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New AT 6" and 8" Classical Cassegrain

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#1026 highfnum

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Posted 06 February 2020 - 09:34 AM

good ronchi

lines are parallel (if not mirror tilt)

not too bumpy (optical roughness)

the lines should not flare out at edge(turn down/up edge)

lines should be straight if they have a slight curve in or out thats a sign of SA under or over corrected

 

there is limit on star test ronchi

DPAC ronchi better but you need

setup


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#1027 Asbytec

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Posted 06 February 2020 - 10:34 AM

I quite agree that the third ronchi looks way better. Guess once I'll have to bother with ronchi, how it works and what it needs. And then maybe I can tell what that means, "looking good". When it is a good assessment for opical resolution can't it be used to better collimate the scope?



You can check your mirror spacing if you put the Ronchi on the known focal plane location and using a star. The Ronchi will null on a star in focus. Looking good is clean straight lines in a star test. Never heard of collimation with a Ronchi. It might be possible, but I question the precision trying to read off axis coma. Interesting question, though. Gotta read and consider it.

Edited by Asbytec, 06 February 2020 - 10:36 AM.

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#1028 macdonjh

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Posted 24 February 2020 - 01:05 PM

I tried the flashlight/ laser aperture test with my Driveway Scope (not an AT classical Cassegrain, but I tried it anyway).  I got interesting results.  I first tried it with my green laser.  The image of the laser on the wall made it look like something in my scope isn't collimated or is off center.  Interestingly, I can still form a pleasing image with it.  

 

Fast forward a week or so to last night.  I used my Hotech collimation laser which is somewhat self-centering in your eye piece holder.  At least much more self-centering than a hand-held green laser.  It still appears some component is not properly centered.  Focuser, primary baffle, secondary mirror?  I'll have to track that down.  My collimator is much fainter than my green laser and what appeared on the wall was reflections from the spider vanes.  Apparently they are much more reflective than their black paint would suggest.

 

I'll be taking that scope apart eventually to install a secondary baffle and correct (if necessary) the mirror spacing.  I hope to fix all of that when I do.



#1029 quilty

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Posted 25 February 2020 - 09:56 AM

you don't need any laser for the aperture test. A simple led torch will do. And even if the eyepiece is focussed or not has little impact, at least at my scope.Guess it matters that the light beam fills the whole exit pupil. You then can perfectly read effective aperture and obstruction.



#1030 mclewis1

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Posted 25 February 2020 - 10:02 AM

For North Americans ... "Torch"  is just another term for a flashlight.   wink.gif



#1031 quilty

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Posted 25 February 2020 - 11:06 AM

Oh ja, for Germans: we thought that a "torch" is just a portable light. Like your famous Maclight.



#1032 quilty

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Posted 31 March 2020 - 05:36 AM

long time no hear! looking back I see me to have been the last one to contribute, so you could blame me for the silence.

Meteorologic spring brought real winter in terms of beautiful snow and freezy nights. So come that just now I cared for cooling down time of the 6 inch cc for the first time.

Surprisingly it takes about 15 long  minutes to produce an acceptable vision after taking the scope from 20°C indoors to zero out of doors. Refocussing is necessary during next hour several times.

To me this is considerably longer than expected, for instance compared to my 5 inch Mak.

Just when doing long time exposures (which I don't do at moment) you wuld have to wait an hour or so or to install an autofocus device.

 

Is this along to your experience?



#1033 Vinny1980

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Posted 16 May 2020 - 10:09 AM

I bought the 8" from Agena Optics couple of weeks ago, and my impressions on the scope are overall very positive. I wrote a preliminary, quick review (in italian) here:

 

http://forum.astrofi...hp?f=3&t=106313

 

thay you can try to translate with Google..good luck smile.png

A star test is attached, which shows very good correction from SA and a slight amount of astigmatism (not worrying, for sure).

Im preparing a full review on my website, hopefully (if weather cooperates, which now is far from true) with a Roddier test attached.


Edited by Vinny1980, 16 May 2020 - 10:38 AM.

