A combined focal extenders / Plossl-Nagler test on the Sun

[Piero]

Today, I decided to test the following combinations on the Sun with my Lunt Herschel Wedge solar filter, with and without a green filter #58:

- TV Plossl 20mm + Bresser SA 2x (36x, 1.7mm exit pupil),

- TV Plossl 20mm + TV Powermate 2.5x (45x, 1.3mm exit pupil), and

- TV Nagler 7mm T6 (51x, 1.2mm exit pupil). 

I do not want to declare a Winner or a Looser, because all the above combinations are very good. The findings below are minor although detectable to my eye. 

Date: 19/09/2015

Time: 11:00-12:45

Temperature: 16C (13 km/h)

Seeing: 2 - Slight undulations

Transparency: 5 - Clear

Telescopes: Tele Vue 60 F6

Eyepieces: TV Plossl 20mm, Nagler 7mm, Vixen SLV 5mm, Bresser SA 2x, Powermate 2.5x

Filters: Lunt Herschel Wedge, Orion Green Filter n.58

Magnifications: 18x, 36x, 45x, 51x, 72x, 144x +/- GF58

General observation.

One large sunspot surrounded by a well defined penumbra region was visible near the centre. Three other medium size sunspots were observable at West, and a little chain of six sunspots was located right at the centre. Granulation was visible, better at lower powers even without GF58. The seeing was not sufficiently good for pushing the magnification at 144x. At 72x all the details were still acceptably crisp and defined.

Test 1: Nagler vs Plossl+focal extender

- Without GF58, independently of the used focal extender, the Plossl 20mm controlled the light transmission better than the Nagler 7mm. Although the light transmission with the Nagler 7mm was noticeably higher, this somehow spread through the field of view. The sunspots and the penumbrae were more defined and crisp with the Plossl. Faint sunspots surrounding the main one were detectable with the Plossl 20mm, whereas these were almost invisible, or not visible at all, with the Nagler 7mm. Granulation and faculae were detectable with some effort with the Plossl 20mm, whereas they were not obvious to spot with the Nagler 7mm.

- With GF58, the observation for all these features was simpler, although, to my eye, the Plossl 20mm combined with any of the two focal extender achieved the best results.

Test 2: Plossl+BresserSA2x vs Plossl+PM2.5x

Between the Bresser SA 2x and Powermate 2.5x, I did not find differences aside from the magnification and the slightly longer in-focus travel required for the Bresser SA 2x. Both the combinations were able to separate and distinguish three little pairs forming a chain of sunspots located at the centre. Two faint sunspots were also detectable near this chain with both the combinations. Of note, despite the increase magnification, these pairs were not easily visible and rather appeared as elongated areas with the Nagler 7mm. 

I do not know why the Plossl 20mm outperforms the Nagler 7mm, even when a focal extender is employed. My feeling is that this is due to a better control of the light transmission. Although this was obviously higher with the Nagler (different number of lenses?), the image was just clearer and crisper with the Plossl 20mm, whereas the light seemed to spread in a more uncontrolled way with the Nagler 7mm.

All these differences were minor or subtle, and required a careful observation though.

[michael.h.f.wilkinson]

Interesting comparison. It sounds like it is less a matter of transmission than of scatter control. I wonder how much the larger FOV influences this perceptual difference.

[Floater]

Good work, Piero. A well-detailed comparison.

I was lucky enough to have an unusually long session with my solar kit earlier today and I, too, saw the features which you describe so well. I viewed with my SolarMax and TV76+Quark, comparing views and messing with the tuning of the Quark. I then viewed through the Herschel wedge and tightened in on ARs 2418 and 2415. Highest useful magnification was x80-90-ish but that may have as much to do with the fact that focus, as ever, is fairly critical and my single-speed focuser lacks the necessary finesse. Don't know.

Now, I fear I may be venturing into deeper water than I should but, just to throw this into the mix, Daystar specifically state that complex EPs (such as Naglers, of course) are not recommended for viewing with the Quark filter. And, once again quite specifically, recommend plossls. I wonder if this advice for viewing in Ha might apply more generally to solar observation. In this context, of course, I'm thinking of white light. Any comment, sun lovers and/or eyepiece experts?

I doubt if I can even hang on to the edges when things like light transmission, light scatter or (worse!) wavelengths are mentioned but I'm happy to try and, perhaps, learn a bit more.

Whatever, it was a joy to be able to get my scopes pointing at a big, happy Sol again. It wasn't as busy a sun as I've seen but well worth browsing over. And over again and again.

Clear skies and sunny days.

[YKSE]

I tend to agree with Michael, it's more likely scatter light than transmisison making this difference. Less glass eyepieeces like Supermonos, Othos have some followers among dedicated planetary astronomers on the other side of pond, mostly by the reason of less scatter of these bright objects.

[Piero]

I don't think it is light transmission either, as this was "higher" with the Nagler 7mm than with the other two combinations. I was not sure that 'spread of light around the field of view' was called "scatter light", that is why I preferred to describe it with words. I believe the Nagler controls light scatter a bit less than the plossl. This is also supported by the fact that the Vixen 5mm (lower exit pupil and light transmission than the Nagler 7mm) actually showed more contrast and controlled this scatter light a bit better in a more similar way to the Plossl 20mm. 

I took a couple of photos today as well. 

Sun at 36x. The big spot surrounded by a penumbra region is visible South. Little chain of sunspots located at the centre too. Apologise for the low definition.. I am improving (slowly!).

Another image I took, this time at 144x, showing the big sunspot surrounded by the penumbra region.

[Dave In Vermont]

I've been wondering about doing a similar experiment with my mega-$£$£ MallinCam FR + a Meade F/6.3 and a Meade F/3.3. Perhaps tomorrow when the Sun is scheduled to emerge from the current rain. This is the MallinCam OA-10, which I have in mind to use. See images below. Anyone have any ideas and/or suggestions for this experiment?

I have a series of spacers & adaptors for fine-tuning the degree of focal-reduction. And some nice orthoscopic EP's to draft for this. Using more glass from EP's like Nagler's might be problematic, eh? My guess would also be scatter causing problems with contrast or similar. Interesting.....

Thanks for putting this on my list of To-Do's, Piero!

Until later -

Dave