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Short 400-450mm FL ED or triplet for AZ GTi

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#1 elakrab

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Posted 19 September 2019 - 03:20 AM

Hi,

 

Given the success of the redcat51 + AZ GTi experiment, I'm considering raising my game and look for something in the 400-450mm focal length range that is not very heavy. I'm considering expending 500-900€ (flexible, but definitely below ~1000-1100€) for that.

 

Ingredients:

- 400-450mm focal length. Shorter would make little difference with the redcat, longer may be hard to guide/ride mainly with the AZ GTi (I have also a NEQ6 at my hometown, but it will mostly go with the AZ GTi). 

- Decent color correction: I was thinking on either a triplet or a FPL53+lanthanum doublet.

- Full frame coverage (flat, no extreme vignetting, no color kinks). 

- I need something to be able to use filters, ideally in a configuration similar to the one of he redcat51 (inside). 

- Around F5.8-F6.6

- Ideally no flexure (I experience that sometimes with the redcat and the 70-200 F4 and it is a bit annoying).

 

I have the possibility to order here in Europe and also in the US in one month.

 

So the candidates I was checking are.

 

Doublets:

  • Lacerta 72 f6, 2.5kg. 629€ Pros: Really inexpensive, Cons: FPL53+NBM51 seems to be inferior to FPL53+lanthanum.
  • Evostar 72 f6. ~350€. Ok, this one is absurdly cheap for a FPL53+Lanthanum. Where is the trap, no Full Frame due to the 2-inch focuser?
  • TS 72 f6, 2.2kg + ??. 529+165€ (with the TS 1.0X flattener). Pros: FPL53+Lanthanum.  
  • AstroTech 72 f6, Not sure if it is equivalent to the TS or the Lacerta. Does it use NBM51? Lanthanum?. Inexpensive, 2" focuser. I would need to find a 1.0X flattener. No FF due to 2" focuser?
  • WO ZS73. Virtually identical again, but not sure if it uses lanthanum. More expensive than the previous ones but the nice WO finishing and dedicated flattener.

Triplets:

  • TS 70mm f6 triplet: Does not show up anymore in the TS webpage, but still can be found elsewhere. 
  • GT71. f5.9 triplet, but the dedicated corrector is a reducer and leaves a focal length that is not very different than the redcat's one. 

Cuadruplets:

  • TS71SDQ. f6.3. I don't really know how to read the spot diagrams, but it doesn't look to be super great. Is it just a doublet + corrector? The advantage is no playing with distances.
  • Sharpstar 71. f6.3. The same as before.

Any opinion? suggestions? recommendations? I am open to other brands of coruse, but only if they pouring money to just buy the brand. 


Edited by elakrab, 19 September 2019 - 03:39 AM.


#2 astroburning

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Posted 19 September 2019 - 05:52 AM

I'm looking for a similar setup myself.

Evostar 72 is on top of my list, but I'm not sure about the FPL53 it's supposed to have. Experts can add some light.


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#3 Hesiod

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Posted 19 September 2019 - 06:59 AM

Your "ingredients" are a somewhat hard to mix in, especially the full frame coverage and lack of flexures.
2" focusers are not a great plan IMHO for full frame cameras, both optically and mechanically (I would add that a telescope equipped with such focuser could not ve designed to perform well on full frame sensors)
Rather, have already looked for a second hand 400mm lens (e.g. canon ef400 f/5.6)?

#4 elakrab

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Posted 19 September 2019 - 10:28 AM

The problem with the lens is that I have flexures with my 70-200 already + I have no way of using interference filters for pseudo narrowband, right?

#5 Hesiod

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Posted 19 September 2019 - 11:44 AM

Flexures may be addressed by replacing the mounting hardware, but take note that a refractor with somewhat weak focuser may add further sources of flexures.

 

As for filters, provided that you really want to use them with your camera (if it is not modded I would rather use a broader "light pollution" filters such as the CLS or an IDAS), those are available also unmounted, as "clip" for a number of DSLR body (e.g. Canon's APS-c, some Canon's full frame, some Sony's Alpha, and also some Nikon models), and in more traditional photographic sizes.