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#1034 ATY

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Posted 16 May 2020 - 12:58 PM

I have my Orion version of this 8" Classical Cassegrain for about a week now and I must say I am very impressed. I have had many telescopes come and gone in the past 30 plus years. These scopes ranged from Newtonian, Dob, SCT, APO refractor, and from 4" to 14" in sizes. 

One morning at 4 a.m. PST I had the Explore Scientific 82 degree 6.7 mm eyepiece in for 358x magnification on the moon and the sharpness and contrast were unreal. There was no image breakdown at all and the contrast was just excellent the black was black and the white was white. I have never seen the moon so razor sharp and high contrast through any my previous telescopes.

I think the open tube design, the eleven light baffles,  no front corrector plate, fixed primary mirror, Crayford style 2-speed focuser all contributed to great images.

The scope was slightly out of collimation when I received it and I re-collimated it using my Cheshire eyepiece. I did have to adjust both the secondary mirror and the primary mirror. Since the secondary mirror was center spotted so collimation was quick, easy and accurate. Orion did provided a peep hole kinda end cap for collimation but don't count on it. It was a joke. The peep hole wasn't even drilled center to the end cap and the small black donut ring tapped on the inside was also off center. Even doing the collimation a in bright room or outside you still couldn't quite see the black donut reflection so spend an extra $20 and buy yourself a Cheshire eyepiece. 

One thing I still have to get use to is the fact that this is a fixed primary mirror design so focusing is done by moving the eyepiece in and out, Orion included one 2" extension ring and two 1" extension rings, and a 3" travel drawtube to deal with back focus requirement for all sort of eyepieces and equipment you may want to hang at the back. But that also produces a very long train at the back of the scope and makes the scope back end heavy. I have a used a clamp on 2.5 lb counter weight on the bottom D-style dovetail plate.

The Orion or GSO 6" and 8" CC or RC designs are basically the same structure with the differences are in primary mirror figure and focal ratio. I am a visual astronomer and I rarely do astro-imaging so I picked the CC with f/12 for a little better contrast visually vs. the f/8 RC.

All in all for $1000 you have a very portable, decent aperture, and very good optic telescope for lunar and planetary viewing. My two thumbs up.


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#1035 glend

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Posted 16 May 2020 - 06:38 PM

my procedure was based on document in this thread

i believe spacedude4040

 

place star at center

look carefully for coma

adjust main mirror to get rid of coma as best as posible

then ajust secondary if it shows astigmatism 

put star in middle

if you still have coma adust

main mirror 

repeat above until you squeeze out errors 

 

i wasted a lot of time i should read before doing

i made matters worse so 

i had to undo errors 

if you do right it should be no more than 20 minutes 

Absolutely, I agree this sequence works very well on the CC. I would follow up with a Ronchi if I had one.



#1036 Vinny1980

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Posted 20 May 2020 - 03:46 AM

I ve read with interest what has been said about the clear aperture of these scopes, so yesterday I disassembled my 8" and made some measurements and calculations to understand things better. Here's my preliminary findings:

  • First, I m not sure if the "torch" test is always realiable, for various reasons another user has underlined in the other topic. I mean, one should take its results with a grain of salt, crossing its outcomes with complementary tests;
  • A very important point IMHO has been overlooked here (or maybe I ve missed it, so long thread!). An undersized secondary mirror would not be able to "see" the whole primary, when you look at the primary reflection in the secondary to collimate the last. Of course you have to put your eye at focus, which at design spacing is about 25 cm from rear cell. The GSO I own seems ok in this respect (but this is very critical, and needs further check);
  • Its quite straightforward to measure the RoC of the primary and then its focal ratio. From my measurements, the primary works around f/3;
  • It's also quite easy to measure the effective focal lenght with a plate scale solving or (as I did) on a planetary image, for which a software can calculate the actual sampling. The scope resulted to work at f / 11.5, very close to the advertised;
  • Last, and more important, putting all these info in ATMOS software I got a coherent design for an f/3-f/11.5 Classical Cass with 35% real obstruction and the real M1/M2 spacing I ve measured (around 42 cm).

The only elements which seems not coherent are the dimensions of the secondary baffle, but AFAIK not all manufacturer respects them.

Unfortunately here is cloudy since 2 weeks, anyway it would be interesting to experiment about 1) the spacing of the mirrors, which one can easily modify, in particular moving the little mirror and 2) removing the plastic baffles which maybe make some obstruction to the entry pupil.

