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$15,000 and a long focal length

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#1 mlrtime99

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Posted 04 April 2020 - 08:38 AM

You've just been given $15,000 and want to obtain the longest focal length for your Bortle 2 observatory which already had a AP 1600GTO mount.  What do you buy?


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#2 rgsalinger

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Posted 04 April 2020 - 09:15 AM

Planewave CDK14 or AG Optics IDK 14. If that's too much of a stretch get the 12.5" versions. I have the CDK 12.5 with accessories and tax it came to around 12K USD. I use a full frame camera and get round stars across the entire field night after night. Scope has not needed collimation since I got it 4 years ago.

Rgrds-Ross


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#3 mlrtime99

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Posted 04 April 2020 - 03:57 PM

Ironic, these were the two I had narrowed my selection down too but wasn't able to find many reviews for either of them and wasn't sure if there was a quality, lesser known maker out there.



#4 rgsalinger

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Posted 04 April 2020 - 04:44 PM

If you go the 14" SCT route then by the time you buy the necessary fans, focuser, focal reducer and heaters you have spent a lot more money than you expected and you still don't have an integrated system or optics as good as a corrected Dall Kirkham design. I love the PWI software and the auto focus that comes with the scope is top notch. I say this based on benchmarking it against the usual suspects like Focusmax and the SKYX @focus2.

 

There was a part of me that wondered about the expense but after 4 years I'm really glad that I bought it. PW used to be 90 miles from my house which is why I chose it over the AG. The 12.5 AG, though, is lighter and F6.7 if that matters but you have to buy the focuser and the temperature control system separately. I'm not sure which one I'd buy now that PW is in Michigan. 

 

Rgrds-Ross


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#5 Cotts

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Posted 04 April 2020 - 08:46 PM

I'd buy Damian Peach's 14 inch SCT.   

Proven planetary performing telescope...

 

Think he'd go for it?

 

Dave


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#6 L. Regira

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Posted 04 April 2020 - 08:59 PM

I'd buy Damian Peach's 14 inch SCT.   

Proven planetary performing telescope...

 

Think he'd go for it?

 

Dave

You also have to buy his quality night sky.


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#7 mlrtime99

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Posted 04 April 2020 - 09:04 PM

I'd buy Damian Peach's 14 inch SCT.   

Proven planetary performing telescope...

 

Think he'd go for it?

 

Dave

Fair point... I should have added for DSO imaging :)



#8 Cotts

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Posted 05 April 2020 - 10:37 AM

Fair point... I should have added for DSO imaging smile.gif

I'm not sure a "long focal length" telescope is ideal for Deep sky imaging....

 

Dave


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#9 macdonjh

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Posted 05 April 2020 - 11:28 AM

  • Long focal length, CFF 400mm classical Cassegrain (it may fit within the budget, if not, I'd keep my 350mm classical Cassegrain and spend your money on something else)
  • Shorter focal length, CFF 300mm Ritchey-Cretien

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#10 rgsalinger

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Posted 05 April 2020 - 12:01 PM

I like the look of the CFF RC scopes but the CC scopes are intended, at Fl5, to be visual only scopes per the website. Since the OP didn't say "for imaging" the CC scopes might very well fit the bill. They don't seem to be distributed in the USA but that may or may not bother someone. Love to see a report on the RC's as they seem well priced assuming that the optics are top notch. The GSO RC's are also well priced and seem to work for most people. I'm not an RC fan because they can be tricky to collimate but once collimated (I use a 24" RC from time to time) they can be wonderful telescopes for imaging.

Rgrds-Ross



#11 Raginar

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Posted 05 April 2020 - 03:56 PM

What Ross said. Some day that’s what I’ll get.

#12 macdonjh

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Posted 06 April 2020 - 08:49 AM

Actually, in Post #7 the original poster did clarify his intention as deep sky imaging.  While I agree with Cotts, a slow scope wouldn't be the choice of many for deep sky imaging, I posted the CFF classical Cassegrain because I have one and think it's great (though I don't think I'd image DSO with it).  Based on having to good CFF mirror sets, I wouldn't have any problem buying an RC from CFF, either, if I were to start imaging DSO (no danger of that in the foreseeable future).



#13 jakecru

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Posted 06 April 2020 - 11:29 AM

I am not an expert in this AP subject what so ever, so feel free to ignore this comment. It seems to me a 12.5" Planewave + a 100 mm APO such as the Espirit 100 or a Tak 100 could work as a dual threat for long and short fl deep sky work. Maybe the 100 mm would be light enough to piggy back the Planewave 12.5 (or even an edge hd 1400 with 0.7 reducer and hyperstar capability, and forget the refractor). 



#14 luxo II

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Posted 06 April 2020 - 08:01 PM

For imaging this https://www.teleskop...ht-Focuser.html



#15 rgsalinger

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Posted 06 April 2020 - 09:38 PM

luyxo had an interesting alternative. You certainly could go for a big refractor if you could find one under 15k that had a flat field the way that the CDK's do. My AP155 looks wonderful on my Paramount but it won't cover a full frame camera!

Rgrds-Ross


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#16 luxo II

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Posted 06 April 2020 - 09:41 PM

Or you could find a 10" f/12 rumak like mine (it has a flat field), put a 0.5X reducer in it and cover a 50mm circle at f/6. The challenge is finding one...


Edited by luxo II, 06 April 2020 - 09:42 PM.


#17 bobhen

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Posted 07 April 2020 - 07:09 AM

I'm not sure a "long focal length" telescope is ideal for Deep sky imaging....

 

Dave

HERE is a link to some images. Scroll down and click on the images taken by Roland Christen with his 10" F14.6 Mak.

 

Long FL delivers image scale that can reveal small details. You do need decent conditions and a really great mount.

