A lunar session and some optical testing

[davesellars]

It was a beautifullly clear and transparent evening of the 14th March. So far the 4" refractor had only seen a very brief (minutes) look at the Moon the previous Monday before clouds flanked in and shut off play on one one of the best night's seeing conditions in quite some time.

I was unsure though the seeing was going to be that great... transparent sky normally means the seeing is not going to be brilliant. I let the scope cool down about 15 to 20 minutes while I got ready but it was freezing outside - the temperature differential meant realistically the scope needed 30 minutes and better yet 45 minutes to an hour to be completely stable.

After some light viewing of the Moon (seeing was OK but far from great) I decided to test optics by splitting Alnitak which was easily achieved at 178x and just about possible with 142x. M42 was starting to get a little too far down into the murk and while the 4 main stars were nice and clear and probably just the E star, I could not discern the F star.

Seeing conditions probably limited the testing but I did a star test. Inside focus showing perfectly diffracted rings and collimation was dead on as far as I could see using a 4mm eyepiece for 178x. Outside focus showed some difference and was slightly mushy but rings still apparent and looked to be not a bad result giving under-corrected optics. Probably to repeat if I can be bothered on a night of great seeing and have let the scope cool down more than 1 hour to be sure.

It is understood that prism diagonals can be beneficial to under-corrected optics in refractors (English, N., 2011. Doublet Apos. Choosing and Using a Refracting Telescope, pp.123) by essentially nulling the under-corrected spherical abberation with the prism which is naturally over-corrected. Perhaps, when funds allow I may invest in one of these particularly when the gas giants again make their display once again where I think they will benefit the most from use of the prism diagonal.

My 80ED actually displays the same under-correction (will come to this later...). But for now, the "proof of the pudding" is in the eating... and so, without further ado I pointed back to the lunar surface for rather more critical observation.

The seeing initially allowed for 178x with my 4mm TMB clone however , the best view was at 142x with the Pentax XW 5mm The lunar surface was shimmering but not much at these level and there were sufficient periods of time of clarity to pick out detail.

I only managed the most central craterlet in Plato. I have not managed to see any before with the 80ED and the seeing was far from optimal for this.

The Mons Hadley region was quite spectularly three dimensional with great contrast and colour rendition with the white highlights on the mountain range.

An outstanding region at the terminator from the Herodotus / Aristarchus craters and showed superb contast of pitch black in the gulley to the freshly illuminated walls from this upwards. This entire region resplendent in features hightlighted by the sun-rise across the face.

Schikard was watched at various time during a 4 hour period. Initially with the 4" refractor the back wall of the has not yet illuminated however the raised basin central to the crater was slightly illuminated certainly giving the impression that this crater is far from flat. The nearby Nasmyth crater perceived depth of abysmal blackness inside the crater against the full illuminated walls was quite something at this stage of illumination.

Moving upwards once again to Gassendi the central peaks here were very easily seperated as disinct peaks with the 4" refractor along with considerably detail of the surface of the crater with its many undulations, ridges and paths around it. At the current illumination it seemed quite optimal for picking up many of its features and not washed out.

From Gassendi to Mersenius holds considerable interest. A "small" mountain range that almost connects the two craters and several rilles the easiest of these directly shoots off from Gasendi banking around Mare Humorum. However, more interesting to me was that of the rille that cuts across the mountain range between Gassendi and Mersenius. With the 4" refractor this rille was followed in its entirety as it seems to wend its way through a low mountain range through to Letronne. Later, with the 80ED I attempted the same however the rille was lost after the main deeper section. Mersenius itself showed at least 4 or 5 craterlets - again the current illumination and contrast probably helping with this.

Sinus Iridium was viewed as point of comparison later with the 80ED. With the 4" refractor the detail at the edges of the bay seemed considerably more refined and with greater depth to the view.

I had actually brought the 4" in around 10:30 and left the mount out as quite frankly it was two darned cold to grapple with it. I had something to eat and warmed up and later had the idea to get out with the 80ED... It seemed like a good idea at the time!

The 80ED at least cools down very quickly... It was probably ready after 15 minutes pretty much stable or there abouts and was observing again by 11:45.

