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Equipment Discussions >> Electronically Assisted Astronomy

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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6
      #5700984 - 02/26/13 03:49 AM

Currently I have a Canon 550D DSLR and ST80 F/5 refractor.



I'm thinking to buy a CCTV Lens for video astronomy like this manual IRIS Tamron 20-100mm Variofocal Lens F/1.6 ( http://www.tamron.co.jp/en/data/cctv/13vm20100as.html )for an astro video camera with 1/3" Sony sensor size, but I have the following dillema:

a) will the Tamron 20-100mm+Lntech camera gather light much faster than a 550D+ST80+x0.5 focal reducer?

b) what will be the maximum aperture for Tamron when set at 100mm focal length? Will it remain F/1.6?

c) what will be the image scale for the combos on large DSOs like M31?

d) what will be the image quality difference between the 2 combos?



According to this site calculations http://creativemediatech.our.dmu.ac.uk/tag/crop-factor/ , the crop factor between the APS-C and 1/3" is 26.8 / 6 = 4.55.

If you'll ask why the combo between Lntech and Tamron 20-100mm lens, I tell you it's because of weight, ease of use, space, image scale, more light gathering per time unit (I think in favour of the Lntech+Tamron).


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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5701009 - 02/26/13 04:33 AM

Sorry, I made a mistake at point a). There should be no x0.5 focal reducer, but just 550D+ST80.


e) what differences (like speed, image quality) will be between Tamron 20-100mm+Lntech camera and ST80+x0.5 reducer+Lntech camera?


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5701152 - 02/26/13 08:28 AM

That CCTV lens' front lens size is 32mm (thus for fl=20mm, it's f/1.6).
For fl=100mm setting, the f-ratio is 8! <-- sorry my mistake. It's not that low.

(That's why I said in the previous thread that you have to read marketing spec. carefully.)

Clear Skies!

ccs_hello


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5701269 - 02/26/13 09:44 AM

Quote:

Currently I have a Canon 550D DSLR and ST80 F/5 refractor.



I'm thinking to buy a CCTV Lens for video astronomy like this manual IRIS Tamron 20-100mm Variofocal Lens F/1.6 ( http://www.tamron.co.jp/en/data/cctv/13vm20100as.html )for an astro video camera with 1/3" Sony sensor size, but I have the following dillema:

a) will the Tamron 20-100mm+Lntech camera gather light much faster than a 550D+ST80+x0.5 focal reducer?

b) what will be the maximum aperture for Tamron when set at 100mm focal length? Will it remain F/1.6?

c) what will be the image scale for the combos on large DSOs like M31?

d) what will be the image quality difference between the 2 combos?



According to this site calculations http://creativemediatech.our.dmu.ac.uk/tag/crop-factor/ , the crop factor between the APS-C and 1/3" is 26.8 / 6 = 4.55.

If you'll ask why the combo between Lntech and Tamron 20-100mm lens, I tell you it's because of weight, ease of use, space, image scale, more light gathering per time unit (I think in favour of the Lntech+Tamron).




You're better off buying a .5x focal reducer for your short tube refractor to use with a video cam. You'd be operating at F/2.5 and FL=200mm .
A 1/3" Sony icx672 CCD would have an image scale of about 6.3 arcsec/pixel and M31 would almost fit on the sensor (field of view of just over 1 deg with the reducer ) .
Your Canon would have a field of view of 2x3 degrees without a reducer with the same ST80 and an image scale of 2.3 arcsec/pixel. M31 would be framed just right .
The Lntech with the .5x reducer would gather 7.5 more light per pixel than the 550D without the reducer .
Forget about tiny cheap slow C-mount video lenses .There are no miracles and there is no substitute for aperture .


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nytecam
Postmaster


Reged: 08/20/05

Loc: London UK
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5701286 - 02/26/13 09:56 AM

Quote:

I'm thinking to buy a CCTV Lens for video astronomy like this manual IRIS Tamron 20-100mm Variofocal Lens F/1.6 ( http://www.tamron.co.jp/en/data/cctv/13vm20100as.html )for an astro video camera with 1/3" Sony sensor size, but I have the following dillema:

a) will the Tamron 20-100mm+Lntech camera gather light much faster than a 550D+ST80+x0.5 focal reducer?

b) what will be the maximum aperture for Tamron when set at 100mm focal length? Will it remain F/1.6?

c) what will be the image scale for the combos on large DSOs like M31?

d) what will be the image quality difference between the 2 combos?

