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New AT 6" and 8" Classical Cassegrain

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#1001 quilty

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Posted 04 February 2020 - 07:58 AM

That's reasonable. flowerred.gif  To blame the test instead of device. Or blame the guy providing the test. This square thing is all in here, some pages ago and I'm quite sure of my estimation except when using a primary of 460mm fl instead of 500, do I loose 40 or 80 mm Backfocus? And to extimate the effect on sa. But that much is sure by (rough) measurement: Any mm distance shift takes roundabout 15 mm backfocus, if it is square f2 or f1xf2 I don't care. And that makes it easy to compensate a backfocus loss by a shorter primary.

But Whow! quite a lot. 50mm distance shift and the scope still works, good news again, that's what I hoped for. And 500+ magification at which you really are watching objects! Not too bad for a reflecting scope! I was watching 300x maximum through my 5 inch Mak but just to better separate stars. At any imaging object it's performance ends at about 250x. 



#1002 quilty

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Posted 04 February 2020 - 08:39 AM

Sorry guys, a problem arises from posting faster than thinking. I'm afraid It's all vain. My new estimation is that when shortening the primary focal length by 40mm there might result a backfocus loss of 180mm (which could be conpensated by shortening the mirror distance by 12mm leading to additional secondary vignetting by 4mm which is about 12mm on the primary mirror. Here we are again. So there seems to be no way than to change the focal lengths of both mirrors in the same time. And therefore I do a stop here and hope to do no more postings except in case of a good idea.



#1003 highfnum

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Posted 04 February 2020 - 09:31 AM

im very close after doing the great fiddle dance of getting this scope aligned up

 

seeing was very good last night i did compare of cc8 with my c925

im close now 

It still could use some more tweek!

cc8 first then c925

Capture 2_3_2020 7_27_33 PMcc8.jpg

Capture 2_3_2020 6_40_20 PM925pf.jpg

 

 

 


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#1004 Spacedude4040

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Posted 04 February 2020 - 01:10 PM

im very close after doing the great fiddle dance of getting this scope aligned up

 

seeing was very good last night i did compare of cc8 with my c925

im close now 

It still could use some more tweek!

cc8 first then c925

attachicon.gifCapture 2_3_2020 7_27_33 PMcc8.jpg

attachicon.gifCapture 2_3_2020 6_40_20 PM925pf.jpg

 

Very nice imageswaytogo.gif 

 

Same camera right, did you adjust either image by inlarging the original and if so which one ones made larger or smaller.

Thanks

Mike



#1005 quilty

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Posted 04 February 2020 - 01:33 PM

if you ask me I surely prefer the first one. What is it? Copernicus? Great. What about image processing, the cc one is much darker? And what means "tweek"? 



#1006 highfnum

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Posted 04 February 2020 - 01:57 PM

same camera asi120mm

what you see is at prime focus for each

no enlargement 

yes copernicus 

first one darker cc is f 12

 

c925 f 10

tweek means collmination adjustment 



#1007 highfnum

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Posted 04 February 2020 - 01:58 PM

each stack 700 registax

same wavelet



#1008 aneeg

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Posted 04 February 2020 - 06:08 PM

CC is the sharpest.

 

Arne


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#1009 highfnum

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Posted 04 February 2020 - 06:28 PM

so maybe my efforts are paying off

it may be that superior cool down

of cc8 had a play in being better


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#1010 quilty

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Posted 04 February 2020 - 06:43 PM

once again.:supposed photographic quality and seeing was the same this is the proof for the cc's superiority to the sc type, isn't it? An 7,34" cc displays way better than an 9,25?" sc type. That's what this thread is all about. 



#1011 quilty

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Posted 05 February 2020 - 05:21 AM

and "tweek" sounds so ugly as it is.


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#1012 highfnum

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Posted 05 February 2020 - 07:54 AM

yes this scope was no fun to align (if you are not used to CC) although I did learn from it

if a beginner gets one out of alignment and just starts moving mirrors around 

oh boy!


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#1013 dweller25

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Posted 05 February 2020 - 09:49 AM

yes this scope was no fun to align (if you are not used to CC) although I did learn from it

if a beginner gets one out of alignment and just starts moving mirrors around 

oh boy!

What was your procedure the bring your scope back into collimation ?



#1014 quilty

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Posted 05 February 2020 - 12:32 PM

I guess it's too early for a statement like this, therefore I choose to ask:

This was my fourth time to "watch" through a sc type scope, the others were c11 and c8. And never I was much impressed by what i saw. Low contrast and low definition.

The CC type scopes are notorious for beeing delicate at collimation. 