I did similar observations as above. The time meant though that Schickard was now almost fully illuminated with the very top of the back wall of the crater just peeping though. The basin itself though was fully illuminated. Amazing how quickly actually light moves across the surface (something I had not really observed much before in the space of one session). Most of the finer / smaller details that I had observed were there in the 80ED although particularly noted was the loss of some low contrast areas of rilles and some of the smaller craterlets in Mersenius. However..... those small details that were existing in the 80ED were so much more difficult with the 4" showing them very obviously.

OK, the the upshot is this... There is a substantial visual difference between the 4" and the 80ED - this goes in hand with the substantial increase in light gathering and most importantly resolution for lunar observation. On better seeing I feel that the scope can certainly handle more than my 4mm eyepiece as the image was far from dim nor lacking contrast at this stage.

So, I'm happy with this scope! I will have to keep in mind that the scope requires a reasonable amount of cool-down time or be a little prepared for the 4" to reach its best optical ability for lunar and planetary observation in the future although this is more of an issue in winter.

[notFritzArgelander]

Nice! Glad the 4" pleases.

[WilliamPaolini]

I would not worry about any apparent undercorrection, because the scope was probably not fully thermally acclimated yet. The optics need to be fully thermally acclimated and thermally stable before you can make any reasonable assumptions about the correction. Very good seeing also important. Nice report btw :-D

[Bigzmey]

Great lunar session Dave! As Bill says I would not base my conclusions just on one session. Seeing and acclimation have strong effects on the star tests.

Nevertheless, getting a good quality prism is a good idea. They produce noticeably less light scatter compared to a typical dielectric diagonal which benefits lunar, planetary and doubles. Once I discovered that I have switched to prisms in my ED fracs and Maks.

Here is one which is surprisingly good optically and well made mechanically for the price.

https://www.altairastro.com/altair-2-in ... 2038-p.asp

That's the one I was using in my 7" Mak for my all time personal best 0.3" (partial) split of STF 1233 AB during the last session.

[John Baars]

Very nice lunar observation report!
The views of the Moon in a good 4 inch refractor can be stunning!
I understand you are quite happy with this 4 inch gem.
I suppose a VROD would be well earned!

[davesellars]

Great lunar session Dave! As Bill says I would not base my conclusions just on one session. Seeing and acclimation have strong effects on the star tests.

Nevertheless, getting a good quality prism is a good idea. They produce noticeably less light scatter compared to a typical dielectric diagonal which benefits lunar, planetary and doubles. Once I discovered that I have switched to prisms in my ED fracs and Maks.

Here is one which is surprisingly good optically and well made mechanically for the price.

https://www.altairastro.com/altair-2-in ... 2038-p.asp

That's the one I was using in my 7" Mak for my all time personal best 0.3" (partial) split of STF 1233 AB during the last session.

Thanks! This seems to be an excellent price - actually about the same price as the 1.25" prisms from Baader etc. Good to know this performs very well.

[davesellars]

Yep, I wasn't worried about the star test. The conditions were nowhere near sufficient for a really accurate result especially with the extensive cool-down. Most importantly the scope is dead-on collimated and the results of sharpness already seen in the lunar features, complete lack of CA and and pin-point stars already state to me that the optics are excellent.

[davesellars]

Very nice lunar observation report!
The views of the Moon in a good 4 inch refractor can be stunning!
I understand you are quite happy with this 4 inch gem.
I suppose a VROD would be well earned!

Thanks John!!! Yeah, this is a great refractor - definitely worth the "upgrade" from the 80ED ...although no way I'm parting with the 80ED...

[helicon]

Congratulations Dave on the great lunar/gear report and on winning today's Visual Report of the Day!

[davesellars]

Congratulations Dave on the great lunar/gear report and on winning today's Visual Report of the Day!

Many thanks Michael!!!

[Bigzmey]

Great lunar session Dave! As Bill says I would not base my conclusions just on one session. Seeing and acclimation have strong effects on the star tests.

