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

[John]

33 minutes ago, Deadlake said:

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?

Having checked the spec of the Baader T2 prism I use with my Tak, it appears that it is the BBHS version so I'll try it in the 130mm triplet sometime and see if I can notice any differences over the Astro Physics Maxbright that I use with that scope currently.

I probably need a night of exceptional seeing so that might take some time !

(Can't seem to get "seeing" of any kind lately )

Edited February 1, 2021 by John

[Deadlake]

13 minutes ago, John said:

Having checked the spec of the Baader T2 prism I use with my Tak, it appears that it is the BBHS version so I'll try it in the 130mm triplet sometime and see if I can notice any differences over the Astro Physics Maxbright that I use with that scope currently.

I probably need a night of exceptional seeing so that might take some time !

(Can't seem to get "seeing" of any kind lately )

Wednesday is looking good, hopefully it will hold.

Martin

[JeremyS]

15 minutes ago, John said:

Having checked the spec of the Baader T2 prism I use with my Tak, it appears that it is the BBHS version so I'll try it in the 130mm triplet sometime and see if I can notice any differences over the Astro Physics Maxbright that I use with that scope currently.

I probably need a night of exceptional seeing so that might take some time !

(Can't seem to get "seeing" of any kind lately )

Sounds like a Pup splitting night, John 🤞

[JeremyS]

59 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

 

That’s my dilemma with the TSA 120, John. My BBHS mirror is wonderful, but might the prism be even better.....?

[Dantooine]

22 minutes ago, JeremyS said:

That’s my dilemma with the TSA 120, John. My BBHS mirror is wonderful, but might the prism be even better.....?

About a year ago I had the dilemma of the bbhs mirror or the bbhs prism. It was the thought in my head that the mirror would make my refractor partly a reflector. This played havoc with me so I got the prism. It may sound daft to most people but it settled me 😎

[John]

I'm not bothered about having a mirror in the system, as long as it's a good one.

Likewise, I'm happy to use lens based eyepieces in my 12 inch dobsonian

 

 

[mikeDnight]

Personally i think there will be greater differences brought about by seeing conditions and experience than by diagonal choice. A good observer will see more using a £99 mirror diagonal than a less experienced observer would from a £500 prism. That's the reality, as the greatest variable is the observer himself. The same applies to eyepieces and even to telescopes. W. F. Denning in his book Telescopic Work for Starlight Evenings notes " What one man sees through a 5 inch glass, another man needs a 10 inch." Of course that's not to say that one diagonal isn't technically better than another.  Sometimes an experienced observer can fine tune his already superb optic by buying a high end  eyepiece or diagonal, but not everyone could see the advantage. 

[Dantooine]

10 minutes ago, mikeDnight said:

Personally i think there will be greater differences brought about by seeing conditions and experience than by diagonal choice. A good observer will see more using a £99 mirror diagonal than a less experienced observer would from a £500 prism. That's the reality, as the greatest variable is the observer himself. The same applies to eyepieces and even to telescopes. W. F. Denning in his book Telescopic Work for Starlight Evenings notes " What one man sees through a 5 inch glass, another man needs a 10 inch." Of course that's not to say that one diagonal isn't technically better than another.  Sometimes an experienced observer can fine tune his already superb optic by buying a high end  eyepiece or diagonal, but not everyone could see the advantage. 

It just means I can’t blame my tools.. just myself 🤣

[chiltonstar]

This is of course true, but I don't think you can beat the laws of physics - agreeing with @Vlaiv in an earlier post. Coming back to the original theme, for much of the recent Mars season, I had my 4" f13 (Vixen objective) frac out alongside my 180 Mak and there was a consistent benefit in using the larger scope on nights with good to excellent seeing. On poor nights, the frac gave a better (more contrasty) view but when the seeing was good enough,  extra detail was apparent with the Mak even though the contrast was slightly less.

When I bought the Mak it was after using a long focus frac for nearly 30 years - I wanted a larger frac but couldn't afford (or lift) it and the Mak seemed the next best thing. I haven't regretted the decision, and have not been limited by the cooling problems that some describe - every scope type is a compromise and in the real world you have to seek solutions to mitigate problems. You can reduce dT/dt issues with cladding and storage at ambient temperatures for example.

Chris

[Alan White]

18 hours ago, alex_stars said:

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 😉

Love this comment Alex, a great idea, I think many of us will like to do the same.
But sadly I have lost my Marbles.....

On the new scope, great choice, hope it works well for you.

