A tale of two diagonals.

[ukskies]

When I purchased my Takahashi DF just before Christmas I decided to buy a quality 2" diagonal for it. Many choices were out of stock so limited my options but FLO had the excellent 2" clicklock dielectric in stock so I ordered one. All my early viewing has been done with this diagonal.

I read the reports by Bill P on several diagonals tested and following this I felt that a prism diagonal might suit the FC100 more than the mirror one. In particular I fancied the T-2 Zeiss prism diagonal but FLO showing no stock. I tried RVO not expecting anything but my email was answered with a yes in stock. I ordered it immediately.

Now I've only managed one short session with it so far due to the ever present clouds and on that night a veil of thin cloud was covering the entire sky. The Moon and Venus were visible through this however so I set up to try the new prism on the Moon. I scanned the Lunar surface for quite a while at 150x and it was beatiful and sharp, loaded with high contrast details. I swung to the limb of the moon and suddenly noticed there was no false colour.I usually can see a very tiny yellowish green halo at the limb but it wasn't there. I decided to try more power and still could not provoke any false colour so I turned to Venus very low to the horizon and turned the focus wheel, it did have a little false colour but it was a tiny amount much less than I expected. There were larger low level clouds by now making a viewing a hit and miss operation requiring patience however I stuck at the eyepiece and went back to Luna. As I increased power to 333x the highest I'd been to this point I noticed that the details still stayed sharp as a razor, the view was dimmer but the contrast and sharpness remained. I eventually got to over 500x times before the sharpness was not there it held up at over 400x very well indeed.

I decided to grab the 2" clicklock to compare but the clouds had increased and obscured everything.

I plan a side by side comparison as soon as I get a clear night and will update this thread once I have but the prism on this night showed no false colour where previously there has been and allowed me to go to higher powers than the mirror has allowed before.

[John]

Interesting.

I use a Baader T2 Zeiss prism diagonal with my FC100-DL F/9 and it works really well. I also have 2 inch Tele Vue and Astro Physics mirror diagonals but I've not seen any significant differences one way or the other when comparing them with the T2 prism unit. Mostly I use the prism with the DL and 1.25 inch eyepieces and reserve the 2 inch mirror diagonals for when I want to use my 2 inch eyepieces. I fancy that the smaller form of the T2 prism suits the slim lines of the DL rather better but that is rather a superficial point of view 🙄

It will be interesting to hear what, if any, differences you see when you compare the T2 prism with the 2 inch clicklock 🙂

When bright targets are low atmospheric dispersion can cause some false colour of course but I guess you know that. I'm a little surprised that your DF shows any false colour around the moon - I thought they were pretty much colour free at focus ?

 

 

 

 

 

Edited March 31, 2023 by John

[jetstream]

4 hours ago, ukskies said:

I eventually got to over 500x times before the sharpness was not there it held up at over 400x very well indeed.

Excellent!

This statement shows the "power"  of great optics. I respect your note on false colour, many wont mention anything like this- it could also be eyepiece related as many including TV will "blue fringe" at the field stop, even in reflectors. Others will fringe in orange etc. I always include a yoga like "straight through" observation in my tests of things. I had a poor diagonal many years ago and found it this way.

Congrats for a very fine optic that will take extremely high mag! What eyepieces do you use?

Gerry

Edited March 31, 2023 by jetstream

[ukskies]

6 hours ago, John said:

Interesting.

I use a Baader T2 Zeiss prism diagonal with my FC100-DL F/9 and it works really well. I also have 2 inch Tele Vue and Astro Physics mirror diagonals but I've not seen any significant differences one way or the other when comparing them with the T2 prism unit. Mostly I use the prism with the DL and 1.25 inch eyepieces and reserve the 2 inch mirror diagonals for when I want to use my 2 inch eyepieces. I fancy that the smaller form of the T2 prism suits the slim lines of the DL rather better but that is rather a superficial point of view 🙄

It will be interesting to hear what, if any, differences you see when you compare the T2 prism with the 2 inch clicklock 🙂

When bright targets are low atmospheric dispersion can cause some false colour of course but I guess you know that. I'm a little surprised that your DF shows any false colour around the moon - I thought they were pretty much colour free at focus ?