Nevertheless, getting a good quality prism is a good idea. They produce noticeably less light scatter compared to a typical dielectric diagonal which benefits lunar, planetary and doubles. Once I discovered that I have switched to prisms in my ED fracs and Maks.

Here is one which is surprisingly good optically and well made mechanically for the price.

https://www.altairastro.com/altair-2-in ... 2038-p.asp

That's the one I was using in my 7" Mak for my all time personal best 0.3" (partial) split of STF 1233 AB during the last session.

Thanks! This seems to be an excellent price - actually about the same price as the 1.25" prisms from Baader etc. Good to know this performs very well.

I also have T2 prisms from Baader, and Altairastro performs quite close to them.

[Thefatkitty]

Congrats on the APOD Dave, and that was a great lunar report! Your new scope seems to perform really well as I can imagine, and I quite liked the detail you saw and described. If it would only stop snowing/raining here, I'd be doing the same..!!

Interesting other folks views on the prism diagonal. I am not an overly experienced observer with other than what I have. See my sig file However, I do have two dielectric diagonals (W/O 1.25" and ES 2"), and the prism from my stock Vixen-made C80 blows both of those out of the water, both for velvety black skies and extreme clarity on planets and spots on the Sun. The difference is noticeable. This scope is from the late 70's/early 80's, and when I first got it I could see some dirt on the diagonal through the eyepiece, so I took it apart, cleaned the prism in diluted water; the black felt with cold water and put it all back together.
All original parts, and probably my best piece of glass Nothing wrong with the other two, but the prism, to me, is the better view. If you can find one of these online I would highly recommend it.

Glad to hear you're happy with the scope, and all the best to you and yours

[Unitron48]

Sounds like a great lunar session...and well written report, Dave! Nothing beats a good quality 4 inch refractor

Congrats on a well deserved VROD!

Dave

[John Baars]

Congratulations on the VROD!

[Butterfly Maiden]

A well written report Dave, and congratulations on receiving the TSS VROD for your contribution.

[davesellars]

Many thanks Vanessa!

[davesellars]

Interesting other folks views on the prism diagonal. I am not an overly experienced observer with other than what I have. See my sig file However, I do have two dielectric diagonals (W/O 1.25" and ES 2"), and the prism from my stock Vixen-made C80 blows both of those out of the water, both for velvety black skies and extreme clarity on planets and spots on the Sun. The difference is noticeable. This scope is from the late 70's/early 80's, and when I first got it I could see some dirt on the diagonal through the eyepiece, so I took it apart, cleaned the prism in diluted water; the black felt with cold water and put it all back together.
All original parts, and probably my best piece of glass Nothing wrong with the other two, but the prism, to me, is the better view. If you can find one of these online I would highly recommend it.

Many thanks - I'll keep my eye open on the used market of which here is normally very active. Good to hear another real confirmation that the prism is more beneficial on these objects than using a mirror diagonal.

[Makuser]

Hi Dave. You have a very nice well written and informative report here. It was a good read and I am glad that your 4" refractor meets your expectations, and is quite useful on your lunar observing. Thanks for taking the time to put this up on here Dave and congratulations on receiving the TSS VROD Award today.

[davesellars]

Many thanks Marshall! It's always a pleasure to write-up and share an observation report.

[John Baars]

(...)
Interesting other folks views on the prism diagonal. I am not an overly experienced observer with other than what I have. See my sig file However, I do have two dielectric diagonals (W/O 1.25" and ES 2"), and the prism from my stock Vixen-made C80 blows both of those out of the water, both for velvety black skies and extreme clarity on planets and spots on the Sun. The difference is noticeable. This scope is from the late 70's/early 80's, and when I first got it I could see some dirt on the diagonal through the eyepiece, so I took it apart, cleaned the prism in diluted water; the black felt with cold water and put it all back together.
All original parts, and probably my best piece of glass Nothing wrong with the other two, but the prism, to me, is the better view. If you can find one of these online I would highly recommend it.

It is known that in older refractors manufacturers used prisms that matched exactly with the lenses in the instrument. I.e. Slightly undercorrected refractors were equipped with slightly (natural) overcorrected prisms. That is why a modern zenit-mirror in such an instrument is not better than its original prism.