Refractor for planets, mostly visual, 4" or 5"

[jetstream]

1 minute ago, Deadlake said:

Or the Baader prism one? 😀😆

Actually no! lol!😀

As much as I like this prism I cannot recommend it for doublet ED scopes- its too much money to gamble on it working with a particular ED scope. The Baader BBHS is "guaranteed" to work with any refractor IMHO.

[Deadlake]

4 minutes ago, jetstream said:

Actually no! lol!😀

As much as I like this prism I cannot recommend it for doublet ED scopes- its too much money to gamble on it working with a particular ED scope. The Baader BBHS is "guaranteed" to work with any refractor IMHO.

If you get lucky with the chromatic shift it brings? 

[jetstream]

Just now, Deadlake said:

If you get lucky with the chromatic shift it brings? 

When a lot of top refractor users talk about " this scope" has better color on Jupiter than "that scope" etc I think its down to the glasses used, f ratio etc and who knows what else. I do think a prism can also massage colours- but its a lot of money to gamble on.

By all means try the prism!

In my refractors the APO triplets like it- my one ED scope does not. This is not to say all ED doublets won't get along with it. Personally I'm not a gambler and I love known characteristics etc.

[johninderby]

I tend to use prisms in my f/10 plus acopes but mirrors for anything much less than F/10. Apparently below  f/7 ofr f/8 is where prism s can cause chromatic problems.

Edited February 1, 2021 by johninderby

[mikeDnight]

I'm mainly a lunar and planetary observer, and although I've used many mirror diagonals over the years, for five years I used a relatively cheap Takahashi 1.25" prism diagonal. In my 100mm F7.4 Tak, there was no visible colour on the lunar limb and the planet's were vibrant and alive. So you don't need to empty the bank to get a good prism for planetary. Even on the Tak Sky 90 the Tak prism works beautifully, and although there may be some CA at F5.6, its not obvious, and views are razor sharp with no noticeable scatter.  When I bought a FC100DZ, I treat it to a BBHS prism, which is exquisite.  It also works perfectly well on SW 100ED &120ED's.

[Deadlake]

17 minutes ago, jetstream said:

When a lot of top refractor users talk about " this scope" has better color on Jupiter than "that scope" etc I think its down to the glasses used, f ratio etc and who knows what else. I do think a prism can also massage colours- but its a lot of money to gamble on.

By all means try the prism!

In my refractors the APO triplets like it- my one ED scope does not. This is not to say all ED doublets won't get along with it. Personally I'm not a gambler and I love known characteristics etc.

You mean you end up with both.... 😃

[jetstream]

2 minutes ago, Deadlake said:

You mean you end up with both.... 😃

Yes, but...

had I known the prism could be fussy I might not have bought it!

Sure glad I did though. Through all this discussion I must say that I have a cheap SW mirror diag that give vg views- its one downfall is scatter though. I had an expensive mirror diagonal that was a piece of junk years ago, so bad in fact I questioned my 90mm APO! In came the prism and voila- the 90mm gives wicked lunar/planetary views (for a small scope).

Knowing what I know now and if I could only have one diag it would be a top mirror diag like the Baader BBHS.

[alex_stars]

23 minutes ago, jetstream said:

Excellent and don't forget one of these

https://www.teleskop-express.de/shop/product_info.php/info/p7951_Baader-2--BBHS-Sitall-Diagonal-Mirror-with-1-10-lambda-surface.html

Thanks for pointing that out. I have been looking into that and what to use on the 125 mm F/7.8. A guy on a german forum (original link in German, last post by Gerd-2) calculated the optimal glass path to be added to the 125 mm F/7.8 to get the best colour correction out of the system. Here the short summary:

If you add 65 mm of glass path to the 125 mm F/7.8 you can achieve the following Strehl values:

• 480 mm ... Strehl 0.84
• 644 mm ... Strehl 0.85
• Polystrehl with 65 mm glass path in the optical path is 0.95

There you go, if that is really possible, then you have a great system at hand.

Regarding the Baader prisms:

• The standard 1.25" prism version with T2 threads (details here) adds about 35 mm of glass path
• The BBHS 1.25" prism version with T2 threads (details here) adds about 47.5 mm of glass path
• The BBHAS2" prism version with Clicklock (details here) adds about 100 mm of glass path

So given our German friend is correct, the BBHS 1.25" prism would be closest to the 65 mm that would be needed. But be aware, if you plan on a bino-viewer, that would add more glass to the optical path and then you might end up over-correcting and possible end up being worse that you started off with.

I personally will start off with my diagonal mirror and then see how the colour representation is. Afterwards I work from there...

CS

Alex

 

[jetstream]

5 minutes ago, alex_stars said:

calculated the optimal glass path to be added to the 125 mm F/7.8 to get the best colour correction out of the system

I'm not sure about this- it it in reference to imaging?

If it were me I'd use a good mirror diag , pop a good eyepiece in the scope and enjoy the good views.

[alex_stars]

30 minutes ago, jetstream said:

I'm not sure about this- it it in reference to imaging?

If it were me I'd use a good mirror diag , pop a good eyepiece in the scope and enjoy the good views.

