14 replies to this topic

[Mauro Da Lio]

Finally I decided to buy the Baader 8-24 Zoom. The field is smaller than the Delos (apparently between 45 and 65°) depending on the focal length. Coupled with the NV which has 40°, the field is enough.

 

Optically it looks very good to me. It does better using a Paracorr. It is necessary to play a bit with the focus of the NV: I found out that it is better to focus the NV on the very short distance (then you focus with the telescope). This way the Baader is almost perfectly parfocal and the corrected field is great. Whereas if you set NV to infinity and focus with the telescope, there is some vignetting, it is not parfocal and has astigmatism at the edges.

 

Zooming allows you to change magnification on the fly. Changing eyepieces by moving the NV is cumbersome and wastes a lot of observing time. The most commonly used focal lengths are 20 to 12 mm. Between 12 and 8 there are internal reflections that are not seen visually but only with NV. Anti-reflection treatments are obviously optimized for visual, and these reflections may originate in the infrared. If I really want to get the optimum between 12 and 8 mm, I use the 9 mm ES. Otherwise the image quality (with NV mounted) is identical to that of regular eyepieces.

 

High magnification, between 12 and 8 mm: I have seen they are very useful for seeing inside small planetary nebulae. Last night I resolved Cat's eye NGC 6543 by observing the central star and several inner lobed structures. More difficult was NGC 6210 which is smaller, but again there was a hint of internal structure. For comparison, with the 60 cm dobson it is difficult to see internal structure on these small planetaries, because of the size and the high contrast with the sky background (the same reason that you see details on Ganymede when projecting onto Jupiter, but not when projecting onto the sky background).

 

Globulars: the more you zoom in the more resolved they become, down to about 12 mm. M13 and M92 were perfectly resolved in stars (I dare say as in 60 cm). SQM 18.5.

 

Galaxies also seems to like somewhat high magnification, Say around 16 mm. The Spindle Galaxy, M102, was visible against a SQM 18.5 sky, albeit no details were resolved.

 

Another thing is nebulae that require filters. Even just a UHC-S makes one prefer magnifications around 24 mm (52x) or, better, the Pentax 40. But that is another story and, fortunately, there are not only emission nebulae.

 

I look forward to trying it out under a dark sky.

 

Translated with DeepL.com (free version) and minimally re-edited.

Attached Thumbnails

• 

Edited by Mauro Da Lio, 28 June 2024 - 02:34 AM.

[ytserrof]

Wish there would be similar in the 26-67 mm FL range.

[Highburymark]

I’ve noticed the same internal reflections below 12mm - but agree it’s a good option between 12mm and 24mm. I prefer fixed focal length Delites, Naglers and Panoptics but as a single high magnification solution, the BHZ is great

[ButterFly]

I am in the testing phase for the paint mentioned in this thread: Extra thin flocking.  So far, it's looking great, and I should be applying the paint to the inside offending surface soon.

 

For reference, the surface causing the problem at higher magnifications is the tube housing the field lens group, just below the "T" of the main body at its objective side.  At higher magnifications, the field group moves down that tube, closer to the objective, and exposes a top portion of that tube on the inside of the eyepiece to very high incidence angle light, and it becomes specularly reflective.  There is very little mechanical tolerance in that tube, so ordinary flocking materials are too thick.  A few, very thin, layers of this paint looks like an ideal solution.

 

 

I found out that it is better to focus the NV on the very short distance (then you focus with the telescope). This way the Baader is almost perfectly parfocal and the corrected field is great. Whereas if you set NV to infinity and focus with the telescope, there is some vignetting, it is not parfocal and has astigmatism at the edges.

This is a great idea that I wish to try.  Zooms are designed to be parfocal for some level of nearsightedness correction, and the Baader is definitely not designed to be parfocal for corrected vision!

 

How short is a "short distance" here?  Should I focus the device at one meter, two meters?

[Mauro Da Lio]

If you change the focus on the PVS-14 and then refocus with the telescope focuser, the light path changes. There is some zoom effect and, also, the edge aberration vary. You have to try with your eyepieces. 

With Delos there is also the possibility of varying the PVS-14 to eyepiece distance. However it did not look to make significant changes. RafCamera sells spacer rings, in case.

 

The reason why stray light is visible in the PVS-14 but not to the eye seems to be due to the different aperture of the entrance pupils: 6-7 mm for the eye versus 24 mm for the PVS-14. As better shown in the attached figure the strai light is blocked by the iris. However it is not blocked with the large entrance pupil of the PVS-14.

 

The exit pupìil of the Zoom, at 24 mm and F/5 scope is 4.8 mm. Hence we are using only a small central part of the PVS-14 objective lens. Thus, I made a baffle (figure right) to block the stray light. The diameter of the hole is 8 mm. enough to let the exit pupil of the eyepiece to enter. At the same time it block the stray light. The baffle is inserted between the PVS 14 thread and the adapter thread. Picture 2.

 

Attached Thumbnails

• 
• 
• 

[Mauro Da Lio]

Here si the blocking effect on the Zoom.

Attached Thumbnails

• 
• 

[ButterFly]

Super easy to make the baffle.  I have an iris diaphragm I use for 1x, and tried with that to be absolutely certain that ring was the cause.  It is, so I really want to darken it.

 

I do notice veiling glare visually though.  Phobos is unreasonably hard with the BHZ.  General background sky also doesn't darken nearly as much as it should when zooming.  With the APM, the background darkens by a lot, at only a 2x zoom, versus not much at all, with a 3x zoom.  Any time the eye pupil is big enough to cover that ring (about 2x the exit pupil), that veiling glare is very noticeable.  Even with my dob, it's f/4.78 with a P2, so when at 8, the exit pupil is about 1.7mm, with that ring starting at around 3.3mm!  My eye pupil is bigger than that.  I'll take care of it if I can.

