Higher power EP for Sax 150 x1200 reflector

[CR500]

G,day Fellas. It is my first post here, so be gentle:)
I have just bought a Saxon 150mmx1200mm reflector for my daughter and I to use. It comes with a 20mm and 10mm eye piece. How much shorter length eye piece can I effectively use with this scope to get closer views of planets without going over the scope capabilities?
I dont mind spending a bit more to get better quality, without going straight to a TV Ethos etc.

[Kunama]

Hello Rod (and daughter) and welcome.
At the moment you have 60x with the 20mm and 120x with the 10mm.
You could possibly go to a 7mm or 6mm with good seeing conditions.
They would give you 170x (7mm) or 200x with the 6mm. I would not expect much more than that.

[mental4astro]

Hi Rod, :welcome: to IIS!

There's a "golden rule" when it comes to the maximum practical magnification for a given aperture. It is:

50X per inch of aperture

Your 150mm reflector is a 6" scope, so its maximum magnification before the image starts to degrade is 300X.

HOWEVER, the atmosphere is the ultimate dictator of what can be used on any given night. Average conditions will see a maximum useable being 120X, regardless of how large your scope is.

Good conditions will allow 200X to 250X. Exceptional conditions upto 400X, but these are very rare and need an appropriate sized scope.

A 4mm EP will give you your maximum 300X, but the number of times you'll get to use it is very small per year. As Matt mentions, a 6mm would be used more often if you are looking for higher magnification.

I suggest you also look for EPs other than plossls. These see the eye lens (the lens of the EP that you look into) get smaller as the focal length shortens and the eye relief (the distance you need to place your eye from the eye lens) get smaller too. It gets to the point that you need to place your eyeball on the eye lens the size of a pinhole for a 6mm plossl.

The best short focal length EPs at a great price are the TMB Planetary Type II range. These can be found on Ebay too. There are other fine short focal length EPs, but these are also more costly, so the way you want to go is upto you.

One of my favourite EPs is my 9mm TMB! It gives me 250X in my 8" and 17.5" scopes (both have the same 2000mm focal length), and is my most used EP with the Moon and planets.

[CR500]

Thanks for the info. I didnt think I could go much bigger. I will limit it to 6mm.
I have been reading some posts about the 8" dobs. I was wondering if it might be best to get them to delay sending my 6" Saxon reflector on EQ3 mount and instead exchange it for an 8" dobsonian? I thought the EQ mount would be easiest for holding on a planet , but will the 8" Ozscopes Dob be better in the long run for viewing the moon and planets mainly???
p.s. Where I will be viewing from at home will be fairly bright from local street lights.

[Scorpius51]

Have you looked at the Dobs from Bintel and Andrews Communications?

[mental4astro]

Rod, if your EQ3 has a clock drive it is a fine instrument for the Moon and planets. My favourite lunar and planetary scope has been a 5" SCT, and all it had was a clock drive - no GPS, no fancy electronics, didn't even make me coffee. But it gave me spectacular images of the Moon and planets. You really don't need larger for the Moon and planets.

If you can afford the luxury, keep your eq mounted 6" (with a clock drive) for the Moon and planets, and save for a 10" dob for your DSO work. You will have the best of both worlds this way. Many folks have various scopes for different objects for these reasons. Me too. 6 actually. Only 1 of these six, however, I really only use for the Moon and planets as it has a clock drive - the only scope I have with a drive.

But, if the $ is too much to ask, an 8" dob will serve you very well for planetary and DSO's. I've used a few of these, and they are superb for this.

[Regulus]

Rod the 6" is as good for 'star surfing' as anything but the 8" Dob will give more detail in nebular and galaxy views due to the extra aperture.
Sure with a 6mm (recommended btw) you need to move the scope to keep planets/moon in the view cos they can disappear fairly quickly. This is really not a problem, all the OMG and Gasp factors still exist.
PLUS u wont get the Chromatic Aberration prob of red/blue halos around the brighter objects that Achromats produce if it's an Achromat you r talking about (altho reduced somewhat by the length of the scope u have opted for).
They r quick to set up and easy to transport and easy to use.
Trevor

[CR500]

Scorpius. If I was to exchange this scope for a Dob, it would have to be from Ozscopes, where I have ordered this one from.
http://www.ozscopes.com.au/newtonian-reflector-telescope-saxon-f15012eq3.html

Mental4astro. I,m not sure if it has a clock drive, but it will do me for now. It doesnt have all the beels and whistles for this price.
Like you said, I was thinking of having a second bigger Dob later on, such as the 10" one.
This would be for if I go for a bit of a drive to a dark area, no problem fitting a 10 or 12" in my car. I will get started with the smaller one so I know what I,m doing.