 

 

 

 

 

Hi John thanks for your input, it is appreciated. I'm actually wondering at this point if my  particular 2" clicklock might be not be up to snuff.

I too was surprised at the false colour, my first light with the Tak was on Jupiter and mentions the purple halo I observed. Bear in mind it was from my town garden with houses in all directions and central heatings on as it was a very cold night so the seeing wasn't inspiring hence I didn't worry too much. As time has gone on though I do notice some false colour on most bright objects so began to suspect the eyepieces or even my own eyesight, I mean a £200+ diagonal was not likely to be the issue was it? I know the remove the diagonal and view straight through advice that is oft quoted but I believe my days of being capable of getting into position for this have long gone. Maybe a proper observing chair might assist.

However the T-2 Zeiss BBHS does appear to give me completely colour free views albeit on only one occasion so far and only on the Moon.  

[ukskies]

2 hours ago, jetstream said:

Excellent!

This statement shows the "power"  of great optics. I respect your note on false colour, many wont mention anything like this- it could also be eyepiece related as many including TV will "blue fringe" at the field stop, even in reflectors. Others will fringe in orange etc. I always include a yoga like "straight through" observation in my tests of things. I had a poor diagonal many years ago and found it this way.

Congrats for a very fine optic that will take extremely high mag! What eyepieces do you use?

Gerry

Hi Gerry thank you. Ha the yoga straight through position is no longer on my available options list, if I got there I may not be able to get out of it.

I also appreciate your comments about my Tak being a fine optic, I believe you are right and I'm a mixture of pleased and relieved at this. 😆

Finally eyepieces, yes, well, the plan is to purchase some top end eyepieces to go with my top end scope but I have to budget for these so I hope to acquire a few as this year progresses and as funds allow. (It never ends doe it?)

For the moment I have as my main collection a Baader mark IV zoom with associated 2.25 Barlow and a 30mm UFF plus a couple of BST Starguiders in 5mm and 3.2mm and a Celestron 2X barlow. I'm sure the Tak objective is likely begging for better and capable of more but I will have to be patient to test that theory out. I have a few old Plossls but rarely use these nowadays.

[jetstream]

8 minutes ago, ukskies said:

a Baader mark IV zoom with associated 2.25 Barlow and a 30mm UFF

These are vg eyepieces 👍

Yeah my yoga days are becoming fewer and farther between.

[Mr Spock]

I'm not sure how a mirror diagonal can introduce chromatic effects. Maybe there's something else at play here?

[ukskies]

1 hour ago, Mr Spock said:

I'm not sure how a mirror diagonal can introduce chromatic effects. Maybe there's something else at play here?

Yes, my thoughts exactly and another clear night will be needed before I can make any conclusions, if then.

Is it not  however possible that as I am quite sensitive to false colour and maybe the objective is producing a little off axis colour that the mirror is amplifying it whereas maybe the prism is muting it.

Many people have noted more light scatter with dielectric mirrors than with prisms I've read recently.

Correct me if I'm wrong here but doesn't dielectric coating involve getting something like 100 ultra thin layers of coating? From memory a process Baader say is extremely difficult to achieve without surface irregularities?

That said I wanted the 2" mirror primarily for lower powered widefield views with 2" eyepieces, it seems fine for this purpose but I will be looking harder from now on.

[jetstream]

19 hours ago, ukskies said:

Yes, my thoughts exactly and another clear night will be needed before I can make any conclusions, if then.

False colour can also happen at times with certain eyepieces when looking through them at extreme angles.IMHO.

Edited April 1, 2023 by jetstream

[bosun21]

19 hours ago, ukskies said:

From memory a process Baader say is extremely difficult to achieve without surface irregularities?

The Baader Clicklock diagonal has a 1/10th pv mirror so I don’t know about surface irregularities.

Edited April 1, 2023 by bosun21
Typo

[dweller25]

I read somewhere that prisms can overcome under correction in a refractor - maybe that is what you are seeing ?

Edited April 1, 2023 by dweller25

[dweller25]

Found it ….

https://www.cloudynights.com/topic/175204-prism-diagonal-better-color-correction/

[dweller25]

@ukskies

I changed from a William Optics diagonal to the Baader Zeiss prism and was very surprised by how much better the views got. I also have the Badder BBHS diagonal and that is just as good as the prism. Neither add colour to the views.

