Higher power EP for Sax 150 x1200 reflector

[AstralTraveller (David)]

Originally Posted by CR500

.... I will get one of these filters you mentioned ....

I think some confusion has crept in here because the discussion has shifted from reflectors to refractors. Only refractors suffer from chromatic aberration and so only refractors benefit from minus violet filters. You don't need one for a reflector (although they have their own issues).

Actually you only need a minus violet for a poorly corrected refractor. So nice (expensive) triplets don't need a filter (or so I believe) and probably an ED doublet can also do without. For simple achromatic doublets the amount of chromatic aberration is a function of aperture and f ratio. So as a scope gets bigger it needs a longer ratio. According to one discussion I read a 125mm doublet is OK at f/8 but a 150mm needs f/10 to have acceptable colour. That would make the scope 1.5m which is possibly too big, certainly for a portable scope. So the compromise is f/8 and a filter.

[Varangian (John)]

Originally Posted by CR500

Thanks for the info Varangian. I actualy bought one of these last night from an Australian shop so I will look out for the identifying marks you mentioned to see if I have the real deal.

No worries. The fakes and the authentics are now essentially made in the same place in China, many people think they are the same quality with different markings, but there are those that would not touch a TMB with a MM marking on it. They look for the mm marking and the gloss barrel as stated. I have a fake TMB and am pretty happy with it, it is superior to my standard plossls.

[CR500 (Rod)]

Originally Posted by AstralTraveller

I think some confusion has crept in here because the discussion has shifted from reflectors to refractors. Only refractors suffer from chromatic aberration and so only refractors benefit from minus violet filters. You don't need one for a reflector (although they have their own issues).

Actually you only need a minus violet for a poorly corrected refractor. So nice (expensive) triplets don't need a filter (or so I believe) and probably an ED doublet can also do without. For simple achromatic doublets the amount of chromatic aberration is a function of aperture and f ratio. So as a scope gets bigger it needs a longer ratio. According to one discussion I read a 125mm doublet is OK at f/8 but a 150mm needs f/10 to have acceptable colour. That would make the scope 1.5m which is possibly too big, certainly for a portable scope. So the compromise is f/8 and a filter.

So, just to be sure, I dont need one of these filters for my Saxon Newtonian Reflector, but the quality standard of the mirrors will still restrict me to 200X?

[mental4astro (Alexander)]

Rod, my apology about the debate that's developing.

John, I am still confused by your selective quoting. You are just confirming what I had written about the atmosphere being the limiting factor, irrespective of the telescope.

Then the question of the upper limit of magnification I mention, is it a question of the scope's quality? I certainly don't agree with Department Store Specials that say 545X on a dodgy 3" instrument. I limit my statements to my own experience with reasonable quality gear, AND always put the caviat of the atmosphere in my statements. The question asked is on high magnification & I'm seeking to pass on a realistic case, not a fanciful one. If my notion of the 50X per inch is misguided, please point me in the right direction - PM would be good out of this thread.

Of course to ask an achromat to do this is not possible, but the scope in question here is a reflector. I have not read anywhere the OP saying his scope is a refractor. In fact, his original post states it is a reflector. Why all this talk of refractors???

[AstralTraveller (David)]

Originally Posted by mental4astro

Why all this talk of refractors???

I think this started when I noted that the focal length of my refractor is the same as the fl of the reflector and hypothesised that eps that I found useful would also be useful for Rod. I never meant it to go any further.

Originally Posted by CR500

So, just to be sure, I dont need one of these filters for my Saxon Newtonian Reflector, but the quality standard of the mirrors will still restrict me to 200X?

No you don't need a filter. The limit of 200x applies to refractors because they suffer from chromatic aberration whereas a reflector doesn't. So the reflector should be capable of 50x per inch of aperture. However IMHO 300x is very high and you won't often use it. I think starting with a 200x ep is a good idea. See how you go after that. In general buy relatively few good eps and plan to keep them. I have three scopes but only one set of eps. I may sell a scope but I have no plans to part with any eps.

[CR500 (Rod)]

Astralatraveller, thanks for clearing that up, good to hear.
I have bought some good Pentax XW eye pieces (20mm and 14mm so far) to use with my good quality spotting scope and expect to be able to use these with my reflector telescope as well. Like you said, If I do get a bigger Dob, I will be able to use the same good eye pieces.

[brian nordstrom (As avatar)]

, Yep , back on track here Rod , 200x in a nice 6 inch f/8 newtonion is nothing to be sneezed at .
On the rare perfect nights ( 1 or 2 a year ) a good 6 inch newt properly columinated will easily top 400x on the moon or the brighter planets so a 3-4mm eyepiece that will give you that wont be used very often , but its good to have ,,, just in case .
Brian.

[CR500 (Rod)]

Brian, I will keep a lookout for good quality used short length eye pieces, just to try once in a while like you said. I might look at a 5mm eye piece first if I can get a good one for a good price.

[brian nordstrom (As avatar)]

Grab one of those 4-7mm LP planetary's from Andrews , same eyepiece as the William Optics , and only $59 delivered .
I have the 3.5mm one that works well as a HP/EP thats now only used for white light viewing of the sun , good contrast EP's at high power ( only when the seeing is perfect , of course ) .
Brian.

[Satchmo]

I'd recommend something around 6mm or 200X as a good allrounder for a 6" F8 Reflector. Sorry about the red herrings I misread the title of the post. You won't be able to use 300X many nights and at a 1/2mm pupil any dust on the eyelens and floaters in your eye will become annoying.

[CR500 (Rod)]

I have a 6mm TMB planetary coming already, so if it goes
ok, I will try a 5mm Long Perng for those good nights.

[mental4astro (Alexander)]

Nice choice the 6mm TMB. It's in my kit too.

[ausastronomer (John Bambury)]

Originally Posted by mental4astro

John, I am still confused by your selective quoting. You are just confirming what I had written about the atmosphere being the limiting factor, irrespective of the telescope.

Of course to ask an achromat to do this is not possible, but the scope in question here is a reflector. I have not read anywhere the OP saying his scope is a refractor. In fact, his original post states it is a reflector. Why all this talk of refractors???

Alex,

My apologies. Like Mark (satchmo) I didn't read the thread title correctly and with several earlier posts talking about chromatic aberration and achromats, I incorrectly assumed the scope under discussion was the 6"/F8 Synta (Saxon) Achromatic refractor.

Rod,

As others have mentioned the 6" reflector should be capable of 300X on nights of good seeing. More often than not about 200X will be a realistic maximum due to thermal equilibrium and atmospheric conditions.

The 6mm TMB you have on order is an excellent choice.

Cheers,
John B

[mental4astro (Alexander)]

No problem, gentlemen. I'm glad this was cleared up in a civilised manner,