Can of worms - Fracs and Dobs

[JOC]

OK, some interesting comments have come out another thread where I found out that the aperture of a Dob could be stopped down to give some of qualities of a Frac.  It was then said by John:

It's worth remembering that stopping down your primary mirror or objective lens also reduces it's resolution to that of the size of the aperture. So the brightness does reduce but so does the amount of detail that the scope will resolve.

I responded with:

OK, so that makes sense, so then why do frac users deploy those long thin telescopes when they could be using a Dob with a huge mirror and lots of possibility to resolve detail.  Where does the Frac design score over the Dob. design? (Or does that open too many can's of worms?)

It's been suggested that my question requires its own thread and several large boxes of popcorn, but if we're having popcorn could have the sweet toffee butterkist type please - salty popcorn is affront to popcorn world wide.

Question is now launched:

Why do frac users deploy those long thin telescopes when they could be using a Dob with a huge mirror and lots of possibility to resolve detail.  Where does the Frac design score over the Dob. design?

  Need an emoticon with a tin hat!

[John]

I'm hoping that Shane (Moonshane) will chip in at some point because he has quite a bit of experience of using a larger newtonian with an aperture mask and has also more recently become a regular refractor user

Wheres my popcorn ? - oh here it is !

[SilverAstro]

It was my mistake for mentioning the 'frac' word !

What I should have said :- "positioned correctly it can remove the diffraction effects of the vanes and central obstruction " - and left it at that ! Then no need of tin hats (maybe)

Quiet in the back row, stop all that munching

 

[domstar]

Beauty?

[Moonshane]

Some initial thoughts, differences in :

• storage restrictions - some might not have room for a big dob but can slip a frac behind the couch
• cool down times - big dobs take a while to cool sometimes
• seeing conditions - smaller doublet fracs cut through poor seeing 
• cost - you can get a small frac for a lot less than a big dob
• observing preferences – solar is far better with a frac and wedge than a dob and film
• accessories – I like binoviewing which is easier with fracs than dobs
• wide field requirements – bigger aperture normally = less field

As always it’s never simple. I have only relatively recently ‘discovered’ fracs as I got into solar but I only get on with ED or better and now use e.g. my 120ED more than any other scope, closely followed by my 12” f4 dob, then my 100mm PST mod, my 102mm ED, my 6” f11 newt and my 16” f4 dob which I have not used in anger for around 2 years. I am going to be chopping the tube on this last scope soon to allow more easy loading into the car so I will use it more often. Having a number of options is good.

[Racey]

This is gonna run and run... 

[SilverAstro]

"Beauty "

 

[Moonshane]

As John said above, on the specific question of aperture masking bigger apertures, I feel this is worth it if you have a big newt but I'd not buy a big newt to allow you to do it!

I got my best views of Mars with my 16" masked to 170mm f11.

 

 

[happy-kat]

From my minimum use I much prefer the pin point stars my small refractor gives over my reflector.

[Moonshane]

5 minutes ago, happy-kat said:

From my minimum use I much prefer the pin point stars my small refractor gives over my reflector.

So do I but if I want to see a faint extended object at home I prefer to see it with one of my big dobs along with slightly blobby stars rather than not see it along with pinpoint stars in my frac. 

[SilverAstro]

3 minutes ago, Moonshane said:

Some initial thoughts :

seeing conditions - smaller doublet fracs cut through poor seeing 

solar is far better with a frac and wedge than a dob and film

Having a number of options is good   Yep

Very interesting thank you

I have not thought about wedges, will have to Google why they can not be used with a Dob  ( which is what is implied I am thinking ? ) ?

Re. poor seeing yes , but that removes the 'golden' or 'lucky' moments as well ???

,,,,

- No chromatic distortion from a mirror (eypieces beng common to both)

 

[Moonshane]

2 minutes ago, SilverAstro said:

I have not thought about wedges, will have to Google why they can not be used with a Dob  ( which is what is implied I am thinking ? ) ?

You'd overheat your secondary and also probably not have adequate in focus.

