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What 130mm +\- Refractor covers full frame?

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#1 Morefield

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Posted 28 February 2020 - 03:20 PM

I'm considering buying a refractor in the 130mm range to pair with a full frame camera.  I would prefer not to use a flattener so my question is, what scopes are on the market that will natively cover a 42mm image circle?

 

The Takahashi FSQ130 would have been an excellent option but alas, it is not longer being made.

 

Thanks for your help!



#2 Gene3

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Posted 28 February 2020 - 03:41 PM

I use a SVX 130T with a QHY367C (full frame, OSC). The image circle is 43mm and I have not seen any vignetting. I do use a flattener as most who do AP, I think.

Even my prior FSQ106 had a flattener, it was built in as part of the design. I think the FSQ130 was the same.



#3 photoracer18

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Posted 28 February 2020 - 03:43 PM

You realize that the outer stars in a refractor won't be in focus without a flattener at least, although you don't need one that is a focal reducer also. In most cases to cover FF you need a refractor with a big focuser. And often 42mm might not be enough as you may need a 48mm T-ring. 2" focuser won't do it you need one with 2.5" or maybe 3". Most makers offer an upgraded imaging focuser in the 3" to 3.7" although Astro-physics offered a 4" because they were mainly doing medium format film (Pentax 67) in those days. Full frame is a misnomer because it should be called 35mm format just like smaller is called APS-C format or 4/3 format since medium format and large format are both much bigger. My old Borg 150ED was designed for 6x9 medium format with either the flattener or the Super Reducer giving an 88mm flat field.

Your problem is visual refractors are designed to have eyepieces in the optical train while imaging ones generally are not because they usually have flatteners built in.


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#4 Coconuts

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Posted 28 February 2020 - 05:22 PM

My recommendation is the Thomas Back designed triplet 130 mm f/6 APO, lens made by LZOS, available at APM telescopes (link below), along with the Feather Touch focuser, and either the Large M82 Riccardi 1X flattener, or the Large M82 Riccardi 0.75 reducer/flattener (f/4.5).  I have one with interchangable M82 correctors, and love it!  It is well-corrected out to a 52 mm image circle.  It is also less than half the price of the no-longer available Tak 130, and has a solid focuser.

 

https://www.apm-tele...cnc-lw-ii1.html

 

All the best, 

 

Kevin


Edited by Coconuts, 28 February 2020 - 05:24 PM.


#5 Morefield

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Posted 28 February 2020 - 06:01 PM

I should probably describe more about what I'm after.  I'm looking for a essentially a larger version of my Tak FSQ106.  I would like a longer focal length to reduce the image scale to a better match with great seeing.  700mm-900mm would be the goal.  I'm trying to avoid the flattener to avoid the very specific back-focus requirements those come with.  My FSQ is a joy in that I can stick pretty much anything on there and if it reaches focus, we are good to go.  

 

The scope would also be remote and serviced in frequently.  So a solid manufacturer is a much.  And I'm specifically thinking refractor because I want the simplicity of the package in addition to the FL.  Think cleaning, coloration, varmints, etc.  I already have a long FL CDK scope and this one will be for semi-widefield use.

 

There was a lot of discussion about why anyone would want to pay the $15K price for an FSQ130.  Maybe not enough did!  But it seems as though it was unique at its size with having such at huge corrected circle.

 

 



#6 ezwheels

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Posted 28 February 2020 - 10:46 PM

Once you get the spacing correct it is a done deal. A new TEC 140FL with the TEC FRC field flattener (.9x) leaves you with 882mm FL, f6.3. It has about 115mm of metal back distance. I am using this flattener on my 140ED and it is a very nice piece of kit. Unfortunately I do not have my 6200mm camera yet so I can not test the performance at the FF edges, but I suspect it should not be an issue. 

 

The FSQ130 really was a unicorn and unfortunately they could not have found a way to get it closer to 10k in price. That would have probably been in the park for many folks on the edge about getting one. But at its selling price you could get 2 TEC140's and run a dual rig. 


Edited by ezwheels, 29 February 2020 - 10:15 AM.


#7 WadeH237

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Posted 29 February 2020 - 07:14 AM

I'm trying to avoid the flattener to avoid the very specific back-focus requirements those come with.

Personally, I don't view this as a big deal.  It's a one-time setup, and most of the manufacturers of higher end refractors can tell you those very specific requirements (and may even have ready made adapters).

 

If you are looking for a telescope with an integrated flattener (essentially what your FSQ is), and you can't find a used FSQ 130, then you might take a look at the Televue NP127is.  It's a 4 element design with integrated flattener and a focal length of 660mm, so just a bit shorter than you are looking for.  Televue doesn't publish the size of the imaging circle on their web site, but I'm sure that if you call them, they can tell you.

