Best test for a diagonal?

[festoon]

Hello all

I am the owner of two lovely diagonals a William optics 2" diagonal and a Revelation astro 2" diagonal.

Both of them were fairly expensive and I am trying to save for an upgrade to my OTA so one of them may have to eventually go!!!!

The Revelation diagonal feels slightly heavier than the WO and claims the dielectric coatings have a 1/10th wave surface accuracy on a 10mm thick Quartz mirror.

The WO is a again a dielectric and if I remeber correctly claimed 1/10th wave surfaces.

If I wanted to do a side by side comparison, what would you say were good tests of a diagonal? Should you compare wide angle star fields and high magnification detail? Or is a good test Airy disk/star tests with the diagonal in place?

Cheers

Edited March 23, 2017 by festoon

[mikeDnight]

Star tests tell part of the story as they can reveal differences in light scatter levels and brightness. However, I think one of the most stringent tests for any diagonal would be to assess each with regard to the amount of subtle planetary detail it shows, and the ease with which it shows it. If one diagonal reveals subtle nuisances that the other does not then keep that one. If a diagonal shows The planets well then it will show anything well! The trouble is that much also depends on the quality of the telescopes optics, as the diagonal will only ever be able to give as good as it gets. I'd probably stick hold of the 12th wave diagonal if I had to choose without testing!

May be it would be best if you wait until you get your new telescope before making any decisions, if you can. 

Mike

[Cosmic Geoff]

I'd suggest reading some in-depth reports such as the one on Cloudynights which reports on around eight diagonals of different types.  I suggest this because the conclusions of such reports can be counter-intuitive.  Much is made of 1/10 wave accuracy for instance, but is this really the critical factor and does even a cheap diagonal deliver sufficient wave accuracy to deliver a clean Airy disk?  It seems that the main difference one is likely to note between diagonals is the degree of light scatter, which affects contrast.

Some coatings are more durable or long-lasting than others.

After reading the Cloudynights report I acquired a prism diagonal for use in my long focal ratio telescopes.

[DirkSteele]

Wave-front errors on diagonals are often quoted before the dielectric coating is deposited so the final surface accuracy can be quite different.  Dielectric mirrors often have 50 or more layers applied.  If each has an deviation of 3-5% then the overall wavefront error could end up quite substantial.  I would suggest that you compare the image of the two side by side (and if your neck is up to it, using straight through viewing as the true baseline) to assess which produces the best image at the eyepiece (using same EP for each test, and quickly switch out one diagonal for the other).

 

Assess the amount of scatter around bright objects such as Jupiter and the edge of the moon.  Try to evaluate how much low contrast surface detail you can see on Jupiter.  Also compare the vibrancy of any colours seen on the planet.  

 

Try some tight double stars and see if one diagonal makes the split slightly easier at the same magnification.  I was recently able to split one very tough double by not using a diagonal at all, to keep scatter and optical components to a minimum.  I have as yet been able to do the same with a diagonal in place.  The modest amount of additional scatter is rendering the close dim companion invisible.

 

Look at a good open cluster and try to determine which diagonal is showing the faintest stars.  It is possible one will reveal slightly dimmer stars on the limit of perception.  You could also perform a similar test on an emission nebula to see if one allows slightly more detail to be seen.

 

You will need to be aware that atmospheric conditions can change from minute to minute so keep switching back and forward to try an eliminate those effects.

[John]

Having owned those two diagonals myself, unless one is out of collimation, you may not be able to tell any differences between them in all honesty.

 

 

Edited March 23, 2017 by John

[DirkSteele]

2 minutes ago, John said:

Having owned those two diagonals myself, unless one is out of collimation, you may not be able to tell any differences between them in all honesty.

 

 

Yep! That is probably the truth. 

[Grotemobile]

Just make sure the mirror surface is kept clean.

That is the most important thing, I think. 

[festoon]

Thank you for the inputs on what might be a good test. I will try a range of these tests looking at scatter on the moon and detail on Jupiter. Also try splitting a tight double or picking out faint stars in a cluster.