According to this site calculations http://creativemediatech.our.dmu.ac.uk/tag/crop-factor/ , the crop factor between the APS-C and 1/3" is 26.8 / 6 = 4.55.

If you'll ask why the combo between Lntech and Tamron 20-100mm lens, I tell you it's because of weight, ease of use, space, image scale, more light gathering per time unit (I think in favour of the Lntech+Tamron).


Much of the info you request like angle of view [check M31 against starchart] is already on the Tamron page you link.

Under b]No - the f/ratio @ 100mm fl can't be f/1.6. The gross diam of lens mount is 49mm whilst 100/1.6=62.5mm. It's difficult to guess the lens construction but if the gross lens aperture is 49mm - ~5mm [=lens mount] = say 44mm then f/ratio @ 100mm fl = 100/44 = ~f/2.3 eg still 'fast' and may suck-in LP if present ! If available try cheap s/h fixed focus camera lenses with M42/Pentax screw from old SLRs and couple to video cam with adapter. Good luck


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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: nytecam]
      #5701587 - 02/26/13 12:32 PM

@nytecam: I've done the calculations like you and that's why I was thrilled about using the Tamron+Lntech. I thought Tamron has ~45mm front glass element and at 100mm focal length it will have F/2.2-2.3 which is quite fast. On the other hand, it seems that ccshello is partially right too by saying that at 20mm Tamron has F/1.6 and 32mm front glass element, but at 100mm we have an F/3,12 (100mm/32mm=3,12).

On the other hand, it seems strange to have a lens diameter of 49mm with a front glass element of only 32mm!

Now, who's calculations are wrong, ccshelo's or nytecam's?




@ccshello: You are right, but I have read the marketing specs and I haven't understood them very well because I don't have your knowledge/experience.

@mattflastro: We have to try to find a way to cut corners in video astronomy at least.


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5702175 - 02/26/13 05:59 PM

Quote:

@nytecam: I've done the calculations like you and that's why I was thrilled about using the Tamron+Lntech. I thought Tamron has ~45mm front glass element and at 100mm focal length it will have F/2.2-2.3 which is quite fast. On the other hand, it seems that ccshello is partially right too by saying that at 20mm Tamron has F/1.6 and 32mm front glass element, but at 100mm we have an F/3,12 (100mm/32mm=3,12).

On the other hand, it seems strange to have a lens diameter of 49mm with a front glass element of only 32mm!

Now, who's calculations are wrong, ccshelo's or nytecam's?




@ccshello: You are right, but I have read the marketing specs and I haven't understood them very well because I don't have your knowledge/experience.

@mattflastro: We have to try to find a way to cut corners in video astronomy at least.



None of the calculations you quoted is correct.
At a focal length of 20mm F/1.6 means 20mm/dia =1.6 , => dia=20mm/1.6 = 12.5 mm effective diameter.
The drawing that shows the lens dimensions shows an O.D. =49mm . From the photo you can see the front lens is much smaller than the barrel O.D. .
However , this is irrelevant since the front lens is not the one that defines the aperture . If this lens had its effective aperture defined by the front lens, it would be a very fast lens, let's say 20mm f.l. divided by 35mm front lens dia. = 0.57 . Wow, an F/0.57 lens, I wonder why do they call it F/1.6 ????
Some much smaller lens burried deeep inside is the one that determines the effective aperture and it's 12.5mm.
At the long end you get 100mm/12.5mm = 8 . So ccs_hello was correct at the long end with F/8 .
Again, you are chasing the wrong idea with a tiny lens hoping to get more light gathering than your ST80 . No matter what you do, the ST80 will get way more photons . All you need is to funnel them to the camera sensor with an appropriate reducer to get the image scale your target needs.
A .5x reducer sized for the 1/3" ccd's is inexpensive and represents better corner cutting than wasting the same amount of money on a tiny peephole lens .


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5702338 - 02/26/13 07:38 PM

probably you noticed that I do not have too great a confidence on a consumer grade CCTV lens.

With the $109 price tag, a 20-100mm f/1.6 (no way 100mm will be at F1.6), and the front lens diameter of just (approx.) 35-40mm, I'd say you'll be disappointed.

These consumer grade lenses play games on marketing spec.
That lens may at a range of focal lengths that have f/1.2. At 20mm end, some field lens' aperture is not used. At 100mm end, the max. possible f-ratio will be 100mm/35mm = approx 3-ish. This is just a optimistic number. Can be much lower than that.