[ukskies]

58 minutes ago, bosun21 said:

The Baader Clicklock diagonal has a 1/10th pv mirror so I don’t know about surface irregularities.

Hi bosun. Yes I realise now that sentence wasn't complete and seems out of context when isolated. I read, as I wrote above, that Baader claim that dielectric coating a mirror diagonal is no easy trick because as the layers build up it is difficult to control surface irregularities because the many layers of coating required can build up in an uneven manner. What I didn't post above is that their 2" clicklock however does not have this issue because they claimed to be the first to crack the problem of how to control the build up of layers.

I was also aware that Baader claim one tenth lambda surface accuracy for their mirrors as that's the reason I chose that particular diagonal.I have however read both of Bill P's diagonal reports on CN which gave me food for thought and led to my purchase of the prism diagonal as I couldn't seem to a clean image at high power with the mirror at low and even medium powers it was fine.

Skies are clearing here right now and I have to attend a birthday party otherwise I would have more input from further testing.

[ukskies]

17 minutes ago, dweller25 said:

@ukskies

I changed from a William Optics diagonal to the Baader Zeiss prism and was very surprised by how much better the views got. I also have the Badder BBHS diagonal and that is just as good as the prism. Neither add colour to the views.

Hi, thanks David.

Bill P found the BBHS mirror to be an amazing experience reporting seeing details on Jupiter if memory serves, that he'd never seen before.

Anyway I will test again and report back soon. Thanks everyone.

[bosun21]

4 minutes ago, ukskies said:

Hi bosun. Yes I realise now that sentence wasn't complete and seems out of context when isolated. I read, as I wrote above, that Baader claim that dielectric coating a mirror diagonal is no easy trick because as the layers build up it is difficult to control surface irregularities because the many layers of coating required can build up in an uneven manner. What I didn't post above is that their 2" clicklock however does not have this issue because they claimed to be the first to crack the problem of how to control the build up of layers.

I was also aware that Baader claim one tenth lambda surface accuracy for their mirrors as that's the reason I chose that particular diagonal.I have however read both of Bill P's diagonal reports on CN which gave me food for thought and led to my purchase of the prism diagonal as I couldn't seem to a clean image at high power with the mirror at low and even medium powers it was fine.

Skies are clearing here right now and I have to attend a birthday party otherwise I would have more input from further testing.

 I didn’t know that about Baader cracking the coating problem. I also owned the Clicklock diagonal which I used with my Starfield 102. I found it to be a good diagonal with no false color I could see. Although I didn’t push the magnification as high as you did. I am now using a Williams Optics 2” diagonal which claims to be 1/12th lambda. I didn’t notice any discernible difference between the two. I have only used two prism diagonals , one being a Baader and the other the Takahashi 1.25”. Both were good.

[Highburymark]

1 hour ago, dweller25 said:

I read somewhere that prisms can overcome under correction in a refractor - maybe that is what you are seeing ?

My thought too 👍.

My three diagonals are all T2 Baaders - Zeiss prism, Amici prism and BBHS mirror. I swap them around so sometimes am not even aware of which I’m using. The Zeiss prism does show slightly less scatter on bright objects, but they are all very close. The Amici is superb - highly recommended if you prefer lunar binoviewing to present a correct image.

[mikeDnight]

False colour when observing Venus can also be due to atmospheric dispersion even in colour free systems. So an ED may show more colour than a fluorite or triplet apo, merely because the image can lose critical focus momentarily because of the turbulent atmosphere.  Complex wide angle eyepieces can often show lateral colour more readily than narrower field eyepieces. Its often better to observe Venus while its still quite high and set against a blue sky.  

[ukskies]

Sunday late afternoon the clouds began abating opening up to lots of clear patches. I was getting ready for a session. 

I set the tripod and mount out in the garden in the full daylight performing a rough polar alignment and got the FC100 case and the eyepiece case also out in the garden to acclimatise.

I waited, what seemed like forever for Venus to appear in the still blue sky and then I was finally away. I am not a skilled Venus observer hence I am only reporting what I actually noticed in the eyepiece.