If I had a binoviewer, I guess I could have placed both diagonals in a the same time for the tests!

All I need now is a clear night sky and a clear night in my calendar to do the tests

[festoon]

56 minutes ago, John said:

Having owned those two diagonals myself, unless one is out of collimation, you may not be able to tell any differences between them in all honesty.

 

 

Thnak you John...hopefully there will be no difference! Just out of interest...how can a diagonal change the collimation if it is a flat? probably I'm not understanding something

[John]

7 minutes ago, festoon said:

Thnak you John...hopefully there will be no difference! Just out of interest...how can a diagonal change the collimation if it is a flat? probably I'm not understanding something

Sometimes the mirror is not held at exactly 90 degrees to the optical axis of the scope due to manufacturing tolerances or even the diagonal getting bashed at some point. It's not usual but it has been known and will affect the performance of the optical system as a whole.

I'd not make this your number one concern though.

 

[festoon]

1 minute ago, John said:

Sometimes the mirror is not held at exactly 90 degrees to the optical axis of the scope due to manufacturing tolerances or even the diagonal getting bashed at some point. It's not usual but it has been known and will affect the performance of the optical system as a whole.

I'd not make this your number one concern though.

 

Thank you John

I guess you could pick that up by using a test with and without the diagonal (using an external focuser for an SCT) and looking for movement of the position of the star. So if the star was dead centre in the eyepiece without the diagonal. Then after the diagonal is added...is the star still in the centre of FOV.

[John]

14 minutes ago, festoon said:

Thank you John

I guess you could pick that up by using a test with and without the diagonal (using an external focuser for an SCT) and looking for movement of the position of the star. So if the star was dead centre in the eyepiece without the diagonal. Then after the diagonal is added...is the star still in the centre of FOV.

The best test for this with a refractor is actually to use a well collimated laser collimator. You put it in the scope without the diagonal in place 1st and check that the laser beam exits the objective lens more or ledd right in the centre. This demonstrates that the optical axis of the objective and the focuser are all alighned as well (not always the case !). Then you add the diagonal and note any variation in the exit point of the laser when shone through the whole system. If the laser exits the objective somewhere else then the diagonal is not quite doing it's job. With an SCT you can't really do this so star testing would be required with carefull examination of the intra and extra focal images and then you need to work out which optical componant might be responsible for any issues that you see - not easy !

You need to be careful not to get too paranoid about all this though otherwise you stop enjoying your equipment and spend all your time testing and worrying about it

Edited March 23, 2017 by John

[Timebandit]

 

Hi. From what I have read both the diagnals you mention are of good quality . I think you are going to be hard pressed to see which one is visually better(unless collimation has been knocked out). I have the William optics diagonal and it is very good in quality. I also have a televue 2" diagonal and have tried testing these against each other to see if there is a clear winner. I have also tried testing the views with the diagnals in and without the diagnals in (straight through). To my eyes the view produced are all very good , without a clear noticeable winner in any instance. Obviously the atmosphere condition can have so much effect of the view , far more difference in atmosphere interference of view , compared to possible diagonal difference. 

In my opinion put these diagonal against each other on a clear crisp night with great seeing conditions. Test them on planetary and double stars to see if you can see a difference. I suspect visually you are going to struggle to see any difference between them at all . And it can also be sometimes the scope or eyepieces are a weaker link in the optical chain compared to the diagnals you are using. If after your test of the diagnals you can see no difference (to what I suspect will be the outcome). Then the best way to decide which one to keep, is to flip a coin and let fait decide , otherwise you will just be stuck on the fence ☺

[Highburymark]

I struggle to see any difference between the WO and the cheaper Skywatcher 2" diagonals. Presume the Revelation comes from the same factory.

[John]

The Revelation is made by GSO I think. It's a nice diagonal.

[festoon]

Yes the revelation is a GSO. It has a good quality feel and the reason I bought it was after good reviews here and on CN ?

[YKSE]

Having a good read of BillP's review of diagonals here, should give you some idesa what to look for when doing comparison.