I am not saying the I blindly trust the manufacturer about all what they say, since I ve seen more than a mess on these cheap instruments. But, until now, I did not find solid reasons to be seriously worried.

It s also maybe possible that the primary is not paraboloidal, but only optical testing could assess this. It could be hardly decryptable from a Company which has manufactured more than decent Newtonians and RC, though.

My 2 cents about what -performance wise- have proven to be very good telescopes, and, at least, an attempt to make popolar again the glorious Cassegrain design.


Edited by Starman27, 21 May 2020 - 11:45 AM.

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#1037 Terra Nova

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Posted 25 May 2020 - 06:17 PM

I love the idea of the 8", at least everything but the weight. I wouldn't need the Losmandy D-style mounting rail and would want to switch it out for the Vixen rail. Getting rid of the wide D rail would shave about two pounds off the weight. Anyone know if this is possible without physical alterations? The radius blocks on the bottom would need center holes to do so and Losmandy plates usually mount with two bolts at each end rather than one. Given that the effective aperture is less than the actual, the 8" has become much more appealing.


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#1038 eros312

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Posted 25 May 2020 - 07:36 PM

People have just been mounting it upside down.  There's a Vixen rail on the top and the focuser rotates. 


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#1039 Bill Barlow

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Posted 26 May 2020 - 09:21 AM

I ve read with interest what has been said about the clear aperture of these scopes, so yesterday I disassembled my 8" and made some measurements and calculations to understand things better. Here's my preliminary findings:

  • First, I m not sure if the "torch" test is always realiable, for various reasons another user has underlined in the other topic. I mean, one should take its results with a grain of salt, crossing its outcomes with complementary tests;
  • A very important point IMHO has been overlooked here (or maybe I ve missed it, so long thread!). An undersized secondary mirror would not be able to "see" the whole primary, when you look at the primary reflection in the secondary to collimate the last. Of course you have to put your eye at focus, which at design spacing is about 25 cm from rear cell. The GSO I own seems ok in this respect (but this is very critical, and needs further check);
  • Its quite straightforward to measure the RoC of the primary and then its focal ratio. From my measurements, the primary works around f/3;
  • It's also quite easy to measure the effective focal lenght with a plate scale solving or (as I did) on a planetary image, for which a software can calculate the actual sampling. The scope resulted to work at f / 11.5, very close to the advertised;
  • Last, and more important, putting all these info in ATMOS software I got a coherent design for an f/3-f/11.5 Classical Cass with 35% real obstruction and the real M1/M2 spacing I ve measured (around 42 cm).

The only elements which seems not coherent are the dimensions of the secondary baffle, but AFAIK not all manufacturer respects them.

Unfortunately here is cloudy since 2 weeks, anyway it would be interesting to experiment about 1) the spacing of the mirrors, which one can easily modify, in particular moving the little mirror and 2) removing the plastic baffles which maybe make some obstruction to the entry pupil.

I am not saying the I blindly trust the manufacturer about all what they say, since I ve seen more than a mess on these cheap instruments. But, until now, I did not find solid reasons to be seriously worried.

It s also maybe possible that the primary is not paraboloidal, but only optical testing could assess this. It could be hardly decryptable from a Company which has manufactured more than decent Newtonians and RC, though.

My 2 cents about what -performance wise- have proven to be very good telescopes, and, at least, an attempt to make popolar again the glorious Cassegrain design.

Have you compared the 8CC to your Edge8?  Is one better than the other for visual use?  Thanks.

 

Bill



#1040 ATY

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Posted 26 May 2020 - 11:52 AM

I love the idea of the 8", at least everything but the weight. I wouldn't need the Losmandy D-style mounting rail and would want to switch it out for the Vixen rail. Getting rid of the wide D rail would shave about two pounds off the weight. Anyone know if this is possible without physical alterations? The radius blocks on the bottom would need center holes to do so and Losmandy plates usually mount with two bolts at each end rather than one. Given that the effective aperture is less than the actual, the 8" has become much more appealing.