Since I first posted this thread I have been mulling things over and forming a plan of attack. This was to simplify the issue by limiting the eyepieces initially to only two and the Barlows to just the Baader 2.25. I also decided to refer to the BBHS prism as T-2 and the clicklock mirror as 2".

Venus at 148X, 5mm BST with the T-2. Very luminous, faint purple glare/flare around the planet nothing too bad tho. Gibbous phase was very easy to see plus occasional hints of darker patches/shading in the cloud tops. Atmosphere unstable at the low altitude of Venus, the planet appeared quite sharp but wobbled a little at times. Quick swap to the 2" plus 5mm BST showed a liitle darker shade of purple in the flare around the planet but not a lot different plus a very noticeable red flare to the South Pole.

In with the 3.2 BST For 231X, the planet obviously appeared larger and so did the flare but only in the same proportion as in the 5mm, more of the same just bigger with more obvious movement to the image due to the atmosphere. The darker shading in the Venusian cloud tops appeared to be slightly more elusive though, not being visible for most of the time. Back in the T-2 went and the story was unsurprisingly the same, a faint purple flare but the red South Pole was not there, same as the 5mm exhibited. I really preferred the image scale at 231X but the low contrast detail was more easily visible at 148X this particular night. 

I persevered with Venus whilst waiting for the increasing darkness to reveal other targets and concluded that the T-2 showed slightly less chromatic aberration than the 2" however it wasn't by a lot and the view though either one was fine and showed pretty much a similar level of low contrast detail.

Eventually i could see an extremely low Sirius so swung the Tak onto it. Same order, 5mm for 148X with the T-2 focused on the airy disc and the star showed two (I think) diffraction rings which were constantly moving and breaking up and spoiled the view a bit. Swap in the 2" and again the view was more similar than different but more so than on Venus to the point that it was difficult to see any difference but careful study swapping diagonals twice more revealed a very slight one the T-2 was ever so slightly cleaner. Same story with the 3.2 but the image in both diagonals was more alike than different.

A quick look at the ever diminishing Mars sitting quite high to the south, The Red planet in the 2" at 148X was small, sharp and very red. No glare or false colour was visible. Changing to the T-2 the view appeared identical, if there was a difference I could not detect it. 3.2 for 231X revealed the same story.

The moon was up by now but I could not view it as it was still too Easterly and obscured behind the house next door. I looked up at the Twins, they were sitting very high above me so I thought the view would be less affected by the seeing and I quickly located Pollux. It appeared yellowish white in focus in the 3.2, the faint companion was not visible. The star showed a nice tight airy disc with a single unstable diffraction ring, breaking away from the star. In with the 2" and again a very similar view presented.

On to Castor and the double star rewarded me with a very pretty view, the fainter companion showed a tight airy disc with a single diffraction ring that didn't break away whereas the brighter components diffraction ring was breaking up quite a bit. The view was the same in the T-2 except it looked a tiny bit cleaner/clearer. I swapped a couple of times to double check but it was definitely slightly preferable in the T-2 in my eyes.

I picked up the whole rig now and moved it as far away from the house as I could to view the moon. The T-2 was already in so I slipped in the 5mm and the view was very bright and bleached of detail. I slewed to the south and then used the high contrast shadows to focus accurately, the focus popped and so did the detail. I don't think I'll ever stop gasping at the Lunar views through this scope. In with the 2" and very much the same view. I swapped diagonals again several times and same with the 3.2 and the views were extremely close but the view in the T-2 was just, for want of a better word, nicer but not by a lot. There was no trace of false colour when i viewed the limb of the moon in either eyepiece or diagonal.

It was getting late so I quickly popped in the 2.25 Barlow with the 5mm for 333X and the 2" was just as sharp as the T-2 but again the T-2 was ever so slightly nicer maybe prettier? I can't think of a descriptive term that would explain the difference as it was very subtle but something about the T-2 view just made it appear nicer to my eyes. 

4 hrs I had been out by this point and it was very cold by this time after 11 o'clock so I called it a night. As I  did not notice any false colour slewing around the limb of the moon in either eyepiece or diagonal I went indoors rather pensive.

By the next morning (Monday) I was convinced that I had not re-created the conditions which produced the false colour on my previous outing and planned a second session for that very night.

Edited April 4, 2023 by ukskies