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

[festoon]

9 minutes ago, YKSE said:

Having a good read of BillP's review of diagonals here, should give you some idesa what to look for when doing comparison.

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

What a great review - thanks for posting ? Makes me want a quality prism ?

[F15Rules]

Prisms are great on slower scopes, I used one on my F15 refractor and it's control of light scatter was superb. They also can last much longer as they don't have the delicate coatings that the fancy dielectrics do. However, I have heard that on shorter focus refractors they can introduces some chromatic abberation (CA)...I haven't seen that myself though.

Of the two mirror diagonals in your original post, I agree that there is likely little if any difference in them. The WO costs quite a bit more new, is "prettier", but no better optically IMO, so if it was me I'd keep the Revelation one and sell the other for an extra £30 or so and put that towards my next better scope 

Dave

[festoon]

9 hours ago, F15Rules said:

Prisms are great on slower scopes, I used one on my F15 refractor and it's control of light scatter was superb. They also can last much longer as they don't have the delicate coatings that the fancy dielectrics do. However, I have heard that on shorter focus refractors they can introduces some chromatic abberation (CA)...I haven't seen that myself though.

Thanks Dave..appreciate the input

Out of interest, do you have a particular prism that you can reccomend at 1.25"? I know I'm meant to be saving for a new OTA...but the article that YKSE posted really sells the prism! Might be a nice addition for viewing Jupiter this spring

[John]

4 minutes ago, festoon said:

Thanks Dave..appreciate the input

Out of interest, do you have a particular prism that you can reccomend at 1.25"? I know I'm meant to be saving for a new OTA...but the article that YKSE posted really sells the prism! Might be a nice addition for viewing Jupiter this spring

The Takahashi 1.25" prism is very good optically despite it's plasic housing. The prism is excellent in it. I use the Baader Zeiss T2 diagonal with my F/9 Tak refractor and thats superb but quite expensive !

I think it's the one that got a very glowing report in that test

[festoon]

5 minutes ago, John said:

The Takahashi 1.25" prism is very good optically despite it's plasic housing.

Thanks John - the Tak prism is reasonably priced too

[F15Rules]

+10 for the Tak prism, I had two of these over the years and rate them very highly.

I also really like the helical retaining ring, although not everyone does. My only watch out would be that heavy or large bodies 1.25" eyepieces might stress this (plastic) ring a bit and I would NOT use binoviewers with it..But it came as standard often with earlier Tak scope bundles I think, and was ideal for use, for instance, with Tak LE, TV Plossls, Orthos etc.

The Baader unit John refers to is about as good as it gets, a 2" capability as well, and built like a tank - with or without the Zeiss prism. (The Baader prism is also excellent).

A last one for consideration and even better (mechanically) than the Tak is the Baader 32mm aperture prism set (the 32mm true aperture allow the prism to fully utilise all of the 1.25"/31.7mm aperture of 1.25" eyepieces). It includes T2 connectivity (useful for connecting other components such as Baader binoviewers), a 1.25" nosepiece and a lovely helical focus 1.25" eyepiece holder with 5mm travel microfocussing..Ideal for high power fine focusing if your main focuser is single speed only. It's this one..http://www.365astronomy.com/Baader-Prism-Diagonal-T-2-90-degree-32mm-Set.html 

Great value even at the post Brexit vote price of £119..

Hope that helps.

Dave

Edited March 24, 2017 by F15Rules
Info added

[festoon]

Just bought the baader T2 prism diagonal set...could not resist!!!!!!

Now I'll have to sell one of the 2" dielectric mirror diagonals and inclined to sell the WO simply as I'll get more second hand for it!

[Timebandit]

39 minutes ago, festoon said:

Just bought the baader T2 prism diagonal set...could not resist!!!!!!

Now I'll have to sell one of the 2" dielectric mirror diagonals and inclined to sell the WO simply as I'll get more second hand for it!

 

 

Funny how you posted a question about looking to sell one of the diagnals you have.

And now us friendly chaps on here have somehow managed to get you to spend your money on another diagonal?