A lunar session and some optical testing (Starfield 4" refractor)

[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.

[callisto]

I enjoyed reading that... thanks for sharing 

 

Mark.

[Kon]

Nice report and excellent session.

[Mr Spock]

Good report. My 102mm is excellent. There's no image difference inside to outside of focus and no colour either. 

I have a 2" Barlow fitted for planetary work. That gives x1.85, so, a 6mm is x220. Great on the moon - surprisingly sharp. 

[Stu]

Great report @Davesellars. Everyone should have a decent 4” apo! 👍

[RobertI]

Nice report, glad you’re enjoying your scope. I really enjoy using mine and with binoviewers it’s awesome for solar system. 👍

[Davesellars]

Just now, Stu said:

Great report @Davesellars. Everyone should have a decent 4” apo! 👍

Definitely!!!   I had regretted rather selling my 120ED (was necessary at the time and its bulk meant it hardly got used back then as I need to walk with it to my dark site) - However, I think the performance of this scope really is not that far off the 120ED in what it was showing... I guess a few sessions on the planets later this year will give me a better comparison.  However, I do have the big 'ol light bucket anyway for when the conditions really allow.  Many times I don't have the time to get the 12" out beforehand & cool down. etc  I think this 4" refractor may well become my main scope of choice for many nights.

[Paz]

Great report, my 4" refractor is my most used scope. Other scopes can match or beat it at their specialities but it's the best combination of quality, flexibility to observe different targets, and ease of use.

[Davesellars]

3 hours ago, Mr Spock said:

Good report. My 102mm is excellent. There's no image difference inside to outside of focus and no colour either. 

I have a 2" Barlow fitted for planetary work. That gives x1.85, so, a 6mm is x220. Great on the moon - surprisingly sharp. 

 

That's good to know.  I was thinking about possibly purchasing the BST Starguider 3.2mm which gives me that same power approximately or find a suitable 3mm which gives 238x.

[Stu]

On 15/03/2022 at 21:49, Davesellars said:

Definitely!!!   I had regretted rather selling my 120ED (was necessary at the time and its bulk meant it hardly got used back then as I need to walk with it to my dark site) - However, I think the performance of this scope really is not that far off the 120ED in what it was showing... I guess a few sessions on the planets later this year will give me a better comparison.  However, I do have the big 'ol light bucket anyway for when the conditions really allow.  Many times I don't have the time to get the 12" out beforehand & cool down. etc  I think this 4" refractor may well become my main scope of choice for many nights.

Agreed! I have had three 120EDs and they are excellent scopes, I just found that the Tak gives up very little to it on most targets I observe, and gets far more use due to its small size and convenience.