[Mauro Da Lio]

Just tested on the sky. The Baffle works. I will make another one, more precise and with 7 mm diameter.

[ButterFly]

Just tested on the sky. The Baffle works. I will make another one, more precise and with 7 mm diameter.

As small as the largest exit pupil you get with your telescope is ideal.  The NV objective doesn't move like an eyeball does when looking at a different field angle, so a central block will suffice for all field angles.  For the smallest focal length, you'll still get a little bit of glare, but you'll still cut off a lot of that ring.  All of those circles you see without a telescope scale with the exit pupil, so

 

The outer rings always follow the exit pupil, so your stray light images are a little misleading.  Those two sets of parallel black lines can be considered to be views at a given filed angle.  The stray light for some filed angle enters parallel to those sets, just beginning (about) twice the distance away from the edge of the exit pupil.  The outermost edge of that bright ring is the top (closest to the eyepiece body) of the tube housing the field group, while the innermost edge of the bright ring is the lowest part of the tube.  That cutout you see in the bright ring is the rail the screw holding the field group in its helical track rides in.  The lower the focal length, the more of that tube gets exposed, so the closer the inner diameter of the bright ring gets to the actual exit pupil.

 

A pupil block isn't a perfect solution, but if it works well enough for your liking, then it works well enough!  It's also super easy to do and undo, so it's work a try.

[ButterFly]

The greens lines I added is a more accurate representation of what's going on for some particular field angle.

 

 

This makes it a little easier to understand why:

 

the bright spot starts in the middle (the exit pupil and the corresponding outer ring is always centered);

 

is always brightest in the middle (one can catch the entire outer ring when it's centered because less of it gets cut off by the outside of the eye lens); and,

 

why the spot gets brighter for larger field angles as one zooms in more (the inner diameter of that outer rings gets closer to the actual exit pupil, so less of it gets cut off by the outside of the eye lens - in the picture, the green lines get thicker, while getting closer to the exit pupil; Added: all while the exit pupil and everything else is getting smaller).

Edited by ButterFly, 06 July 2024 - 07:08 PM.

[Mauro Da Lio]

At 8 mm I could see a very faint circle from stray light. It disappears at about 10 mm.

I will make a new baffle with 7 mm hole. Since it may non be at the exact oyopil distance I keep time margin. Fo the 8 mm I couldn't use another baffle but then if I have to swap the baffle it is maybe better to swap eyepieces.

[Highburymark]

Thanks for the useful advice - hopefully it means my eyepiece case can be a little lighter when I go on vacation to Menorca in a couple of weeks.

[Bearcub]

Finally I decided to buy the Baader 8-24 Zoom. The field is smaller than the Delos (apparently between 45 and 65°) depending on the focal length. Coupled with the NV which has 40°, the field is enough.

 

Optically it looks very good to me. It does better using a Paracorr. It is necessary to play a bit with the focus of the NV: I found out that it is better to focus the NV on the very short distance (then you focus with the telescope). This way the Baader is almost perfectly parfocal and the corrected field is great. Whereas if you set NV to infinity and focus with the telescope, there is some vignetting, it is not parfocal and has astigmatism at the edges.

 

Zooming allows you to change magnification on the fly. Changing eyepieces by moving the NV is cumbersome and wastes a lot of observing time. The most commonly used focal lengths are 20 to 12 mm. Between 12 and 8 there are internal reflections that are not seen visually but only with NV. Anti-reflection treatments are obviously optimized for visual, and these reflections may originate in the infrared. If I really want to get the optimum between 12 and 8 mm, I use the 9 mm ES. Otherwise the image quality (with NV mounted) is identical to that of regular eyepieces.

 

High magnification, between 12 and 8 mm: I have seen they are very useful for seeing inside small planetary nebulae. Last night I resolved Cat's eye NGC 6543 by observing the central star and several inner lobed structures. More difficult was NGC 6210 which is smaller, but again there was a hint of internal structure. For comparison, with the 60 cm dobson it is difficult to see internal structure on these small planetaries, because of the size and the high contrast with the sky background (the same reason that you see details on Ganymede when projecting onto Jupiter, but not when projecting onto the sky background).

 

Globulars: the more you zoom in the more resolved they become, down to about 12 mm. M13 and M92 were perfectly resolved in stars (I dare say as in 60 cm). SQM 18.5.

 

Galaxies also seems to like somewhat high magnification, Say around 16 mm. The Spindle Galaxy, M102, was visible against a SQM 18.5 sky, albeit no details were resolved.

 

Another thing is nebulae that require filters. Even just a UHC-S makes one prefer magnifications around 24 mm (52x) or, better, the Pentax 40. But that is another story and, fortunately, there are not only emission nebulae.

 

I look forward to trying it out under a dark sky.

 

Translated with DeepL.com (free version) and minimally re-edited.

Which telescope you are using?

[Mauro Da Lio]

Sumerian Alkaid 10" F/5.

[sixela]

The Spindle Galaxy, M102, was visible against a SQM 18.5 sky, albeit no details were resolved.

Weird. I find that in a 16" at f/6-f/8-ish effective the narrow dust lane is unmistakable in an NVD (and much easier than in glass because of the eye's poor resolution in a purely scotopic regime used with glass eyepieces).

[edit] Ah, I was thinking 16". Haven't tried it with a 10", might still be too small at f/8.

Edited by sixela, 19 July 2024 - 11:01 AM.