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Focal reducer x0,7 + NV D better ?

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#1 gliese

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Posted 22 September 2021 - 05:59 AM

NV D night vision device + focal reducer: (I give my opinion, knowledge, thank you for possibly correcting):

What are the advantages / problems with a good correcteur and focal reducer ?

My opinion: 

1) If I choice NVD +  Paracorr2 (televue) 1.15x + telescope 500mm (20") and  f / d 4 + plossl 55mm: Magnification = 2000x1.15 / 55 = 41.8 . Exit pupil = 500 / 41.8 = 12mm versus the diameter of 26mm diameter lens of the NVD , therefore loss of more half in diameter (and 4x on the surface !!)

 

2) If choice NVD + corrector and focal reducer (x0.7): Magnification = 2000 x0.7 / 55 = 25.5 . Exit pupil = 500 / 25.5 = 19.6mm, i.e. a significant gain / improvement in brightness ( according to me),
and therefore an increase in the SNR (Signal / Noise) ratio?

0.7x is a maximum, because at 0.5x risk of focusing impossible eyepiece , and superior optical aberrations  , and full light field halved if reducer x0.5 , compared to without a reducer (if I understood everything correctly ) ... But maybe not at all annoying for the visual, and very annoying for the photography

 

3) if choice NVD + Russell 85mm eyepiece + Paracorr : 2000x1.15 / 85 = 27 . Exit pupil = 500/27 = 18.5mm but with loss of field due to the narrow eyepiece field ; And aberrations, distortions due to the optical formula and the very long focal length.

My conclusion :
I will see the purchase of this corrector reducer (which would therefore perhaps (?) ,also replace the paracorr 2 ) :

https://www.optics-p...r-0-72x/p,63440
or  https://starizona.co...-coma-corrector
(and : "flattener" or "coma corrector" will correct the peripheral faults that we see in the NVD ... ?)

Thank you very much for your tests and knowledge.


Edited by gliese, 22 September 2021 - 06:03 AM.


#2 a__l

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Posted 22 September 2021 - 07:27 AM

You have a lot of methodical errors.

Prime focus. For a 2000mm focal lengths and a ~ 19mm sensor NV, the field of your telescope is approximately 0.5 deg.

 

Afocal TV 67 plossl 40 deg+P2: field approximately 1 deg.

Prime focus +0.7x reducer/corrector: field 0.7 deg

Afocal TV67 + P2 will be brighter, therefore preferred.

 

Afocal Russell 85mm+P2: field approximately 1 deg (limitation for 2" at 40 deg).

The scale will be smaller and the details are less visible (than TV67), I do not recommend it.

 

https://www.optics-p..._bar_0_select  

There is not a word in the description that this is a coma corrector. Therefore, it will not work for you.


Edited by a__l, 22 September 2021 - 07:45 AM.


#3 Jeff Morgan

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Posted 23 September 2021 - 12:39 AM

You are making this harder than it really is.

 

The analogy is conventional eyepieces in your telescope. You build an eyepiece collection to give a range of magnifications and/or true fields. Not every eyepiece is optimal for every target. But the variety of targets available demands flexibility.

 

In NV, the same concept applies.

 

Thus, the afocal TV 67 Plossl does not really compete with the NVD + 0.7x reducer.

 

Rather, the NVD + Reducer is just another step in the magnification progression.

 

Whether your device is Prime or Afocal only, progressions are a concept you should embrace.


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#4 a__l

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Posted 23 September 2021 - 05:44 AM

The analogy is conventional eyepieces in your telescope. You build an eyepiece collection to give a range of magnifications and/or true fields. Not every eyepiece is optimal for every target. But the variety of targets available demands flexibility.

 

 

No, it’s not like that.
Glass eyepieces are quick and easy to change in the focuser.
In order to change afocal to prime, you need to do a many of manipulations. Therefore, I plan observations like this, first objects for afocal TV67, then objects for prime. Or vice versa. For 0.7 reducer, I have no time left and I do not need it.
My telescopes are roughly what the OP says (~2000mm f/3.3 and ~1800mm f/4).
Another option is to have two complete NV kits, but this is an expensive pleasure and I don't see the need for it.

 

I will explain that afocal NV TV67 is a long and heavy train. There is no desire to often pull/insert into the focuser at night. But it has clear advantages over 0.7x, so I use it.

