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Choose between 140mmf6.5 Apo or 200mmf15 dall kirkham

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#1 Meticon

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Posted 20 September 2021 - 03:24 PM

I am not crazy.
I just want to build a scope light and portable, hence I don't want bulky Newtownians or Dobsons, and I want to build it myself. I set the maximum lenght to 1mt and money is not a problem. So I came up with these two designs, an Apo 140mm or a dall kirkham 200mm f15.
There is no other choice, and I will be observing 80% of the time from city (moon and planets) and the rest from rural sky, I guess bright DSOs. Field of view is not important.

What would you choose and why? Remember, only these two options on the table..

I won't be making optics, just mechanics

Edited by Meticon, 20 September 2021 - 11:25 PM.


#2 Jeffmar

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Posted 20 September 2021 - 03:32 PM

Is the dall kirkham really an f/15? 



#3 Meticon

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Posted 20 September 2021 - 03:36 PM

Is the dall kirkham really an f/15?


Yes, 3000mm focal lenght, I am working on the project on paper along with the Apo, but will pull the trigger only on one

Edited by Meticon, 20 September 2021 - 03:36 PM.


#4 spereira

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Posted 20 September 2021 - 03:45 PM

Moving to ATM, Optics & DIY.

 

smp



#5 Jeffmar

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Posted 20 September 2021 - 03:50 PM

It’s a tough call.Good luck with your decision



#6 PrestonE

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Posted 20 September 2021 - 03:57 PM

"and I want to build it myself"

 

In saying this, does that mean you will be making the optics?

 

If so, a CDK would likely be easier for you as with an APO you

will have 6 optical surfaces to contend with and likely have to

make test plates to do some of those surfaces...

 

If your not doing the optics, both are just about as easy to do

just the mechanical s...

 

Best Regards,

 

Preston


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#7 photoracer18

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Posted 20 September 2021 - 04:54 PM

A DK is just about the same weight as a Newt. They both have a big mirror at the back, a small one at the front. Both can be lighter weight strut type tubes. Optics are harder to grind that a Newt but easier than a Classical Cass or Ritchey-Chretien. But the usable field is narrower due to coma.

As for a refractor that is a whole other league. Some ED glasses are so hard you have to grind them with diamond dust. And there is only one source for each high end brand. FPL-53/55 are only made by Ohara in Japan, FCD100 is only made by Hoya in Japan, OK-4 is only made by LZOS in Russia and Calcium Fluorite is only grown by Canon Centon in Japan. Plus the mating element glasses you will be grinding 4 surfaces for a doublet and 6 for a triplet. Most of the top makers these days use CNC optical grinders, but they are too expensive for anyone without a lot of money. Don't even consider doing this unless you already have a good deal of experience because its easy to ruin a blank and have to replace it, as some elements are both very hard and very brittle (Calcium Fluorite for one). A very daunting project compared to a mirror although each surface being spherical makes the figure easier. Also the shorter the F-ratio of an APO the harder it is to get it fully corrected (and the larger the optics also). That is why only a few companies make their own, Astro-Physics, TEC, now Stellarvue, and Takahashi off the top of my head. Brands like ES, AT, and TS don't, they get theirs made by others China. And the Chinese are getting better all the time.



#8 spaceoddity

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Posted 20 September 2021 - 05:12 PM

A 140 mm apo refractor is not a scope I would consider to be light and portable, especially when you consider the mount and tripod that will be required.



#9 rhetfield

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Posted 20 September 2021 - 06:03 PM

Newt only looks big and heavy until the guy who built the lightweight fast 13" posts pictures of his artistry.
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#10 maroubra_boy

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Posted 20 September 2021 - 06:43 PM

Meticon,

 

With the criteria you have put forward, for me the choice is the Dall-Kirkham.

 

I too do most of my observing from home ad most of it is also the Moon and planets.  At home I also have other scopes, including refractors, Newts & dobs. but only a few of these I use from home and only as a grab'n'go, occassionally.  The scope I mostly use from home is a 9" Mak, and previous to this scope it was other Maks and SCT's.

 

And from home, this Mak is no slouch on DSO's.  Globular clusters and planetary nebulae are cannon fodder for this scope.  Out at a dark site, this scope is very, very good.  With a 3100mm focal length it is very similar to the 3000mm you have calculated for your DK - if anyone brings up a narrow FOV then they have no idea what they are talking about.  If a 3° TFOV is what they want/expect/demand that is fine but the reality is there are few objects that require more than a 1° TFOV to view in in the one FOV.  So you can't fit the whole of M31 in the eyepiece?  So bloody what???  Does this mean you are not allowed to view M31?  So does this mean that these people all have scope with a focal length no longer than 800mm?

 

If you do build a DK or even a CC (Cassical Cassegrain), I strongly suggest you put a lot of thought into the quality of the baffling.  This includes the control of reflections coming off the secondary's baffle, the junction of the primary and the baffle tube, and the design of the primary's baffle tube.  THIS is what will provide you with the best level of contrast (along with the quality of the final polish of the optics).  My 9" Mak has outstanding contrast and even this scope is out done in contrast when I compared it to a 250 Mewlon, a VMC260L Vixen and an APM-Wirth 10" Mak.  These scopes may all be different in their design, but there has been a lot of thought and excellent design in the baffling.  I have never seen any  other scopes with as good contrast as these three scopes.  Oh, and so you know, I did a side-by-side comparison between my 9" Mak and my friend's 10" Mak - same eyepiece and same diagonal used in both.  Both scope have the same outstanding Intes optics.  The difference in contrast was quite stark and entirely from the design of the baffles.  Remember, my 9" Mak has good contrast, but this 10" is something else.

 

Below are two sets of sketches I have done of planetary nebulae and GC's.  Each was from a series of PN and GC sketching marathons I did from my home in Sydney in the last 12 months.  I've included them do demonstrate the excellent capability of a Cassergrain that is well made on DSO's under light polluted skies.  Of course I also used an Oiii filter with the PN's, but neat with the GC's.  I used white soft pastel, charcoal and white gel ink on A3 size black paper for both sets.  And yes, I sketch on to black paper at the eyepiece.

 

Alex.

Attached Thumbnails

  • Planetary Neb marathon 2 LR.JPG
  • GC marathon 2 LR.jpg

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#11 maroubra_boy

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Posted 20 September 2021 - 06:46 PM

Oh yeah, a well made Cassegrain is also outstanding on the Moon.  The sketch below was done using a 7" Intes Mak.  My 9" Mak resolves details a little finer again...

 

Alex.

Attached Thumbnails

  • Marius Hills LR.JPG

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#12 Meticon

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Posted 20 September 2021 - 11:25 PM

"and I want to build it myself"

In saying this, does that mean you will be making the optics?

If so, a CDK would likely be easier for you as with an APO you
will have 6 optical surfaces to contend with and likely have to
make test plates to do some of those surfaces...

If your not doing the optics, both are just about as easy to do
just the mechanical s...

Best Regards,

Preston

I won't be making optics, just mechanics.


I guess I am just worried about the 35% obstruction I get for the DK..

Baffles come from Atmos and Mike's CassDesign, I will flock all interior and exterior with black velvet..

Edited by Meticon, 20 September 2021 - 11:35 PM.

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