[Moonshane]

3 minutes ago, SilverAstro said:

Re. poor seeing yes , but that removes the 'golden' or 'lucky' moments as well ???

Agreed, this is why when I have the chance I use large newts for planetary myself although I find my 120ED gives better views more often

[SilverAstro]

1 minute ago, Moonshane said:

You'd overheat your secondary and also probably not have adequate in focus.

Ah , , , I'm thinking , ,

would the wedge not replace the secondary ?

oh! maybe the refracted (or do I mean reflected) waste beam may singe the tube somewhere else /

, , , still thinking !

 

[Moonshane]

Just now, SilverAstro said:

Ah , , , I'm thinking , ,

would the wedge not replace the secondary ?

oh! maybe the refracted (or do I mean reflected) waste beam may singe the tube somewhere else /

, , , still thinking !

 

The modifications required would be more expensive than a used 120 achromat

[cloudsweeper]

Fracs are great - widefield views are wonderful.  But for faint extended objects, aperture is necessary - surface brightness alone is not enough - more magnification is required to raise the overall brightness.  It's the difference between a one-LED torch and a ten-LED torch.

Doug.

[SilverAstro]

Maybe need two wedges for a Dob (Newt) because of sheer excess from the big mirror, is ther a limiting 'frac size they can be used with ?

 

[Alien 13]

You can always use a smallish newt without having the mirrors silvered for solar, might still need an additional filler though.

Alan

[SilverAstro]

1 minute ago, Alien 13 said:

 always use a smallish newt without having the mirrors silvered for solar

but still reduce resolution on fine detail ?

[Moonshane]

You can see pretty much all available detail in white light at about 4"

[Peter Drew]

51 minutes ago, Moonshane said:

As John said above, on the specific question of aperture masking bigger apertures, I feel this is worth it if you have a big newt but I'd not buy a big newt to allow you to do it!

I got my best views of Mars with my 16" masked to 170mm f11.

 

 

My thoughts exactly, but if you have a big enough Newtonian , I can get a 12" F10 off axis unobstructed aperture from my 30" mirror. A 12" F10 "apo" for the price of a piece of cardboard is not to be sniffed at!  

[ronin]

positioned correctly it can remove the diffraction effects of the vanes and central obstruction

If you think about this it means the mask has to fit between 2 vanes so that means a maximum of 1/4 the actual mirror area. As the mask would be round and the gap between vanes triangular you are going to be down to say half so you get about 1/8 the aperture. Why buy a 10" mirror to be left with the light gather of a 3.5" mirror.

[SkyGibbon]

34 minutes ago, ronin said:

 

 

If you think about this it means the mask has to fit between 2 vanes so that means a maximum of 1/4 the actual mirror area. As the mask would be round and the gap between vanes triangular you are going to be down to say half so you get about 1/8 the aperture. Why buy a 10" mirror to be left with the light gather of a 3.5" mirror.

Because you acquire an 80mm f/15 APO cheaply that way.

[JOC]

So I'm already understanding with Astronomy that a lot (most) of the apparent magnification is down to the EP's - do the frac users see their objects as apparently large (or significantly, smaller or larger) than the Dobs do - If someone had an F6 Frac (that may not even be possible b.t.w. I don't know enough) would they get the same view through the same EP as I would through my F6 Dob?  I imagine their overall view of the sky must be potentially smaller even with the least magnification - their OTA tubes are not as wide - I still struggle to find a given star that I know with my Dob - the Frac users must have a really tough time as surely they don't see as much of the sky in one go - or do they?

[SilverAstro]

40 minutes ago, ronin said:

 Why buy a 10" mirror to be left with the light gather of a 3.5" mirror.

As Skywatcher might say,, still 36% more Light Gathering than a 3" achro ( I dont want to upset any apo chums)  

 

46 minutes ago, Peter Drew said:

My thoughts exactly, but if you have a big enough Newtonian , I can get a 12" F10 off axis unobstructed aperture from my 30" mirror. A 12" F10 "apo" for the price of a piece of cardboard is not to be sniffed at!  

Best reason I've heard so far to get an even bigger Dob