The CC8 comes with both Losmandy and Vixen dovetail rail installed. Losmandy on the bottom and Vixen on top. To use the Vixen rail just mount the scope upside down. BUT there is just one slight problem, there is no pre-drilled and tapped finderscope mounting holes on the bottom. I looked at my CC8 and I don't think you can switch places with the two rails because the mounting holes are not at the same place. Of course you can still mount a finderscope on the now underside.


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#1041 Vinny1980

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Posted 26 May 2020 - 02:08 PM

Terranova, Bill, and others,

first of all, let me say it's quite straightforward to arrange the scope the way you want. The bars are mutually reversibile, and you can laterally switch the finder as well. That's what I did for first with my scope, so no need to worry about that so much. Moreover, removing the Losmandy bar, the scope weights only 6 kg, and (unless the SCT's) is easily balanced in Dec with no additional weights on the bar.
I haven't performed a visual comparison with the C8HD, but Ive compared the star tests. The optical quality of the GSO is not inferior to that of the SCT, which makes round, not astigmatic stars but has some spherical aberration whose correction is excellent in the Cassegrain. To be fair, consider the C8HD needs 3 correctors lenses to achieve its performance while the Cass does all with 2 mirrors only.
I performed additional tests with my GSO 8", and since Im not aware of other reliable methods, I tried the "torch" check also. On my C8, it gave a perfect 200 mm circle, while on the Cass I got some 190 mm. If you observe the primary mirror reflection into secondary from the visual back (one does that in order to collimate the secondary), I carefully checked that you can see the whole primary only if your eye is close to the telescope. If you move your eye to 25 cm from the rear cell (i.e.the value of the design backfocus) you miss a part of the primary, and this defintely matches with an undersized secondary.
Last, but not least, as I said the baffles are too long with respect to my simulation, so one or both of them could cut the light cone giving a net effect of primary diaphragming. The proof of the pudding should be to shorten them and see what happens to clear aperture.


Edited by Vinny1980, 26 May 2020 - 02:29 PM.

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#1042 glend

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Posted 26 May 2020 - 03:28 PM

While I respect Vinny's view point, i don't know about encouraging folks to mod their CCs by cutting down the baffle tubes. As pointed out, GSO has done an excellent job of correction with just its two mirrors, and I suggest there maybe very good reasons for the baffle tube lengths as delivered.  It would be interesting to check out the 10" CC to see how that scope handles baffling and the image circle. 

If anyone plans on volunteering to chop baffle tubes, please post up your experiences here.


Edited by glend, 26 May 2020 - 03:29 PM.


#1043 Vinny1980

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Posted 27 May 2020 - 01:15 AM

Yes, it's true GSO did very good job, and longer baffles would be a design mistake. Anyway the baffles could be glued again, so it's likely I ll do the proof by myself.

For those interested in reading the full review (unfortunately only in italian, at the moment) here is the link:

 

https://www.vincenzo...ical-Cassegrain

 

I see, unofortunately, too many people scared by the collimation of the Cass, which is very simple if you don't proceed mechanically but spend some time in understanding what you are doing. I devote a paragraph on this topic. 


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#1044 Riccardo_italy

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Posted 27 May 2020 - 03:45 AM

Ciao Vinny, grazie per la recensione // thanks for the review!

 

I think nobody has tested if a reduction in the baffle size can restore the 200mm aperture, that's interesting and you should report back!


Edited by Riccardo_italy, 27 May 2020 - 03:46 AM.

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#1045 Aitrus3

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Posted 28 May 2020 - 11:36 AM

Man, slogging through 42 pages to not really get an answer for why I came to this thread...

I am thinking of switching scopes due to health reasons. I currently have a WO Z71ED (20th anniv.) with Flat6A. It's a beautiful little refractor and I love it to death. However, health issues have me unable to be awake at night. The odd day of insomnia I am up at 2/3am, but otherwise it's bed at 8, up at 4.  So my beautiful little refractor gets no time star time. Consequently I am thinking of switching from the Z71 to either the RC6" or the CC6" for planetary/moon photography and maybe, if I ever feel better, some brighter DSO objects. Keeping in mind, my budget is around $750 CAD assuming I get optimal resale value of my Z71+Flat6A. I'm using my Pentax K3II camera at the moment. I've seen images from the RC6" and they are something that I am impressed with, and are images I would be extremely happy with for galaxies/planetary nebulae.
Would I be better served though going for the Orion 6"CC given it's 1836mm? The moon images I saw were quite good.
Dew isn't much of an issue, cold temps are though. Live in the Okanagan region of B.C., so semi-arid desert but winters still get to -25C.