 

Edit:  If I were in your place, I would probably go with a TEC 140 and a flattener.  In fact, I'm pretty sure that I will end up there in a few years.


Edited by WadeH237, 29 February 2020 - 07:16 AM.


#8 buckeyestargazer

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Posted 29 February 2020 - 09:38 AM

As others have said you are really limiting your choices by not looking at a scope + flattener.  I have a CFF135 f6.7. The CFF dedicated flattener gives a generous 55mm image circle and a generous backfocus distance of 101mm.  It's a fantastic scope.  Eventually I'm planning to use a full frame sensor with the scope.  


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#9 ManuelJ

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Posted 29 February 2020 - 01:34 PM

There is no telescope on the market with your requirements. So you must use a flattener/reducer.

I personally use a 130 GTX, and I wouldn't trade it for nothing. Back focus distance was a pain, but once set, it's done. In astronomy nothing is easy.

#10 Tim

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Posted 29 February 2020 - 02:31 PM

Personally, I don't view this as a big deal.  It's a one-time setup, and most of the manufacturers of higher end refractors can tell you those very specific requirements (and may even have ready made adapters).

 

If you are looking for a telescope with an integrated flattener (essentially what your FSQ is), and you can't find a used FSQ 130, then you might take a look at the Televue NP127is.  It's a 4 element design with integrated flattener and a focal length of 660mm, so just a bit shorter than you are looking for.  Televue doesn't publish the size of the imaging circle on their web site, but I'm sure that if you call them, they can tell you.

 

Edit:  If I were in your place, I would probably go with a TEC 140 and a flattener.  In fact, I'm pretty sure that I will end up there in a few years.

Televue Website indicates 52mm diagonal chip 

 

https://tinyurl.com/qraqhw4


Edited by Tim, 29 February 2020 - 02:33 PM.


#11 Morefield

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Posted 29 February 2020 - 03:27 PM

As others have said you are really limiting your choices by not looking at a scope + flattener.  I have a CFF135 f6.7. The CFF dedicated flattener gives a generous 55mm image circle and a generous backfocus distance of 101mm.  It's a fantastic scope.  Eventually I'm planning to use a full frame sensor with the scope.  

A 101mm backfocus for the flattener would help a lot with my issues.  I don't have an exact backfocus requirement yet, but the FLI CFW10+Camera and adapters will take up around 55mm and I want a large OAG and need a rotator.  101mm should cover it.  



#12 Morefield

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Posted 29 February 2020 - 03:32 PM

Televue Website indicates 52mm diagonal chip 

 

https://tinyurl.com/qraqhw4

Looking at the photos and stats I don't see an OAG in the image train.  I'm assuming the corrector at the back has a fixed backfocus requirement, right?



#13 Morefield

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Posted 29 February 2020 - 03:38 PM

Once you get the spacing correct it is a done deal. A new TEC 140FL with the TEC FRC field flattener (.9x) leaves you with 882mm FL, f6.3. It has about 115mm of metal back distance. I am using this flattener on my 140ED and it is a very nice piece of kit. Unfortunately I do not have my 6200mm camera yet so I can not test the performance at the FF edges, but I suspect it should not be an issue. 

 

The FSQ130 really was a unicorn and unfortunately they could not have found a way to get it closer to 10k in price. That would have probably been in the park for many folks on the edge about getting one. But at its selling price you could get 2 TEC140's and run a dual rig. 

Interesting - I like that back focus.  I can't seem to find the flattener you referenced. Any links?  Thanks!



#14 UniversalMaster

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Posted 29 February 2020 - 03:48 PM

Televue Website indicates 52mm diagonal chip

https://tinyurl.com/qraqhw4


It seems the NP127 still needs a flattener for larger chips, see LCL-1069 here:
http://www.televue.c..._table#LCL-1069

#15 Tim

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Posted 29 February 2020 - 04:18 PM

Yes medium format you would. Full frame DSLR as the original poster wanted to use probably not. A phone call to Al wouldn’t hurt before making a buying decision.

#16 OhmEye

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Posted 29 February 2020 - 08:08 PM

https://www.teleskop...7--Focuser.html


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#17 ezwheels

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Posted 29 February 2020 - 10:13 PM

Interesting - I like that back focus.  I can't seem to find the flattener you referenced. Any links?  Thanks!

https://www.telescop...ducer-frc-0-9x/


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#18 UniversalMaster

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Posted 01 March 2020 - 12:56 AM

Yes medium format you would. Full frame DSLR as the original poster wanted to use probably not. A phone call to Al wouldn’t hurt before making a buying decision.


