165 replies to this topic

[ButterFly]

Ok, max.field of view...would that be possible by using the PVS-14 on telescope without any eyepiece? Or is the 55mm Plossl + 67mm reducer as wide as it gets?

A PVS14 with its objective gives you a 1x view, only brighter. Look through your scope without an eyepiece. That's what you'll see, only brighter. The sky won't be in focus.

A 67mm 40 degree is about as wide as it gets for a 2" focuser. It's the focal length of an eyepiece with the largest field stop and a forty degree AFOV. It's about 100mm for a 3" focuser, and about 40mm for a 1.25" focuser, both forty degree AFOV. That's all the angle the PVS14's objective can see.

[patindaytona]

A PVS14 with its objective gives you a 1x view, only brighter. Look through your scope without an eyepiece. That's what you'll see, only brighter. The sky won't be in focus.

A 67mm 40 degree is about as wide as it gets for a 2" focuser. It's the focal length of an eyepiece with the largest field stop and a forty degree AFOV. It's about 100mm for a 3" focuser, and about 40mm for a 1.25" focuser, both forty degree AFOV. That's all the angle the PVS14's objective can see.

When that 67mm adapter is used with the 55mm Plossl, it essentially is turning that 55mm into a 67mm eyepiece? If so, then that setup is giving me the brightest image possible. But no matter what, i cannot get more than 40 degree AFOV with any eyepiece?

Edited by patindaytona, 13 May 2022 - 06:45 AM.

[Deadlake]

I think the field stop is around 47 mm for the TV 55 mm, the same for the TV67 mm????

The only way of making a Baader BBHS 2" play ball with a larger field stop requirement than a 47 mm would be adding a S52 ring and connecting via a M68 to the scope that would give you 5 mm more. 

Or going with a 3" diagonal, however I've not seen many people using one with an NVD.

[GOLGO13]

When that 67mm adapter is used with the 55mm Plossl, it essentially is turning that 55mm into a 67mm eyepiece? If so, then that setup is giving me the brightest image possible. But no matter what, i cannot get more than 40 degree AFOV with any eyepiece?

I think that's correct enough.

[Jethro7]

When that 67mm adapter is used with the 55mm Plossl, it essentially is turning that 55mm into a 67mm eyepiece? If so, then that setup is giving me the brightest image possible. But no matter what, i cannot get more than 40 degree AFOV with any eyepiece?

Hello Pat,

GOLGO13, is correct. The NVG operates at 40° FOV and sqeezes the views down to 40° So what we have to do, to increase the AFOV that we see in the eyepiece of the NVD, is to use longer focal length eyepieces and or focal reducers and faster focal ratio telescopes. I recently purchased a Skywatcher SW 120 F/5 Achro and a Russell Optics 85mm Plossl. In theory this combination should provide one hell of a view of the Milkyway and the Cygnus Nebulae complex. If I can reach focus. "I Hope" This should give a AFOV of 5.67° add  the .08X reducer the AFOV goes up to 7.08° at 7X with the SW 120 F/5 as opposed to using my Tak TSA 120 F/ 7.9 that has an AFOV of 3.78° at 10.6 X and a AFOV 4.72° With the. 08X reducer at 8.47 X with the same eyepiece and focal reducer.  if my math is correct. This is the only way to see more area of the sky with a NVD with a scope. Or just use the NVD by its self at 1X. with a AFOV of 9°.This is also really fun to do.

 

HAPPY SKIES TONYOU AND KEEP LOOKING UP Jethro

Edited by Jethro7, 13 May 2022 - 08:22 PM.

[StarAlert]

Hello Pat,

GOLGO13, is correct. The NVG operates at 40° FOV and sqeezes the views down to 40° So what we have to do, to increase the AFOV that we see in the eyepiece of the NVD, is to use longer focal length eyepieces and or focal reducers and faster focal ratio telescopes. I recently purchased a Skywatcher SW 120 F/5 Achro and a Russell Optics 85mm Plossl. In theory this combination should provide one hell of a view of the Milkyway and the Cygnus Nebulae complex. If I can reach focus. "I Hope" This should give a AFOV of 5.67° add  the .08X reducer the AFOV goes up to 7.08° at 7X with the SW 120 F/5 as opposed to using my Tak TSA 120 F/ 7.9 that has an AFOV of 3.78° at 10.6 X and a AFOV 4.72° With the. 08X reducer at 8.47 X with the same eyepiece and focal reducer.  if my math is correct. This is the only way to see more area of the sky with a NVD with a scope. Or just use the NVD by its self at 1X. with a AFOV of 9°.This is also really fun to do.

 

HAPPY SKIES TONYOU AND KEEP LOOKING UP Jethro

I think you’re going to be disappointed. By my calculations, the max FOV with your SW is 4.6^o. That’s going to be the case whether you use the TV 67mm or the RO 85mm. And a reducer will do nothing for you other than reduce the image scale. You are limited by the 2” field stop. The TV67 is designed to maximized the FOV for a 2” diagonal. 