 

Bob


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#18 kbastro

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Posted 09 April 2020 - 12:01 PM

you can always buy this: https://optcorp.com/...-tube-prc400cgc

 

$15K would make a good down payment,,, lol

 

kb



#19 rgsalinger

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Posted 09 April 2020 - 12:34 PM

A few thoughts.

 

I've never heard of a "Rumak" and I'd be amazed if you could put an .5 reducer on it and still keep a 50mm circle. Is there some example out there of this? 

 

Any F15 telescope is just going to take a long time to collect enough data to get above the read noise. Assuming that you have one that has a large imaging circle it's really just a matter of how much time you want to spend on a picture. In turn, those long exposures mean you need a really dark sky site and a wonderful mount. 

 

I'm sticking to my recommendation of a 12.5" CDK or IDK and would buy the one that gave me either the best price or the easiest shipping. 

 

Rgrds-Ross



#20 luxo II

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Posted 09 April 2020 - 06:31 PM

Ross with all these big scopes there are two issues:

 

1. The seeing at your site - which had better be frequently exceptional, otherwise if its typically 1-2 arc-sec you may as well be using a smaller APO;

2. The image scale vs the pixel spacing of candidate sensors; the largest I'm aware of are 9 or 11 microns. Not a great match at 3m or more focal length.

 

These are why I don't bother attempting imaging with my 10" mak - it is a visual scope, not an imaging scope. The sensor industry isn't helping either - the race to ever smaller sensors with stupid numbers of megapickles is resulting in cameras that match smaller and smaller scopes. APM have (for example) a 20" f/8 maksutov but I can't see it selling easily as there are no commercial sensors to suit it.


Edited by luxo II, 09 April 2020 - 06:36 PM.


#21 mlrtime99

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Posted 09 April 2020 - 08:59 PM

Ross with all these big scopes there are two issues:

 

1. The seeing at your site - which had better be frequently exceptional, otherwise if its typically 1-2 arc-sec you may as well be using a smaller APO;

2. The image scale vs the pixel spacing of candidate sensors; the largest I'm aware of are 9 or 11 microns. Not a great match at 3m or more focal length.

 

These are why I don't bother attempting imaging with my 10" mak - it is a visual scope, not an imaging scope. The sensor industry isn't helping either - the race to ever smaller sensors with stupid numbers of megapickles is resulting in cameras that match smaller and smaller scopes. APM have (for example) a 20" f/8 maksutov but I can't see it selling easily as there are no commercial sensors to suit it.

^This is actually the biggest reason I created this thread.  Finding a scope with the desired focal length really isn't an issue... finding one that works well with the trend in pixel size is another matter.

 

The observatory is in Flagstaff AZ which has Good-Excellent seeing.  I have a 1000mm mak-newt and a RASA 11 but there are objects that I simply can't reach so I'd like to add something in the 2500mm - 3500mm range (telling myself it will be the last purchase wink.gif ).

 

I do have an eye on AG Optics products and the 14" IDK is the current front runner, I just wonder if it has enough reach.  Some part of me really wants to give the GSO RC 16" F/8 3250mm @ $7k or even the 20" RC F/8 4064mm @ 13k for a whirl just for fun.... but then again current, and more importantly future, sensor parameters come into question.


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#22 luxo II

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Posted 09 April 2020 - 11:11 PM

A few thoughts.

I've never heard of a "Rumak"

Fair summary: https://en.m.wikiped...sutov_telescope

The secondary mirror is separate from the corrector providing the designer with two additional degrees of freedom - the mirror radius, and the space between it and the vertex of the corrector. The result is large aperture maks are possible with flat fields - without resorting to aspheric surfaces.

It also opens the possibility of designs that are f/12 with central obstruction at 26%, or f/20 at about 20%.

Most of the Intes maksutov cassegrains were Rumaks, likewise those from Santel, AP and TEC. The larger ones from APM (10”, 11” and 16”) also. These are all primary visual scopes for lunar & planetary though sometimes you’ll see one used for imaging (search astrobin).

The Gregory maksutov design (spot secondary) seen in small maks (Skywatcher, Orion or Celestron and Meade) is fine up to 6” aperture but from 7” up it becomes necessary to aspherical one surface. This approach is used by Ottiche Zen.

Edited by luxo II, 09 April 2020 - 11:18 PM.


#23 rgsalinger

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Posted 10 April 2020 - 03:27 PM

Thanks for the Rumak tutorial! I don't see any for sale in the F8 range though, is that correct? In fact I can't find any on the APM website. I have to think that these are no longer made??

 

Seeing at the site I use is at best 1.5 arc seconds, normally around 2 arc seconds. That's an unofficial measure using a 24" RC that I use from time to time which I know is diffraction limited and it 50 feet from my CDK. My 12.5" can get down to just about that same point. So going bigger at my site makes no sense to me.

 

What I like about the Planewave CDK (other than the full frame flat field) is the integration that comes with it. The focuser, fans and heaters are all controlled by a single excellent and easy to use GUI. You also get full frame autofocus as well as plate solving all in the same product. 

 

I personally would not trust the GSO scopes. I have no logical reason for that other than my antipathy toward RC designs because of the collimation difficulties. That's particularly true if the scope is to be operated remotely as my CDK is these days. 

 

Rgrds-Ross



#24 macdonjh

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Posted 11 April 2020 - 08:08 AM

 stupid numbers of megapickles is resulting in cameras that match smaller and smaller scopes.

funnypost.gif 


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#25 25585

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Posted 11 April 2020 - 10:24 AM

https://www.astrobuy...php?view=157011

 

or

 

https://m.facebook.c...75581295847730/


Edited by 25585, 11 April 2020 - 10:31 AM.



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