The link I posted says "Recommended for cameras with diagonal sizes larger than APS format.", so a full frame would need this if you trust the televue website....

#19 Art Morrison

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Posted 01 March 2020 - 08:24 PM

You probably know this but a TOA-130 with 67 flattener has a 90 mm image circle. I use this with a full frame ASI6200. 


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#20 james7ca

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Posted 02 March 2020 - 01:36 AM

There is an old lens maker's saying that there is a fixed amount of information content that can be put into any optical system (and this limit is only partly determined by the theoretical diffraction limit of the optics). What this means is that you can either have high resolution in a small field or lower resolution in a larger field, such that there is a practical limit based upon the product of resolution and field size -- an inverse relationship whereas as the field size increases the median resolution tends to decrease. Thus, it's either very difficult or near on impossible to have a high-resolution result over a large field.There is no theory or optical law that states this but given limits on cost, complexity, and manufacturability it's generally true for any mass-marketed optical system.

 

That said, YMMV. So, now and then it might be possible to find a single sample (or a few samples) of a scope that exceeds the more typical result that is delivered by a particular design or brand (but consider yourself lucky if you happen to be "blessed" with one of these samples).

 

Okay, so with the above in mind, I've been using a Tele Vue NP127is for several years and I've spent a lot of time in image evaluation and reading on the NP scopes and my opinion is that the Tele Vue NP127is can NOT cover an APS-C sized field without some level of compromise. So, while Tele Vue does make a so-called large field corrector (which I do own) for the NP127is that option is really designed to image at something around three arc seconds per pixel and anything finer will likely result in some disappointment.

 

What these large field scopes offer is the ability to be used with large sensors (obviously), but with some compromises related to image scale and overall resolution. Thus, you can pair the NP127is (or better yet the NP127fli) with a large sensor like the  FLI ProLine 16803 CCD (9um pixels, 52mm diagonal) and with some luck you can get pretty good, large-field results, but at an image scale of about 2.8 arc seconds per pixel.

 

So, the OP says:

 

I would like a longer focal length [...and "full frame"...] to reduce the image scale to a better match with great seeing.

And, what I'm saying above is that those requirements may be difficult to obtain with any scope that you are likely to find in the marketplace. In particular (and in my opinion) the NP127is with its large field corrector will NOT give you the kind of results with a full-frame camera that could make full use of a reduced image scale to "better match with great seeing."

 

In any case, what camera does the OP plan on using that can give both a wide field and high resolution? Furthermore, does the OP really expect to get seeing-limited results with such a combination?


Edited by james7ca, 02 March 2020 - 11:35 AM.

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#21 WebFoot

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Posted 02 March 2020 - 01:46 PM

I imaged for some years with a friend's Astro-Physics AP130 f/6 Starfire.  It was a wonderful scope, and essentially perfect when we added an Astro-Physics flattener (not reducer).

 

If cost is no object, look for one of these used; they come up from time to time.

 

Mark



#22 Morefield

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Posted 02 March 2020 - 02:30 PM

There is an old lens maker's saying that there is a fixed amount of information content that can be put into any optical system (and this limit is only partly determined by the theoretical diffraction limit of the optics). What this means is that you can either have high resolution in a small field or lower resolution in a larger field, such that there is a practical limit based upon the product of resolution and field size -- an inverse relationship whereas as the field size increases the median resolution tends to decrease. Thus, it's either very difficult or near on impossible to have a high-resolution result over a large field.There is no theory or optical law that states this but given limits on cost, complexity, and manufacturability it's generally true for any mass-marketed optical system.

 

That said, YMMV. So, now and then it might be possible to find a single sample (or a few samples) of a scope that exceeds the more typical result that is delivered by a particular design or brand (but consider yourself lucky if you happen to be "blessed" with one of these samples).

 

Okay, so with the above in mind, I've been using a Tele Vue NP127is for several years and I've spent a lot of time in image evaluation and reading on the NP scopes and my opinion is that the Tele Vue NP127is can NOT cover an APS-C sized field without some level of compromise. So, while Tele Vue does make a so-called large field corrector (which I do own) for the NP127is that option is really designed to image at something around three arc seconds per pixel and anything finer will likely result in some disappointment.

 

What these large field scopes offer is the ability to be used with large sensors (obviously), but with some compromises related to image scale and overall resolution. Thus, you can pair the NP127is (or better yet the NP127fli) with a large sensor like the  FLI ProLine 16803 CCD (9um pixels, 52mm diagonal) and with some luck you can get pretty good, large-field results, but at an image scale of about 2.8 arc seconds per pixel.