Edited by Aitrus3, 28 May 2020 - 11:38 AM.


#1046 jgraham

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Posted 28 May 2020 - 01:16 PM

I am currently doing a lot of imaging with an 8" GSO CC8 and I'm probably going to get the RC8 at some point. If my interest were primarily deep sky I'd suggest the RC since that is what it is really designed for and offer an wider field of view (f/8 versus f/12). However, the large secondary of the RC pushes more light out into the diffraction rings making the CC a better choice for high resolution imaging. The longer focal length of the CC makes it a little less suitable for deepsky, but I'm finding that the CC actually does a fine job with deepsky giving a surprisingly flat field with very little coma. Soooo, I'd suggest the CC over the RC for lunar and planetary over deepsky; the RC over the CC for deepsky over lunar and planetary.

 

Food for thought.


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#1047 Aitrus3

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Posted 28 May 2020 - 01:48 PM

I am currently doing a lot of imaging with an 8" GSO CC8 and I'm probably going to get the RC8 at some point. If my interest were primarily deep sky I'd suggest the RC since that is what it is really designed for and offer an wider field of view (f/8 versus f/12). However, the large secondary of the RC pushes more light out into the diffraction rings making the CC a better choice for high resolution imaging. The longer focal length of the CC makes it a little less suitable for deepsky, but I'm finding that the CC actually does a fine job with deepsky giving a surprisingly flat field with very little coma. Soooo, I'd suggest the CC over the RC for lunar and planetary over deepsky; the RC over the CC for deepsky over lunar and planetary.

 

Food for thought.

Awesome, thanks John! Confirmed my suspicions.
I've heard that there are focal reducers one could get with these scopes, but hard to find any info on any with the exception of GSO's reducer that states it's good for cameras <15mm diagonal sensor, ruling out my APS-C. With a FR, I would assume DSO would be a bit better on the CC8. It's a really hard toss up for me. RC decent for DSO and Ok for moon/planet. CC decent for Moon/Planet, ok for DSO. I do hate these decisions. This is why it took forever for me to pull the trigger on the Z71. Also, collimation really does seem scary for someone who has never done it before. Being mostly alone (not many in m area do this kind of thing) it took forever to figure out how to properly polar align, and then I was doing it for half a year with the polar scope out of focus, not realizing the focus could even be changed... I'm impressed I was even able to get it close.



#1048 Terra Nova

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Posted 27 July 2020 - 08:55 AM

Due to the pandemic I’m staying home most all the time and my observing at home now all of the time. I’m interested in doing something new and different. I’ve been using my old ATM 6” F4.5 Newtonian quite a bit this summer, so I think I’m going to order a 6” CC just for comparison sake. It could be fun now that we’re into planet season.


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#1049 macdonjh

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Posted 27 July 2020 - 11:38 AM

Due to the pandemic I’m staying home most all the time and my observing at home now all of the time. I’m interested in doing something new and different. I’ve been using my old ATM 6” F4.5 Newtonian quite a bit this summer, so I think I’m going to order a 6” CC just for comparison sake. It could be fun now that we’re into planet season.

Especially fun if you have a way to mount your future CC6 side-by-side with your 6" Newtonian.  I think it would be fun to be able to look into one eye piece at f/4.5 and then move my head to a different eye piece at f/12.  I can almost get there, but have to change secondary mirrors on my Parks HIT.


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#1050 Bean614

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Posted 27 July 2020 - 03:45 PM

Due to the pandemic I’m staying home most all the time and my observing at home now all of the time. I’m interested in doing something new and different. I’ve been using my old ATM 6” F4.5 Newtonian quite a bit this summer, so I think I’m going to order a 6” CC just for comparison sake. It could be fun now that we’re into planet season.

I, especially,  eagerly await results of your comparison.  I'm considering a very similar move!

Hope you get a great CC sample!




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