Just went through the original German post again, the optimization was calculated for visual use.

However I agree, I'm gonna use my mirror diag (which I think is pretty good) and enjoy Keep life simple.

And if I, one day, can't stand the colours in my apo, then I will start thinking about the glass path again. 😉

[RobertI]

2 hours ago, alex_stars said:

Hi all and once more a big "thank you" for all the contributions.

My mind is set, I will get the 125mm F/7.8 Doublet (FPL-53 & Lanthan). For me, that appears to be the best bet. A doublet to cool fast, as I mostly do visual (so not really a need for a triplet) and the largest APO I can currently afford. So there we are. Ah yes and a set of really nice marbles to place across the meadow behind the house to observe something (thanks @vlaiv). Then I am "seeing" independent and gonna have a lot of fun 😉

I'll report back when I got the scope.Will take until spring though cause I want to see some planets before I form some opinion on the quality of the scope.

CS,

Alex

Looks like a fantastic scope, I’m sure you won’t be disappointed. I look forward to hearing about first light. 👍
 

I was very keen to get a quality prism diagonal for my F7 refractor, but I have to say having read all the above, I think I’ll stick with my mirror! ☺️

[John]

I use a Baader T2 Zeiss spec prism with my F/9 Tak FC100-DL but Tele Vue and Astro Physics mirror diagonals with my other refractors. I have not yet tried the prism in one of my faster refractors - I might one night, out of interest.

 

[Carl Au]

2 hours ago, johninderby said:

I tend to use prisms in my f/10 plus acopes but mirrors for anything much less than F/10. Apparently below  f/7 ofr f/8 is where prism s can cause chromatic problems.

Interesting, I didn't know that 

[Carl Au]

12 minutes ago, John said:

I use a Baader T2 Zeiss spec prism with my F/9 Tak FC100-DL but Tele Vue and Astro Physics mirror diagonals with my other refractors. I have not yet tried the prism in one of my faster refractors - I might one night, out of interest.

 

I have a two inch WO diagonal in mine, lovely quality. I have no complaints at all. 

[johninderby]

1 minute ago, Carl Au said:

Interesting, I didn't know that 

Apparently in faster scopes does give slight chromatic problems but they are very minor and no real problem.

[Carl Au]

1 minute ago, johninderby said:

Apparently in faster scopes does give slight chromatic problems but they are very minor and no real problem.

Right. So what's the advantage of using one.

Which reminds me I need to get my scope in...particularly cloudy indeed.

[Carl Au]

Just now, Carl Au said:

Right. So what's the advantage of using one.

Which reminds me I need to get my scope in...particularly cloudy indeed.

Partially even

[Dantooine]

2 hours ago, Deadlake said:

Or the Baader prism one? 😀😆

I went for the prism one. It was lighter than expected. if I got that there would be no more dreaming about another. 

Edited February 1, 2021 by Dantooine

[johninderby]

7 minutes ago, Carl Au said:

Right. So what's the advantage of using one.

Which reminds me I need to get my scope in...particularly cloudy indeed.

In a long focal length scope a good quality prism gives a bit better contrast and a bit less light scatter..

[Highburymark]

On 30/01/2021 at 17:13, Alkaid said:

I do like refractors also. Which one did you get?

Apologies. Late responding: Tak FC-100DC.

[John]

1 hour ago, johninderby said:

In a long focal length scope a good quality prism gives a bit better contrast and a bit less light scatter..

I've been wondering about moving to a 2 inch Baader BBHS prism diagonal for my 130mm F/9.2 triplet but I'm doubtful that I will notice the difference for the additional £'s spent

 

[Deadlake]

13 minutes ago, John said:

I've been wondering about moving to a 2 inch Baader BBHS prism diagonal for my 130mm F/9.2 triplet but I'm doubtful that I will notice the difference for the additional £'s spent

 

If the F stop of your EPs is not over 33 mm (from memory) you could get the T2 version rather then the 2” version, it’s half the price. Which one by the way > F7 the prism version?

Edited February 1, 2021 by Deadlake

[johninderby]

8 minutes ago, John said:

I've been wondering about moving to a 2 inch Baader BBHS prism diagonal for my 130mm F/9.2 triplet but I'm doubtful that I will notice the difference for the additional £'s spent

 

The term splitting hairs comes to mind. 😁

I mean I think I cam tell the difference in a long focal length scope but not that fussed. Perhaps on a night of perfect seeing?   😁😁😁

Edited February 1, 2021 by johninderby

[Alien 13]

24 minutes ago, John said:

I've been wondering about moving to a 2 inch Baader BBHS prism diagonal for my 130mm F/9.2 triplet but I'm doubtful that I will notice the difference for the additional £'s spent

 

I expect it come down to whether you want to call your scope a refractor or reflector, at the moment your scope is mostly reflector 

Alan

Edited February 1, 2021 by Alien 13

[johninderby]

2 minutes ago, Alien 13 said:

I expect it come down to whether you want to own a refractor or reflector, at the moment your scope is mostly reflector 

Alan

The Baader prism works fine in my grab’n’go reflector. 🙂