 

So, the OP says:

 

And, what I'm saying above is that those requirements may be difficult to obtain with any scope that you are likely to find in the marketplace. In particular (and in my opinion) the NP127is with its large field corrector will NOT give you the kind of results with a full-frame camera that could make full use of a reduced image scale to "better match with great seeing."

 

In any case, what camera does the OP plan on using that can give both a wide field and high resolution? Furthermore, does the OP really expect to get seeing-limited results with such a combination?

 

The camera will be based on the IMX455 chip, though I'm not sure which manufacturer yet.  That is a 3.76 micron pixel.  The seeing I'm targeting is often 1", sometimes less.  With this pixel size @700mm the image scale is  1.11" and @900mm image scale is .86".  Either is under-sampled by a large margin and so a high Strehl ratio is not important.  

 

This setup would be used for both wide field (like 2-2.5 degrees) shots using the full sensor size and highly cropped images where resolution at the center is more important.  Some loss of resolution is tolerable, but I need the field to be flat.

 

If this were a long FL, max resolution setup the decisions and considerations would be much different.

 

Given that the TV NPS setup was designed specifically for the 16803, I agree that it may not be a good choice here.  I would prefer that whatever scope/flattener we choose has published spot diagrams going 22mm out from center. 

 

Kevin



#23 james7ca

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Posted 07 March 2020 - 03:54 AM

...Given that the TV NPS setup was designed specifically for the 16803, I agree that it may not be a good choice here.  I would prefer that whatever scope/flattener we choose has published spot diagrams going 22mm out from center. 

 

Kevin

I misspoke by saying that the NP scopes were "designed to image at something around three arc seconds." What I should have said is that they are probably best used at around that image scale (if you want to image at APS-C or larger fields). In truth, I think the original Nagler-Petzval scopes were designed during the film-era of astrophotography, although they did tweak the design for the NP127is about ten years ago to better serve the requirements for digital imaging.

 

If you want to see how an NP127is performs at an image scale of 1.5 arc seconds per pixel (fairly modest given today's CMOS sensors) you can reference this thread on image quality:

 

  https://www.cloudyni...y/#entry6627920

 

There you can see what kind of image quality you can get with an APS-C format. It's not that bad, but you can definitely see what looks like astigmatism in the corners of the field.

 

Frankly, however, if you can find something that is significantly better than this over a full-frame sensor at under one arc second per pixel then I'd like to see those results. But, as you said you can always drop down to the center of the field when you want better image quality. And, you always have the option to resample to a less demanding image scale, assuming that the pixel count of the sensor allows such treatment -- a 50% reduction on the IMX455 would still give you a 15 mega pixel image and that might actually be a good choice if using the one-shot-color version of the IMX455.


Edited by james7ca, 07 March 2020 - 03:58 AM.


#24 Morefield

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Posted 07 March 2020 - 12:29 PM

I misspoke by saying that the NP scopes were "designed to image at something around three arc seconds." What I should have said is that they are probably best used at around that image scale (if you want to image at APS-C or larger fields). In truth, I think the original Nagler-Petzval scopes were designed during the film-era of astrophotography, although they did tweak the design for the NP127is about ten years ago to better serve the requirements for digital imaging.

 

If you want to see how an NP127is performs at an image scale of 1.5 arc seconds per pixel (fairly modest given today's CMOS sensors) you can reference this thread on image quality:

 

  https://www.cloudyni...y/#entry6627920

 

There you can see what kind of image quality you can get with an APS-C format. It's not that bad, but you can definitely see what looks like astigmatism in the corners of the field.

 

Frankly, however, if you can find something that is significantly better than this over a full-frame sensor at under one arc second per pixel then I'd like to see those results. But, as you said you can always drop down to the center of the field when you want better image quality. And, you always have the option to resample to a less demanding image scale, assuming that the pixel count of the sensor allows such treatment -- a 50% reduction on the IMX455 would still give you a 15 mega pixel image and that might actually be a good choice if using the one-shot-color version of the IMX455.

 

James, my first imaging scope was a NP127 with an SBIG 8300 chip.  I got rid of it because I was not seeing good corner stars.  I was not using a flattener so maybe some corner star issues should have been expected.  I also was pretty green and so some operator error issues could have been mine - specifically that I was using a 2 inch nose piece to connect the camera rather than a screwed or bolted connection.  

 

Anyway, I replaced it with an FSQ106 and never regretted that switch.   That has me thinking about a TOA130+Flatteners for this setup.  



#25 rockstarbill

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Posted 07 March 2020 - 12:48 PM

AP130 f6.3 GT or GTX. You'll need to buy used, but they are exceptional scopes.
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