[chemisted]

Hello Pat,

GOLGO13, is correct. The NVG operates at 40° FOV and sqeezes the views down to 40° So what we have to do, to increase the AFOV that we see in the eyepiece of the NVD, is to use longer focal length eyepieces and or focal reducers and faster focal ratio telescopes. I recently purchased a Skywatcher SW 120 F/5 Achro and a Russell Optics 85mm Plossl. In theory this combination should provide one hell of a view of the Milkyway and the Cygnus Nebulae complex. If I can reach focus. "I Hope" This should give a AFOV of 5.67° add  the .08X reducer the AFOV goes up to 7.08° at 7X with the SW 120 F/5 as opposed to using my Tak TSA 120 F/ 7.9 that has an AFOV of 3.78° at 10.6 X and a AFOV 4.72° With the. 08X reducer at 8.47 X with the same eyepiece and focal reducer.  if my math is correct. This is the only way to see more area of the sky with a NVD with a scope. Or just use the NVD by its self at 1X. with a AFOV of 9°.This is also really fun to do.

 

HAPPY SKIES TONYOU AND KEEP LOOKING UP Jethro

 

I think you’re going to be disappointed. By my calculations, the max FOV with your SW is 4.6^o. That’s going to be the case whether you use the TV 67mm or the RO 85mm. And a reducer will do nothing for you other than reduce the image scale. You are limited by the 2” field stop. The TV67 is designed to maximized the FOV for a 2” diagonal. 

There are problems with both of these posts.  The True Field of View (TFOV) is given by the equation:  TFOV = (57.3 X Field Stop) / Telescope focal length.  StarAlert correctly points out that for a 2" eyepiece and a 120mm f/5 telescope this is roughly 4.6 degrees.  However a focal reducer decreases the focal length of the telescope and consequently increases the field of view.  For a 0.8X reducer the TFOV is now ~5.7 degrees.  Similarly, the 0.8x reducer lowers the focal ratio of an f/5 scope to f/4 which will give a 21.25 mm exit pupil with the 85mm Plossl (85/4) and the fastest possible final focal ratio of ~ 1.2.  For a practical example see my post # 12 in the following thread:

 

https://www.cloudyni...al-with-ovni-m/

Edited by chemisted, 14 May 2022 - 04:45 AM.

[Jethro7]

I think you’re going to be disappointed. By my calculations, the max FOV with your SW is 4.6^o. That’s going to be the case whether you use the TV 67mm or the RO 85mm. And a reducer will do nothing for you other than reduce the image scale. You are limited by the 2” field stop. The TV67 is designed to maximized the FOV for a 2” diagonal. 

 

 

There are problems with both of these posts.  The True Field of View (TFOV) is given by the equation:  TFOV = (57.3 X Field Stop) / Telescope focal length.  StarAlert correctly points out that for a 2" eyepiece and a 120mm f/5 telescope this is roughly 4.6 degrees.  However a focal reducer decreases the focal length of the telescope and consequently increases the field of view.  For a 0.8X reducer the TFOV is now ~5.7 degrees.  Similarly, the 0.8x reducer lowers the focal ratio of an f/5 scope to f/4 which will give a 21.25 mm exit pupil with the 85mm Plossl (85/4) and the fastest possible final focal ratio of ~ 1.2.  For a practical example see my post # 12 in the following thread:

 

https://www.cloudyni...al-with-ovni-m/

 

Hello,

This is not the first time I have been wrong and will not be the the last. This what I get for plugging incorrect numbers into a Astronomy Tools, calculator. Thanks for the correction.  The point of my experiment is to reduce the image scale and aquire the fastest focal ratio possible. Hopefully I will have a chance to see how well this pans out in as soon as I get clear skies without a moon.

 

HAPPY SKIES TO YALL AND JKEEP LOOKING UP.  Jethro

Edited by Jethro7, 14 May 2022 - 06:01 PM.

[chemisted]

Jethro,

 

I have had very good experiences with the 85mm not just with the RC-10 but also my two workhorse refractors.  I hope your experiment goes as well.

 

Ed

[StarAlert]

There are problems with both of these posts.  The True Field of View (TFOV) is given by the equation:  TFOV = (57.3 X Field Stop) / Telescope focal length.  StarAlert correctly points out that for a 2" eyepiece and a 120mm f/5 telescope this is roughly 4.6 degrees.  However a focal reducer decreases the focal length of the telescope and consequently increases the field of view.  For a 0.8X reducer the TFOV is now ~5.7 degrees.  Similarly, the 0.8x reducer lowers the focal ratio of an f/5 scope to f/4 which will give a 21.25 mm exit pupil with the 85mm Plossl (85/4) and the fastest possible final focal ratio of ~ 1.2.  For a practical example see my post # 12 in the following thread:

 

https://www.cloudyni...al-with-ovni-m/

Hold the phone! So you’re saying if I thread my 2” 0.50x reducer in front of my TV67, it will double the FOV? I don’t think it works that way, but I’m going to give it a try today and see what happens. 

Edited by StarAlert, 14 May 2022 - 08:18 AM.

[StarAlert]

Ok. I’m very confused. I just finished measuring the FOV for a TV67 both with and without a 2” 0.5x reducer. As I suspected, the FOV is exactly the same. The FOV is the same regardless of the number of spacers I use with the reducer. The only thing that changes is the image scale.
What am I doing wrong? 