Is it possible that the sc scopes are less sensitive just because they never would allow to see what is really sharp?

Just a question. The edge HD scopes are supposed to to better.



#1015 macdonjh

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Posted 05 February 2020 - 02:18 PM

I guess it's too early for a statement like this, therefore I choose to ask:

This was my fourth time to "watch" through a sc type scope, the others were c11 and c8. And never I was much impressed by what i saw. Low contrast and low definition.

The CC type scopes are notorious for beeing delicate at collimation. 

Is it possible that the sc scopes are less sensitive just because they never would allow to see what is really sharp?

Just a question. The edge HD scopes are supposed to to better.

My guess is many "soft" Schmidt Cassegrains aren't well collimated; especially recently manufactured SCTs, now that quality control is more consistent at Celestron and Meade.  I had a C11 which provided sharp views once I got it well collimated, and worked on a Meade 10" for a club member that was quite sharp and contrasty once it was well collimated.  The latter surprised me, I'd always been underwhelmed by Meade scopes, but perhaps it was the result of their owners more than the scopes.  When looking though somebody else's scope I don't start off with, "Hey, do you mind if I do a quick star test".


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#1016 highfnum

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Posted 05 February 2020 - 02:53 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 


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#1017 highfnum

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Posted 05 February 2020 - 02:56 PM

its possible my c925 was not fully cooled 

cc8 cools down faster



#1018 Asbytec

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Posted 05 February 2020 - 03:29 PM

The edge HD scopes are supposed to to better.

In a sense they are with better field correction. But, it might be misleading to say they are "better", they are still the same quality optics on axis as any other SCT. Just better corrected at the "Edge". 

 

From Celestron, "Aplanatic optics are designed to eliminate both spherical aberration and coma, two major aberrations (image defects) found in telescopes. The end result is sharper images across a much wider field of view than in other designs."



#1019 Ain Soph Aur

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Posted 05 February 2020 - 08:36 PM

its possible my c925 was not fully cooled 

cc8 cools down faster

Not following this thread too closely after the alignment discussion. On your C925 i can highly recommend a reflectix jacket to resolve any cool down issues.



#1020 George9

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Posted 05 February 2020 - 10:17 PM

I basically don't add cool down time any more on my C11 since I added a double layer of Reflectix. I use it more like a refractor: mount it and view. Which is ironic because I considered the CC to improve cool down, and now it turns out that the SCT has the advantage because of its closed design. My friend's 6" CC cools down fast enough, so no complaints, but just funny to have waited for an SCT to cool for 30 years only to find out recently how to solve the problem.

 

George


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#1021 Asbytec

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Posted 06 February 2020 - 12:46 AM

As for a test for optical quality, the images can be misleading and seem to feed into the hysteria of SCT softness. Yet, they say nothing about any waterfront aberrations that may or may not be present in either scope. For that we need an objective test or star test, at least. Apparently both scopes are highly obstructed, too. Imagine that.

As mentioned, it may be a focus problem, a cooling, or a collimation issue. Maybe a seeing issue. One might expect an experienced amateur to get collimation and focus right and seeing was good with both, so the issue may have been thermal as hifnum stated. We don't know the state of cooling, but we can assume the CC did cool more efficiently.

Edited by Asbytec, 06 February 2020 - 12:47 AM.

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#1022 quilty

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Posted 06 February 2020 - 03:02 AM

About Reflectix: Do we agree that it wouldn't help for any open tubed reflector scope? So, Reflectix slows down any temperature shift thus avoiding huge temperature differences on short scale. A big telescope perfectly collimated and focussed at room temperature, will it be the same in winter time out of doors after a long enough acclimatisation time? If not is it just the speed at which the scope runs out of focus while cooling down that annoys? For better understanding.



#1023 highfnum

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Posted 06 February 2020 - 08:31 AM

I did document some steps with ronchi on cc

notice in #6 I  was very off  lines converge  -- (holy $%# boy was I off!)

 

#3 lines are better 

 

i will do another when i have chance on current state which i do not have yet 

gsoron6.jpg

gsoron3.jpg

 

 

 

 


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#1024 highfnum

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Posted 06 February 2020 - 08:37 AM

I did compare to c925 not to prove that's its better but rather to show that's its some what in ball park and can do well when aligned 

 

 

I do not underestimate c925 when in good state 

here is ronchi for c925 (lines nice and clean)

and one of my real good alpine shots 

I got the rille all the way down

c925ronc1.jpg

c925dayeille.jpg

 

 

 


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#1025 quilty

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Posted 06 February 2020 - 08:53 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?




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