 

Another point.
The prime is clearly better picture than the comparable afocal on the N31 (I have no other 30mm eyepieces), so I use it over the afocal.

 

Third point. There are some difficulties in combining prime with paracorr. This can add more time to you if you don't have two paracorr. If it is P2, then I doubt at all whether it is possible to manipulate one such at night by changing the afocal to prime. Except to find a set of rings for afocal TV67. But then another problem arises. Difficulty using glass eyepieces at the same time.

 

Something like this.


Edited by a__l, 23 September 2021 - 06:36 AM.


#5 GOLGO13

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Posted 23 September 2021 - 07:06 AM

I usually do a combination of prime and afocal. 

 

1) Prime without reduction

2) prime with a 2x Barlow for zooming in

3) prime with a .7 reducer

4) afocal with the 67mm setup for further reduction. 

 

All of these cover framing different objects 

 

And also using different telescopes. 

 

1) 8 inch SCT for globular clusters and small planetary nebulas. 

2) 10 inch F4.7 dob for a bit zoomed in. I consider this my best NV scope.

3) 6 inch F5 refractor for a wider field of view especially with the 67mm setup.

 

I'm probably going to sell it, but the 6 inch F4 newt works very well also, but really needs to be used in prime focus. I also was able to get the filter wheel and MPCC working with it at F4. I would not suggest this at all for afocal.  

 

And of course 1x observing and camera lens for really wide fields.  



#6 a__l

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Posted 23 September 2021 - 07:57 AM

GOLGO13, OP asked a question about 20" f/4

First, it makes no sense to use such an aperture on strong light pollution.

You are apparently not using P2. For f/4, this is essential. P2 is one of the best correctors. Certain difficulties arise with him. Or at least P1.

 

For majority of globular clusters and small planetary nebulas, it is better to use glass eyepieces in this aperture. Color appears on stars, and planetary nebulas contain many other elements besides hydrogen.

 

Afocal TV 67 is mainly used for extended hydrogen objects.

 

Prime focus mainly for galaxies with contrasting details.

 

Of course, there are some exceptions.

 

Something like this.


Edited by a__l, 23 September 2021 - 07:58 AM.


#7 GOLGO13

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Posted 23 September 2021 - 09:32 AM

GOLGO13, OP asked a question about 20" f/4
First, it makes no sense to use such an aperture on strong light pollution.
You are apparently not using P2. For f/4, this is essential. P2 is one of the best correctors. Certain difficulties arise with him. Or at least P1.

For majority of globular clusters and small planetary nebulas, it is better to use glass eyepieces in this aperture. Color appears on stars, and planetary nebulas contain many other elements besides hydrogen.

Afocal TV 67 is mainly used for extended hydrogen objects.

Prime focus mainly for galaxies with contrasting details.

Of course, there are some exceptions.

Something like this.


With the MPCC and a filter wheel the 6 inch F4 is well corrected. Personally I don't mind coma when observing nebulas with NV. But for everything else it's very helpful to have a coma corrector.

I have a P1 that works fine with the 67mm in my 10 inch dob. Of course it does add to the crazy tall and heavy stack.

I gave up on trying to use the P2 with NV at prime focus. Would have required a lot of spacers and math.

I agree that globs and planetary nebulas in a dark sky are better with glass. In light pollution other way around.

#8 gliese

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Posted 23 September 2021 - 10:27 AM

Thank you Gemini, especially for having seen that one of the 2 reducers did not correct the coma but just flatten, I had not paid enough attention.

For your field calculations, I don't know which formula you are applying (and that we must apply). And I try especially to understand how to gain signal (therefore in opening, therefore reduction of vignetting, therefore in increase of the exit pupil). And I just have a TV55mm et I don't know that there is a TV 67mm :-( ! .

 

Gemini, you tell Jeff Morgan that the TV67mm would be better in general than the TV 55mm (which I have) and better than the reducer / corrector that I expect to see to buy. that's right , please ?

,
Not sure to have correctly translated / understood Gemini = When you want to do "prime" you need another setting of the Paracorr (I didn't know, paracorr doesn't say anything, put it on "C" or "B"?) And so too difficult to do this easily at night. Is that correct, did I translate correctly?