[chemisted]

Ok. I’m very confused. I just finished measuring the FOV for a TV67 both with and without a 2” 0.5x reducer. As I suspected, the FOV is exactly the same. The FOV is the same regardless of the number of spacers I use with the reducer. The only thing that changes is the image scale.
What am I doing wrong? 

I can only go through the protocol I use.  Focal reduction is determined by the focal length of the reducer and the separation of that optic from the focal plane of the eyepiece.  Using the CCDT67 as an example it has a focal length of 305mm.  The equation that determines reduction is (305 - 100)/305 = 0.67.  That is, to get the specified 0.67X I need to use a spacing of 100mm.  I screw the reducer on the telescope side of the 2" diagonal and slide the RO 85mm Plossl in as normal and am getting very close to the desired reduction with a very obvious increase in the FOV.  Just for fun, I went through this procedure with my Sky 90, RO 65mm and the GSO 0.75X reducer off my back deck where I have a perfect view of a distant radio tower.  The increase in FOV is enormous.  I have neither the TV 67mm nor a 0.5X reducer to try for comparison.

Edited by chemisted, 14 May 2022 - 05:53 PM.

[StarAlert]

I guess I’m confused as to how the CCDT67, with a clear aperture of 44mm can increase the FOV of an eyepiece with a 48mm field stop. This is why I’m not an optician.  

 

I set up a yard stick 60ft from my scope (C9.25). The FOV with the TV 67mm was 14.5”. When I put the 0.5x reducer in front, the scale was MUCH smaller, but the FOV was still 14.5”. Weird. I’ll have to think about this more. 

Edited by StarAlert, 14 May 2022 - 12:02 PM.

[Jethro7]

Jethro,

 

I have had very good experiences with the 85mm not just with the RC-10 but also my two workhorse refractors.  I hope your experiment goes as well.

 

Ed

Hello Ed,

I read your thumbs up on a previous post of yours. The RO 85mm Plossl seems like a niche that I need to fill with my NVA endevours.

 

HAPPY SKIES TO YOU AND KEEP LOOKING UP Jethro

[t.mihai147]

I will try to explain what is my simplified understanding about NV objective usage and telescopes, both prime and afocal.

This could be not 100% correct but I hope my learning from other forum's posts is good enough for real life usage.

 

Standard NV PVS-14 has ~27 mm objective, F1.2 (or close) and is presenting 40* AFOV at the eyepiece.

When I want to choose my target I need to take into consideration telescope or objective focal length (FL), speed (F:) and if I use it prime (with or without focal reducer/barlow) or afocal (which means keep original 27 mm FL objective and add one ocular in front of it).

Afocal usage is very similar with focal reducers/barlows for prime focus.

For example, if you keep 27 mm NV objective (afocal mode) and add 54 mm 2" ocular in front of it, this is similar with 0.5X focal reducer (27:54=0.5X).

 

Finally, the most important parameters in the NV ocular view is total magnification and speed.

For example:

If we take 100 mm F5 telescope and use it in prime mode (no NV objective), we have (100x5):27= 18.5x magnification and F:5 speed. FOV will be 40*/18.5 = 2.16*.

If we want a faster system, we can add a focal reducer or we keep 27 mm original NV objective and add for ex. a 2" 54 mm ocular (this is called afocal mode) for a 0.5X FL reduction (250 mm). This will result in 9.25X magnification, F:2.5 speed and 4.3* FOV.

 

In a similar way we can find the different telescope/focal reducers/barlows/objectives combination that will result in desired magnification, speed and FOV of the NV system.

 

This is just my understanding from reading other posts and from my own experiments, if something needs to be corrected please let me know.

Edited by t.mihai147, 15 May 2022 - 12:05 AM.

[Jethro7]

Jethro,

 

I have had very good experiences with the 85mm not just with the RC-10 but also my two workhorse refractors.  I hope your experiment goes as well.

 

Ed

Good evening Ed,

The sky conditions tonight are nothing to write home about but I sure wish I had these crappy conditions last night for the Eclipse. I set up the SW120 F/5 for first light with the RO 85mm and PVS 14, just to see if I could reach focus, no problem. At first the view was like looking down into a tunnel and then proceeded to adjust the PVS 14's occular focus all the way out to compensate. The eyepiece of the PVS 14 became fully flooded at this point and that was a relief. In the words of Frank Barone, " Holy Crap!!!" this combination is bright, I had to really turn the Gain down on the light intensifier. The target was M81 and M82, I had no trouble finding these tiny DSO's even though the scale was very small. At least I know how to get everything to work and can't wait till the end of the month to try this combination on the Milyway without a pesky moon. Playing in the Milkyway was the reason I purchased both the RO 85 and the skywatcher 120 F/5 in the first place.The RO 85mm is just another tool in the line up, for special targets like my GSO 30mm wide field, the TV55/67mm is still my work horse eyepiece for NVA. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro.

 

P.S. No disappointment here.

Edited by Jethro7, 16 May 2022 - 09:55 PM.