 

 

A last observation: in my opinion probably only the 71q f/6.3 among the models you listed can provide a decent field in the full frame format: the others will have likely worse coma, plus huge field curvature.



#6 elakrab

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Posted 19 September 2019 - 03:16 PM

My 6D is modded, I plan to use them sometimes with a 2 inch idas lps and a l-enhance and the doublets/triplets I counted on getting a flattener.

#7 Toddeo

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Posted 19 September 2019 - 03:22 PM

Astro-Tech AT72EDII Refractor OTA FPL-53 f/6

ASTRO-TECH AT72EDII REFRACTOR OTA FPL-53 F/6
27 Review(s) | Add Your Review
$469.00
Availability: More On The Way
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#8 elakrab

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Posted 20 September 2019 - 12:09 AM

But is it a clone of the TS? Does it have lanthanum element?

Edited by elakrab, 20 September 2019 - 12:09 AM.


#9 Hesiod

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Posted 20 September 2019 - 12:47 AM

6d is one of the models compatible with "clip" filters.
My remark was about the whole "system": very often the optional flatteners for entry-level small telescopes are up only to aps-c or similar sensors.
The fact is that the RedCat has "higher end" features you would not usually get on entry-level small APOs.
To get good results across most of the frame with your 6d I think would be better to look at the most recent offerta with 2.5 or 3" focusers and check the flattener specs.

#10 elakrab

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Posted 20 September 2019 - 01:40 AM

The problem with clip is in full frame canons is as far as I know the vignetting, there is a dark frame, quite annoying with those systems.

Edited by elakrab, 20 September 2019 - 01:40 AM.


#11 Hesiod

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Posted 20 September 2019 - 05:31 AM

Probably a bit, but should not be hard to solve with calibration; take also note that any telescope designed for smaller sensor will do the same, at a much worse level (vignetting+tons of aberrations).
A more serious issue could be that of bright stars' reflections if the flattener has bad coatings: in such case placing the filter before the reducer is the safer option.
Clip filters are handy if want to use standard lenses (albeit far from a necessity, unless plan to shot large nebulae mainly from light polluted sites and have big issues with gradients).
I suggested the lens not because it is the best option, but because it is the safest bet within your budget to met your requirements of full frame coverage.
As far as know there are some nice options at 400-500mm, but in the 80mm size, so maybe are too large for your purpose; also could mean stretching the budget (I am not a big fan of W.O. and their hideous painting schemes, but have given a look to the 81mm plus corrector?)

#12 25585

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Posted 20 September 2019 - 06:38 AM

Right at the limit of your budget, but lightweight  https://www.365astro...1BoCaCIQAvD_BwE

 

https://www.teleskop...-5--Auszug.html


Edited by 25585, 20 September 2019 - 06:43 AM.


#13 elakrab

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Posted 26 September 2019 - 05:18 AM

I'm leaning towards:

 

- TS71SDQ

- 63mm to M48 adaptor

- Filter holder M48.

 

All would sum like 920€. 

 

Reasoning:

- It is supposed to be a 3+1 design (triplet + flattener) instead of a 2+2 doublet. Better color correction?

- Extra backfocus (focuser position) of the 71SDQ with the M48 adaptor should be enough: http://astropical.sp...e/ts71sdq.php  

 

Opinions?



#14 elakrab

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Posted 09 October 2019 - 06:14 AM

Still struggling to find a full combo that accepts M48 filters ... Apparently the M63 to M48 adaptor does not provide an internal thread for filters :(

 

Does anyone have a suggestion of a M63 to M48 adapter (or set of adapters) that could provide such internal place for mounted 2" fitlers? Ideally with a length of ~30-60 mm.



#15 blemasle

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Posted 09 October 2019 - 05:08 PM

A small warning/question : aren't you concerned about the TS-Optics 71SDQ 3.2 kg (especially with a full frame on the end) ?