 



#9 gliese

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Posted 23 September 2021 - 10:29 AM

Thank you Jeff Morgan ,  for your remark to keep in mind , and it is useful to remind me that there is not a very good montage, but several good ones. But I'm afraid of losing precious, expensive electrons / photons (!), And I'm trying to figure out how to avoid it; And if possible also to gain in definition on the periphery of the ocular field, with a clearly visible coma (=?), By adding an additional coma correction (= !?), or different coma corrector from the Paracorr which does not remove everything



#10 gliese

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Posted 23 September 2021 - 10:34 AM

Thank you Golgo13, I do not think enough about the assembly at the primary focus, and therefore be able to use the reducer also with this assembly



#11 gliese

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Posted 23 September 2021 - 10:42 AM

Nobody knows if it is a big handicap, not to light all the objective of the NV (or its entrance pupil if we remove the objective lens to make "prime"), because of a beam of light vignette / reduced by, for example, an eyepiece exit pupil smaller in diameter, than that of the NV aperture (my NV is a " OVNI-M " with 23mm objective lens diameter... so it is very very rare to have an eyepiece exit pupil of this size !! And it is even impossible :-(

 

So significant losses of very many and very expensive electrons-photonsbawling.gifbawling.gifbawling.gif



#12 gliese

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Posted 23 September 2021 - 10:51 AM

Last remark : In reality I have an F / D 3.6  , I had simplified by putting a 20 "   to 4  , to have 2000mm focal length , easy to calculate)  .  Use the Paracorr + a coma corrector reducer .... In the hope of reducing the coma that the Paracorr could not remove  woohoo.gif   ... But I think I'm dreaming sitting in front of my screen 



#13 ButterFly

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Posted 23 September 2021 - 01:08 PM

The TV 55 Plossl becomes a 67 PP becomes from a little lens placed under the volcano top of the 55 Plossl.  Highly recommended!  It maxes out the exit pupil than any scope can present, given the 40 degree limit of NV devices.

 

20" dob typically means truss tubes and poor baffling.  Look at your dob at 1x with your device.  Replace anything that glows in near-IR.  My shroud was replaced.  Then look through the focuser at 1x.  I only have a virtual mirror box, so the back of the mirror is exposed.  Draping something over the back end helps a little.  I added a full around dew shield as well with the same non-glowing fabric I used for the shroud.  The tube itself has an SNR, so feeding it a good SNR to start with helps a lot.  It's not just about more photons (which you have 10,000x+ anyway), but the least BAD photons as well.

 

I have settled on 67PP, 41 Pan, and Baader zoom as my main eyepieces.  The Baader zoom has some internal reflections that show up at its lower focal lengths.  I plan on trying the APM zoom, with native Dioptrx support, when and if it finally is released.  I may either blacken the internal metal of the Baader, or just stick with the APM.  Think in terms of exit pupil to make your life easier when planning steps.  Eyepiece focal length divided by f/ratio, as modified by reducer or barlow, if needed.  You will still see only 40 degrees AFOV at the other end anyway, so it's the same calculation for TFOV and mag steps.



#14 ButterFly

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Posted 23 September 2021 - 01:10 PM

 

I will explain that afocal NV TV67 is a long and heavy train. There is no desire to often pull/insert into the focuser at night. But it has clear advantages over 0.7x, so I use it.

That's also why I hate pants, but yes, there are clear advantages when the wind gets cold.  Different night, different target and all.  It's good to not be dogged and leave yourself options, like putting on pants, or different pants.



#15 a__l

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Posted 23 September 2021 - 05:54 PM

Thank you Gemini, especially for having seen that one of the 2 reducers did not correct the coma but just flatten, I had not paid enough attention.

 

Sputnik (good Russian name), no problem smile.gif

 

 

For your field calculations, I don't know which formula you are applying (and that we must apply). 

For prime, you can use any astrophoto calculator. Accept sensor size Gen 3 NV 17 ... 19 mm.

https://astronomy.to.../field_of_view/

For afocal, the field of view of the eyepiece is divided by the magnification.

 

 

And I try especially to understand how to gain signal (therefore in opening, therefore reduction of vignetting, therefore in increase of the exit pupil). 

 

The exit pupil of the TV67 is about 16mm (focal length 2300 with P2), which is fully captured by the Envis lens. There will be no vignetting here.

Another issue is vignetting 2" paracorr-2. Fully illuminated field for the 2" P-2 for f/3.5  -  26.0 mm, for TV55 plossl field stop 46 мм. So ideally it's better to use 3" P-2. But the price hmm ...

