, it will be 2400mm for the 8" version and 3000mm for the 10" version.
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Old 30-03-2020, 09:20 PM
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Wow, what a snake oil salesman, this guy from Orion..

The CC is a telescope not an astrograph. Even the RC needs a field flattener to become an astrograph.
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Old 31-03-2020, 06:17 PM
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Quote:
Originally Posted by mental4astro View Post
Greg,

This GSO is a "classical" Cassergrain (CC of the purposes of this post). RC's are also Cassegrains... (dah...)

The differences are a few, for which one needs to then make their own judgement as to which optical system best suits their requirements.

* A CC will have a smaller central obstruction & smaller diameter baffle on the primary.

* A CC is typically slower in focal ratio, anywhere from f/10 to f/20

* An RC will often have focal reducers and field flatteners made for them. CC's can have field curvature issues depending on the design of the primary and secondary mirrors. This field curvature has been rectified with other designs, such as the RC, Dall Kirkham, Klevtsov, and even the Schmidt and Maksutov.

* RC's were designed as a photo instrument from the first instance. Visual use is a secondary concern.

There is no one perfect instrument. And every person needs to then evaluate each design on its merits for what their own purposes are. I have tried to give an unbiased summation on the properties of each, and NOT give a pro and con for the very same reason above that each person needs to make their own call on the suitability of any given design. Some people only want photo, others visual and other a bit of each...

What does not change is $$$ will often influence the final quality, or average quality of mass production instruments, an this includes mechanical as much as optical.

Alex.
Thanks Alex.

I do rememberJohanne Schedler's CC and he would have it in 2 configurations, one was with the primary mirror at F3 and the other was a longer F ratio. So he got great wide field as well as galaxy type images from the same scope. The main thing about his images was how tiny his stars were. Routinely they were way smaller than others were getting.

Do these CC's allow a removal of a part that allows the above?

Greg.
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Old 31-03-2020, 06:51 PM
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Quote:
Originally Posted by gregbradley View Post
Do these CC's allow a removal of a part that allows the above?
As a CC, with VERY long focal lengths...

Make the focal length long enough and the radius of the curved field becomes so long that the field curvature peters out to nearly flat at the illuminated area.

Now at f/3, this is as a Newtonian, so the appropriate coma corrector would need to be used to produce a flat field for photo.

Parks still makes three dual focal length Newt/Cassegrain scopes:

Parks 10" f/4 & f/15 HIT Superior System OTA can be had without mount.

Parks 12.5" f/4 & f/12 HIT Superior System OTA can be had without mount.

Parks 16" f/4 & f/12.5 HIT OTA

Alex.
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Old Yesterday, 10:38 AM
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Thanks Alex.

Yes I think Johannes had a corrector lens he put in the system and removes the removable secondary.

Ceravolo has an astrograph that has a widefield and a long focal length corrector lens. 12 inch. A bit similar to the AstroPhysics
Riccardi Honders - different design but a bit similar.

I suppose I could put a Tak 1.6 Extender on my AP Honders and use a smaller sensor or crop.

Greg.
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Old Yesterday, 10:46 AM
glend (Glen)
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Gents I believe these last couple of scopes discussed are way above the average Joe sort of budget. I am still looking to hear from any GSO Cassegrain owners, I know they are here somewhere. Michael from Mt Tambourine I think is one,
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Old Yesterday, 11:16 AM
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mental4astro (Alexander)
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Glen, yes those scopes are beyond the price of the average Joe, but they are mentioned to illustrate examples to questions that have been asked.

As for the average Joe thing, I am not one to push only $$$$$. I certainly am aware of needing to respect the budget of EVERYONE who participates in astro. But I do feel it is necessary to express what potential pitfalls there are to buying BOTH cheap and expensive. Buy cheap, and you will be accessing thousands of pages to "fix" stuff that shouldn't need fixing if designed and built appropriately - not a major problem if you are a tinkerer, but not everyone is and this can lead to frustration and disappointment. I do have a GSO solid tube Newt in my personal collection, but it is heavily modified exactly because of the shortcomings. Read through my many posts here on IIS and you will find I readily offer help on how to remedy many of these problems.