[alex_stars]

3 hours ago, Alan White said:

Love this comment Alex, a great idea, I think many of us will like to do the same.
But sadly I have lost my Marbles.....

On the new scope, great choice, hope it works well for you.

Regarding the observation target, I just had a great session on Jupiter, which is currently at my place about 45 arcsec in diameter 😉 at a nice altitude and local seeing is good. 👍 As an explanation, I placed a 4 cm diameter print-out (1200 dpi) about 170 m away from my scope across the meadow, stabled to a wooden rail on the trailhead.

Now that is what I call a view.

However something interesting to observe regarding the dT/dt (cooling). The first 10 mins the scope was okay, then tube currents set in which made it really hard to focus on "Jupiter" and now the scope is useless for at least an hour (my experience). I can simulate the same focusing problems as on real Jupiter with this setup. Also I can test magnifications, exit pupils and eyepieces and, thanks @vlaiv, test the performance of the scope! I recommend that.

Will go outside in an hour and see how the scope does then.

[Alan White]

@alex_stars Neigh, it cannot be stabled, far to big.... or have you a very large stable 😉

 

[chiltonstar]

19 minutes ago, Alan White said:

@alex_stars Neigh, it cannot be stabled, far to big.... or have you a very large stable 😉

 

Oh I dunno, I had a Mars in a stable once.

Chris

[vlaiv]

45 minutes ago, alex_stars said:

Also I can test magnifications, exit pupils and eyepieces

If you happen to have multiple diagonals - you can check if there is difference between these too

[alex_stars]

Maybe to add to the discussion on what a 4 and 5 inch APO can show in comparison to a 7 inch Mak, here the theoretical MFTs of the scopes.

[DISCLAIMER] the spatial frequency limits plotted below are translated to actual spatial scale limits. For comparison the diameter dimensions of known objects are listed in brackets:

Horizontal resolution is now in actual units (you can check where the Dawes Limit for the 125 mm APO is), so one can compare. Down to a 2 arcsec spatial frequency limit (that's about the size of 1/4 of the GRS in spatial scale), the 125 mm APO will actually be better than the Skymax.

As for the Skymax 180mm, which is more close to 172 mm (however that does not matter much for the MTF), the actual problem is that the central obstruction is larger than expected. So on paper the Skymax is the green line, but in reality it is more like the cyan line, and hence the 125 mm APO does compare for quite a long way (resolution wise). However people are right, for the best of seeing nights (around 1 arcsec and lower), the Skymax will be better.

Edited February 2, 2021 by alex_stars

[Deadlake]

This link was of interest to me:

https://www.damianpeach.com/simulation.htm

The effect of seeing cannot be underestimated. However who are we kidding, if we could we would end up with both scopes a 4-5" Refractor and a large SCT/Reflector for those exceptional nights...

Short answer scope of 10-15 cm in size deal well with poor seeing...

Edited February 2, 2021 by Deadlake

[vlaiv]

6 minutes ago, alex_stars said:

Maybe to add to the discussion on what a 4 and 5 inch APO can show in comparison to a 7 inch Mak, here the theoretical MFTs of the scopes:

It is important to understand how to read such MTF diagram.

Actual numbers and figures are correct, but plotting features like GRS or 2 arc second mark is misleading.

This diagram shows amplitudes in frequency spectrum of image - or rather attenuation for particular frequency component. It has almost nothing to do with actual size of the feature.

Here is example - image in the left - well, that's my doodle of "a feature" on the surface of the planet - right, that is Fourier Transform of that feature image - amplitudes in frequency domain. Bottom - equivalent graph to MTF - X axis is frequency and Y axis is intensity of given frequency component of the feature.

As you see - our feature has frequency components all over the spectrum and if we attenuate some of those frequency components - we will not make feature disappear.

It does not work like that.

 

[jetstream]

17 minutes ago, Deadlake said:

hort answer scope of 10-15 cm in size deal well with poor seeing...

😀

My real world, unscientific tests reveal that when I lower the mag on the 15", with binoviewers, down to about 73x on the moon this combination gives much more detailed views than the TSA120.

These days I just lower the mag on the 15"- if seeing can't support this I go back in the house.

[Robindonne]

20 hours ago, JeremyS said:

That’s my dilemma with the TSA 120, John. My BBHS mirror is wonderful, but might the prism be even better.....?

How nice would it be if shops send out “try before you buy-versions” of these small items to well known customers.   All these dilemmas would be gone. 

[John]

43 minutes ago, Robindonne said:

How nice would it be if shops send out “try before you buy-versions” of these small items to well known customers.   All these dilemmas would be gone. 