Don't forget about the size of your secondary.

 

 

And I just have a TV55mm et I don't know that there is a TV 67mm :-( ! .

 

https://televue.com/..._page.asp?id=36

 

 

Gemini, you tell Jeff Morgan that the TV67mm would be better in general than the TV 55mm (which I have) and better than the reducer / corrector that I expect to see to buy. that's right , please ?

 

You will determine this for yourself. Based on the price of the solution and your preferences.
What I mean by "better" is that the picture will be brighter and the field of view is larger (for afocal TV67).

 

 

Not sure to have correctly translated / understood Gemini = When you want to do "prime" you need another setting of the Paracorr (I didn't know, paracorr doesn't say anything, put it on "C" or "B"?) And so too difficult to do this easily at night. Is that correct, did I translate correctly?

You will have to remove the P2 head (by unscrewing the small screws) and install the astrophoto adapter. Next, calculate the required distance to the NV sensor and screw the corresponding ring onto NV. That is why I am writing about the need for 2 pieces of P2 for NV.

https://www.bhphotov...er_for_2_0.html

 

Further successes with NV smile.gif


Edited by a__l, 23 September 2021 - 06:18 PM.


#16 Jeff Morgan

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Posted 24 September 2021 - 12:35 AM

No, it’s not like that.

Yes, it is like that.

 

Like you, I greatly prefer Prime operation. But for maximum reduction, afocal is the the best game in town. At times, there is just no substitute. 
 

Granted the Hassle Factor is greater when a Paracorr Dob is involved, but one just puts on the Big Boy pants and deals with it. A little organization in the nights target list goes a great distance in that regard. 

 

And lots of us do NV with refractors which is is a simpler world and offers greater true fields.


Edited by Jeff Morgan, 24 September 2021 - 12:35 AM.


#17 gliese

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Posted 25 September 2021 - 12:23 PM

Butterfly, thank you for the new idea that the telescope would have parts that radiate too much in IR. I will test, if I do the galaxies , with a lot of inputs of IR

 

Yes Gemini, I just saw that I am a year late, too confined (?!)  , And not seen the 67mmTV! gaah.gif
With your details given : "Further successes with NV " .... yes! waytogo.gif

The 2 correctors and focal length reducers x0.7 proposed at the beginning, are to be discarded. First because there is already a TV67mm reducer (and cheaper), and also, optical engineers, calculate the correctors according to the focal lengths, the FD and the final receiver ... and a CCD has different optics and curvature of an eye. I just saw that you should not employ a photo corrector for the visual, ... = bad images.



#18 gliese

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Posted 25 September 2021 - 01:05 PM

There are  still improvements with the filters! and no manufacturer (Lumicon, Baader, Chroma etc) offers a specific filter, despite the number of practicing of NV, and despite the often significant purchasing power of their owners :-( !

So we would not hesitate too much to the buy

 

 

It would be necessary perhaps  a BP 5 or 7nm Ha filter , not to amplify the sides, therefore the unnecessary lambda , and thus increase the noise. And also the same thing for the O3. The addition of the 2 luminosity should appreciably increase the SNR. ..... like additions of photos taken with different filters

The emission in H beta is much lower, of memory.

And can to let IR pass? All current filters are "IR cut " for astrophoto :-(    while NV are "50%" in IR! ... Unless the sky background radiates too much in IR, I don't think so.

 

Someone who writes good English, could he contact the manufacturers, to further increase our emotions, our stellar panoramas ? bigblush.gif   praying.gif



#19 gliese

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Posted 25 September 2021 - 01:31 PM

and maybe the  SII  (at 6717 ) , and also Argon (same intensity  than SII  ! )  at 7135    .....



#20 gliese

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Posted 30 September 2021 - 04:59 PM

I am going to catch up with my delay that I had with the TV67mm.

I searched and found:

1)I am going to try a narrow dual band IDAS 7nm Ha and 7nm O3 filter, which theoretically should increase the gain / brightness with such a narrow dual band .

2) And the idea of ​​testing the Nexus corrector reducer 0.75x Starizona, special for Newton. With the aim and hope of also gaining the gain of the SNR, by avoiding the Paracorr 2 which makes lose 15% of F / D ........   + 0.75x reduction of focal length in Plus. Has anyone ever tested?




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