Buying $$$$$ is no guarantee of a trouble free experience either. All scopes are highly technical things (mechanics and optics, mounts, housing, etc), and these all need considered attention too to produce good results.

And I sincerely hope that these GSO Cassegrains do perform optically as hoped. I honestly would love to put one of these 10" Cassegrains through its paces! And if they stack up then blooming brilliant! I would hate to ever become a Brand Snob. Just don't be under any illusion that you will be getting overall Questar quality on a GSO budget.
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Old Yesterday, 12:37 PM
glend (Glen)
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Sure Alex I understand. My previous experience with a GSO RC08 was not a good one, and as indicated it appears the focuser on the CC is identical to the RC, hence my seeking owners to comment. I don't mind a bit of tinkering but I am not going to get as involved as Paul H did in fixing up his GSO RC. My main concern, as outlined by the CN thread on this scope,


https://www.cloudynights.com/topic/6...al-cassegrain/


is the focuser tilt issue that carries over from the RC and cannot be fixed properly without remachining (But can be worked around with the focuser collimation ring which is an optional extra of course). But the focuser alignment potentially needs readjusting whenever you dial the focuser in or out. I used to watch my laser dot wander away from the secondary centre spot on my RC as the focuser was racked in or out of the original setup point.
I would love to try out the GSO 10" truss Cassegrain but i doubt they would let me buy on approval.
Sort of sounds like I have talked myself out of it.

Last edited by glend; Yesterday at 12:52 PM.
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Old Yesterday, 03:32 PM
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There are only a few third party external focusers that have tilt adjustment. You can count them on one hand with fingers to spare. Tilt can be a problem for all cassegrains, not just GSO's, and Newts too. Sometimes it is not focuser tilt that is the source of the fix. Sometimes is the position of the rear cell in the tube that's all that needs adjusting to achieve exactly the same thing - the rear cell may not be sitting totally hard up all around the tube for example.

Then load the focuser up with a camera and associated stuff, and not only is the focuser challenged, but the structural integrity of the OTA is also due to flex. The design of the rear cell of a Cassegrain is vitally important for this reason, as much as the tube of a Newt. Carbon fiber tubes need very careful care to avoid flex (but that's a different topic).

All Cassegrains and Newts will need tweaking when you get them and every now and then, unless you are very bloody lucky. What most compromises the collmation process is the design and build of a scope. Poor choice of materials, components and fabrication will lead to major frustration. HOWEVER, it is good to know what the challenges are so when faced with a difficulty (regardless of what scope you have), it will help you nut out what needs tweaking - be it just collimation or a material aspect of your scope. And if there is mirror shift when focusing, you need to do ALL your collimation and focusing ONLY IN ONE direction in the turning of the focuser. With mirror shift, you CANNOT achieve collimation both inside and outside of focus. You need to work with your scope, not against it

Just don't jump to conclusions before you either dismiss buying something or chucking a dummy spit in frustration and pinging the scope over the fence... What spending $$$$$ will do is reduce the number of potential problems, but won't eliminate them all. I still struggle with collimation sometimes, and it means I need to take a step back for a break or ask someone as I may have missed something as frustration won't let you see past your nose.
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Old Today, 04:44 PM
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Tilt exists in most systems and has to be adjusted for. It could come from numerous sources apart from the scope. Adapters not seating properly or dovetails that the set screw has bitten into not quite in the valley of the metal etc.

Focusers may not be straight or flex, camera sensors may not be level, adapters not 100% square.

The faster the scope the more critical this becomes.

I think just becoming proficient at packing out your camera/filter wheel conquers this fairly easily once you become adept at it.

I have gotten it down to around 10-15 minutes. I also make a note of the packing details on my computer for next time as I often take the camera off and use it on another scope.

There are also several tilt adapters from Teleskop Services if you have the back focus to fit one.

The trend in scopes for the last several years has been towards faster and faster optics. Up to the point where people get frustrated trying to tame the fast scopes and scopes will get a little bit slower.

In my opinion F3.6 to F5 is the sweet spot for imaging. Faster then that, then expect to spend some time doing some fiddly adjustment.

Greg.
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