Do not forget it does not even want to spell it out its vari-f-ratio nature (not a welcome sign), does not want to say it's apochromatic, not saying it's an all glass lenses construction, and not saying if it's fully multi-coated.
I.e., it's a consumer grade lens.
Also means YMMV.


Clear Skies!

ccs_hello


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5702342 - 02/26/13 07:41 PM

RE: FoV question

I'd suggest download the CCD Calculator program
http://www.newastro.com/book_new/camera_app.php
Very handy little program to use.

Clear Skies!

ccs_hello


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ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: ccs_hello]
      #5702679 - 02/26/13 11:23 PM Attachment (14 downloads)

Let me add a Computar f/1.2 (at the shorter fl end) TV Zoom Lens picture:
fl= 12.5mm -75mm (6x zoom)
Front lens diameter is 45mm.
(has motorized zoom, focus, and aperture/iris for remote controls)


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: ccs_hello]
      #5703392 - 02/27/13 12:13 PM

Faster telephoto lenses will retain their f/ratio up to some certain point in the zoom range, thereafter becoming slower with further zooming.

Just because the aperture at minimum zoom is 12.5mm does not mean it remains fixed at 12.5mm at every other zoom setting, resulting in an immediate worsening of the f/ratio as soon as one zooms in from the shortest focal length. For example, it could remain at f/1.6 up to, say, 50mm focal length, where the clear aperture limit kicks in, and the resultant continuous increasing f/ratio with continued zooming thereafter.


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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: GlennLeDrew]
      #5703614 - 02/27/13 02:30 PM

Considering the advice and explanations given I have to say to all of you "Thank you" for not letting me spent money unwisely.
It's clear for me now that I don't understand how the aperture is computed for these cctv lenses at different focal lengths.
To be honest till the end I have to say I'm disappointed I can't cut corners when talking about optics.
Maybe miracles don't come cheap.

Still I have to try to cut one last corner. What happens if you use this combo: Canon 55-250mm lens + adapter from Canon EOS to T2 + cheap x0.5 reducer + Lntech/Samsung? Is it possible to reach focus?


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: GlennLeDrew]
      #5703847 - 02/27/13 05:00 PM

Quote:

Faster telephoto lenses will retain their f/ratio up to some certain point in the zoom range, thereafter becoming slower with further zooming.

Just because the aperture at minimum zoom is 12.5mm does not mean it remains fixed at 12.5mm at every other zoom setting, resulting in an immediate worsening of the f/ratio as soon as one zooms in from the shortest focal length. For example, it could remain at f/1.6 up to, say, 50mm focal length, where the clear aperture limit kicks in, and the resultant continuous increasing f/ratio with continued zooming thereafter.



OF course you are correct , the initial explanation was a bit of an oversimplification .
However, the idea behind that simplification was correct, these lenses are slowest at the long end instead of constant f ratio.
Whether the variation law has a certain type of curve or another is anybody's guess , only the vendor (hopefully) knows.


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5703855 - 02/27/13 05:07 PM

Quote:

Considering the advice and explanations given I have to say to all of you "Thank you" for not letting me spent money unwisely.
It's clear for me now that I don't understand how the aperture is computed for these cctv lenses at different focal lengths.
To be honest till the end I have to say I'm disappointed I can't cut corners when talking about optics.
Maybe miracles don't come cheap.

Still I have to try to cut one last corner. What happens if you use this combo: Canon 55-250mm lens + adapter from Canon EOS to T2 + cheap x0.5 reducer + Lntech/Samsung? Is it possible to reach focus?



You can use it but it's slow at f/4-5.6 .
If you have the lens, can't hurt to try and see if it fits your needs . All you need is a mount adapter from Canon to C mount and they're cheap on ebay.
If you want a faster lens for cheap, your best bet is to look for some older 135mm to 200mm prime lenses for deceased film cameras.
There are lots of Rokkors, Soligors, Vivitars (I'm talking of the old and good ones not the new cheap plastic junk bearing the same names). You can get a 200mm Rokkor f/3.5 for $20 or a Soligor 200mm f/2.8 for about $50 .A Rokkor 135mm f/3.5 might be under $10 . Then you just add your mount adapter and you're set.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: mattflastro]
      #5704095 - 02/27/13 07:32 PM

Squeezing a focal reducer between an SLR type lens and camera might be problematic. Suppose the back focus on the lens is, say, 50mm. The 1.25" format 0.5X reducer has a focal length of just about 100mm, and so in order to work at 0.5X must be located 45-50mm from the CCD. Furthermore, a reducer requires the chip to be moved *closer* to the imaging lens/objective than otherwise, further restricting the working space.