It does happen

[alex_stars]

1 hour ago, vlaiv said:

As you see - our feature has frequency components all over the spectrum and if we attenuate some of those frequency components - we will not make feature disappear.

It does not work like that.

Hi @vlaiv

I never said that any features would disappear in any given telescope. I merely pointed out spatial frequency limits and their respective size in the spatial domain (e.g. so that a 2 arcsec limit in space would translate into a frequency limit of 0.5 cycles per arcsec). Probably the misleading term was "features" and "spatial scales of above 2 arc seconds" would have been better (I corrected that in the original post).

Either way I want to make two points in response to your post:

• Plotting spatial frequency cut-offs on a power spectrum does make sense, else you would doubt the meaning of the Dawes' or Rayleigh limit. I am sure you are familiar with such cut-off limits from signal processing.
• Your doodled feature has obviously many more spatial frequencies than its largest dimension, given that it is of complex shape. Hence the rich FFT and the spread in power spectrum response. No surprise there. However I fail to see the connection to my post here.

Bringing this back to practical observing. I am sure we can agree that the GRS, even though being about 8 arcsec in diameter, has many more higher spatial frequencies due to its internal structure (I never made the claim that it is otherwise). I am sure we can also agree that features do not disappear when talking about resolution and cut-off limits in spatial frequency domain (as stated initially, I never claimed that either). A practical example would be the Cassini Division on Saturn's rings. If I remember correctly it's 0.75 arcsecs wide at its widest location. The Dawes' limit of a 125 mm aperture is at 0.93 arcsecs. It the (never made!) claim would hold true than you would NOT be able to see the Cassini Division in a scope with an aperture below 155 mm. Which we all agree is non-sense.

So in conclusion I don't see what you mean with "It does not work like that", but I am sure you meant well.

CS

Alex

Edited February 2, 2021 by alex_stars

[vlaiv]

14 minutes ago, alex_stars said:

So in conclusion I don't see what you mean with "It does not work like that", but I am sure you meant well.

Indeed - I did not add my comment as direct response for any particular claim. Sorry if it looked like that - it was more an addition to your post than disagreement.

I just wanted to point above out for anyone reading your comment who not necessarily knows about frequency decomposition. That is why I focused on added features on those diagrams and what they might imply for people just getting into all of this.

When we talk about telescope resolution and if something can or cannot be seen in telescope - we don't really explain to people what happens and what they might expect and what that frequency limit means.

I figured that people get the sense of telescope resolution in terms of - particular feature can or cannot be seen and then when we start adding GRS or 2" feature size marker on above graph - it can imply to them that frequencies are related to feature size - and in some far removed sense - they are.

In one of previous posts we touched upon definition of the terms like: resolution, contrast, detail, sharpness and how those are all related in context of telescope image.

MTF that you posted depicts this relationship, but I have a sense that people don't really get what it all means and how it relates to one another.

Just as an example - we see stars in telescope. Stars are possibly smallest angular features that we readily see in a telescope. Angular diameters of stars that we see are tiny micro arc seconds or less (close and resolved stars have milliarcsecond sizes) yet we see those features in telescopes that have Dawes and Rayleigh resolution of 1-2 arc seconds.

I would be happy to discuss all of this in depth with examples, but I'm not sure if we should derail this thread further or if people are interested in such discussion (although I think that most of us would benefit of understanding all of that in order to better grasp telescope performance).

 

[alex_stars]

@vlaiv thank you for clarifying and extending the discussion a bit further. This is well appreciated 👍

I indeed agree with you that we should not further derail this thread and keep it to the more practical aspects 😉

As you might have seen in this thread, I set up a test target Jupiter today (170 m away from my scope). At the risk of sounding strange, I do look forward to sunrise tomorrow and keep on observing what I can see on my target without the atmosphere interfering. This is truly a good exercise to test a scope.

Weather conditions are not good these weeks at my location so this setup is a great way of continue observing 😉

[CraigT82]

17 minutes ago, vlaiv said:

I would be happy to discuss all of this in depth with examples, but I'm not sure if we should derail this thread further or if people are interested in such discussion (although I think that most of us would benefit of understanding all of that in order to better grasp telescope performance).

I for one freely admit that I don't really know how to interpret MTF plots! I would appreciate another thread explaining that 😆

[jetstream]

20 minutes ago, CraigT82 said:

I for one freely admit that I don't really know how to interpret MTF plots! I would appreciate another thread explaining that 😆

Yes a full explanation of MTF vs resolution vs object feature size would be great. @vlaiv?