Apart from the fact that I work with an APS-C, I'm in the same situation myself. I'm leaning towards the TS 72 f6 / Astrotech 72 f6 at the moment (2.1kg but would need to add the flattener weight which I cannot find).



#16 blemasle

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Posted 09 October 2019 - 05:25 PM

Aaaaannd just like that, I'm wondering if waiting for the one day to come cat71 (f4.9 according to the comments) isn't the right thing to do : See this facebook post


Edited by blemasle, 09 October 2019 - 05:27 PM.


#17 elakrab

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Posted 10 October 2019 - 12:57 AM

Aaaaannd just like that, I'm wondering if waiting for the one day to come cat71 (f4.9 according to the comments) isn't the right thing to do : See this facebook post


Looks nice, but 350mm focal length when you already have a 250mm seems pretty redundant (and expensive).

#18 blemasle

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Posted 10 October 2019 - 11:40 AM

Looks nice, but 350mm focal length when you already have a 250mm seems pretty redundant (and expensive).

Yep, I did the maths afterwards and realized it was not a > 400mm APO :( At least now I know don't have to wait to find my next purchase :)

 

As I said on another thread, the AT72 would be my first choice, but I can't find it in Europe...



#19 Doug Culbertson

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Posted 10 October 2019 - 11:45 AM

Yep, I did the maths afterwards and realized it was not a > 400mm APO frown.gif At least now I know don't have to wait to find my next purchase smile.gif

 

As I said on another thread, the AT72 would be my first choice, but I can't find it in Europe...

I’m pretty sure that the TS PhotoLine 72mm f/6 is the same as the AT72EDII, but it appears to be 20 days out from delivery. 



#20 blemasle

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Posted 10 October 2019 - 11:52 AM

I’m pretty sure that the TS PhotoLine 72mm f/6 is the same as the AT72EDII, but it appears to be 20 days out from delivery. 

I was also pretty sure, but not anymore. See this thread : https://www.cloudyni...i-vs-tsapo72f6/


Edited by blemasle, 10 October 2019 - 11:59 AM.


#21 mic1970

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Posted 10 October 2019 - 11:58 AM

I love mine.  

 

 

Astro-Tech AT72EDII Refractor OTA FPL-53 f/6

ASTRO-TECH AT72EDII REFRACTOR OTA FPL-53 F/6
27 Review(s) | Add Your Review
$469.00
Availability: More On The Way
FREE ground shipping



#22 Doug Culbertson

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Posted 10 October 2019 - 01:37 PM

I was also pretty sure, but not anymore. See this thread : https://www.cloudyni...i-vs-tsapo72f6/

It’s really hard to tell with these rebrands. I do like the risers on that Altair 72 Deluxe. 



#23 elakrab

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Posted 10 October 2019 - 04:07 PM

I am currently leaning towards the 71sdq and clones (71/450mm). In the end it is a fpl53 triplet+flattener, but with a native 450mm focal length instead of a doublet at f6 or a triplet working at 350mm. If I manage to get a reasonably priced custom adaptor that takes M48 mounted filters INSIDE the M63/M48 reducer as it should have been for the TS adapter (I cannot believe they forgot that people with DSLR also use filters sometimes), then I am sold ;) . I will keep you posted of my findings.

Edited by elakrab, 10 October 2019 - 04:26 PM.


#24 elakrab

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Posted 10 October 2019 - 04:21 PM

I was also pretty sure, but not anymore. See this thread : https://www.cloudyni...i-vs-tsapo72f6/

the TS has a 2.5" focuser, while the AT seems to have a 2" one.

#25 KevH

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Posted 10 October 2019 - 05:52 PM

the TS has a 2.5" focuser, while the AT seems to have a 2" one.

I suspect the specs are wrong... the page for the AT102ED also states a 2" focuser but it has a 2.5" drawtube... I measured mine.  I'd be surprised if the 72 was any different.




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