And to top it all off, the reducer's relative native f/4 means you will not likely be able to get faster than f/2-2.5 anyway, no matter how hard you try.

Obtain a natively fast lens, such as a 135mm f/1.8-2.


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: GlennLeDrew]
      #5704592 - 02/28/13 12:39 AM

Glenn is right. FR will NOT work on a SLR lens due to it has too short a flange-back focus distance and FR needs to be further into lens body to achieve infinity focus.

Re: Canon 75-300 consumer lens
Fairly *BLEEP* for astro. May be good enough for videocam (due to its low resolution and low S/N to start with) even with its lousy MTF and bad Chromatic Aberration. Its f-ratio is slow anyway.
Some old Manual Focus lenses are okay (good MTF) and cheap. But so-so on CA correction since the low-dispersion (LD) glass has not been mass-produced .
A MF lens with ED optics would be nice, if the price is right.

Morale of the story:
videocams have some headrooms to play price-wise.
On the other hand, high quality optics does not come cheap. There is no free lunch in that department.
(Used market is a different story, of course.)

Clear Skies!

ccs_hello


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: ccs_hello]
      #5704862 - 02/28/13 08:20 AM Attachment (10 downloads)

Add a frontal shot of a Fujinon TV lens.
(14x zoom, f/1.9, very large image circle, heavy, but inexpensive in used market)


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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: ccs_hello]
      #5705040 - 02/28/13 10:20 AM

I really appreciate all your help and expert advice! Problem is I can't find in my country on used market the kind of cheap lenses you told me to look for and buying from Ebay doesn't make things cheap for me due to transportation cost and import taxes.

I have a T2 to 1.25" adapter from Baader like this one (which acts as a precise focuser too) http://www.teleskop-express.de/shop/product_info.php/language/en/info/p1751_B...

What if I use the Baader adapter with a Lntech like camera + 1.25" to C-Mount adapter + cheap x0.5 reducer + UV/IR cut filter + Baader T2-1.25" adapter + this lens http://www.amazon.com/Samyang-500mm-Mirror-White-Mount/dp/B006MKF8KO/ref=sr_1... ?
Will I be able to reach focus with Samyang lens?

A bonus will be price, lack of CA and less weight than a ST80.


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Moromete
professor emeritus


Reged: 02/15/12

Loc: Romania
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5705054 - 02/28/13 10:27 AM

Here are some technical details for Samyang lens http://www.syopt.co.kr/common/pdf/f=500F6.3MIRROR.pdf

I don't know what is diamtere of teh central obstruction. I'm thinking if this lens comes close to a little 66mm F/6 APO but a much lower price.


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: DSLR 550D+ST80 vs Lntech+100mm Varifocal Lens F1.6 new [Re: Moromete]
      #5705412 - 02/28/13 02:01 PM

Quote:

I really appreciate all your help and expert advice! Problem is I can't find in my country on used market the kind of cheap lenses you told me to look for and buying from Ebay doesn't make things cheap for me due to transportation cost and import taxes.

I have a T2 to 1.25" adapter from Baader like this one (which acts as a precise focuser too) http://www.teleskop-express.de/shop/product_info.php/language/en/info/p1751_B...

What if I use the Baader adapter with a Lntech like camera + 1.25" to C-Mount adapter + cheap x0.5 reducer + UV/IR cut filter + Baader T2-1.25" adapter + this lens http://www.amazon.com/Samyang-500mm-Mirror-White-Mount/dp/B006MKF8KO/ref=sr_1... ?
Will I be able to reach focus with Samyang lens?

A bonus will be price, lack of CA and less weight than a ST80.



I think you are speaking general theory , which is true ...generally speaking .
If you actually take the time and look for particular items, you can find better deals .
For example a 200mm Soligor C/D F/2.8 prime lens (this was a good model) like this:

web page

These sell for about $50. This particular one lists shipping internationally for $47, total under $100 . That's less than what you want to pay for that little SCT mirror lens without shipping. It also does NOT need a reducer , cheap or otherwise, because it's already F/2.8 . So it's by far the most economic option.Not to mention the SCT lens is not that wide field .
That is a sharp lens , was actually made by Tokina and whatever little CA it might have , it will be minimal and way under the chroma bleed in PAL/NTSC.


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