[AstralTraveller]

I have a 150x1200 refractor, so the same focal length and magnification. I loooove my University Optics 7mmHD ortho for moon and planets, and for about $120 it's relatively cheap. It is razor sharp and I can see surface detail on Saturn pretty easily. Since it was so good I then got the 5mm version but I've found it less useful. Two reasons. The atmosphere doesn't often support 240x seeing (at least not around here) and it is physically difficult to use. Orthos have small to very small eye lenses and short to very short eye relief (the lens to eye distance). Being decidedly 'old school' I don't mind this - when I started in astronomy you had no choice - but the 5mm is hard even for me. These days many people prefer the long eye relief of some of the newer designs and apparently they work quite well even though they have more glass elements (which should be a disadventage). I haven't used one so I can't comment.

[Varangian]

Hi mate, if you want to go the dob you can compensate for not having a clock drive with a wider field eyepiece like a 5mm Hyperion or Stratus. These EPs are 68 degrees so keep the planet in the field of view longer. They only cost 80-100 bucks second hand as well. Keep your eyes on the classifieds, I've sold two in the past 6 months.

[brian nordstrom]

:) I would stick with the 6 inch f/8 as this size with a 6-7mm eyepiece will give the best views the atmosphere will support ( except the few perfect nights ) .
Use the 6 inch and enyoy what it will show you , and thats years of viewing the night sky .
And in the future if you want to stay sky watching and ' Aperture Fever ' hits go for a 10-12 inch Dob and keep the 6 inch for an extra smaller scope .
Use what you have and enjoy .
I to , as AstroTraveller says have the orthoscopics 5mm -25mm and they are the best in a longer f/l scope like yours , but the Hyperion are very good as well .
I would stay at 200-250x max , thats the sweet spot in most scopes , as I say that the atmosphere will allow .
Learn , take your time , enjoy and most of all :welcome:.
One more thing , do you have a RA ( right ascension , sometimes called a 'single axis drive' ) drive on the mount ? if not this would be a good investement for around the $100 mark , and easy to fit .
Brian.

[CR500]

I have read some good reviews on the 6mm TMB Planetary Type 2 eye piece. Where are the better places to get these in OZ?

[Satchmo]

This is an optomistic rule for a faster achromat. I've owned two of those Chinese 6" Achros and found the image breakdown due to Chromatic Aberration unacceptable above 100X ( 12mm EP ) for deep sky observing . Star look soft with a coloured halo. Using a minus violet filter extended its usefulness a little on the Moon and Saturn to go a little higher. The 4" aperture stop built into the cap was useful to go higher without the yellow filter.

[CR500]

That doesnt sound too good Satchmo. I will see what it is like when the scope body arrives today. I didnt realise it had a 4" stop built into the appature. If the telescope is junk, I might look at keeping the equatorial mount and upgrade the tube.

[ausastronomer]

Hi Rod,

I have to agree with Mark (Satchmo). Your chances of getting that 6"/F8 achromat to 300X with good image quality are "zero". The physics of the objective lens design simply does not allow it.

The first thing I would be getting is a minus violet filter. This will help enormously at minimising chromatic aberration. It doesn't fix it entirely. Even with a minus violet filter in place that scope is going to run out of choof at about 180X to 200X , which is about 30X to 35X per inch of aperture. A good quality telescope of different design, ie newtonian or an apochromat, will push at least 50X per inch of aperture under good conditions. It's not that these 6" achromats are junk, you're just asking it to do something it was never designed to do, or going to be capable of doing. They are excellent at lower to medium power views of clusters and the like.

I think if you get the 6" achromat, try to limit yourself to a 7mm or 8mm eyepiece as your highest power eyepiece. If you get a minus violet filter the 7mm should work fine for 171X. If you don't get a minus violet filter, go with an 8mm at maximum, for an upper limit of 150X . Above this the image quality just isnt going to be good enough for mine.

Cheers,
John B

[mental4astro]

Hey, doesn't anyone READ what is written! Did I say you'd routinely be able to use 300X!!

READ! I'm sick of this lack of concentration.

[CR500]

OK, so it seems that just because its a advertised as a Newtonian Reflector, doesnt mean it will do what the serious scopes will. I will get one of these filters you mentioned, any recommendations where from? I just bought a TMB planetary 6mm eye peice last night, looks like I will be using it on my next scope?

Instead of getting a new 6", I will likely keep it and get a bigger Dob.

[ausastronomer]

Alex,

I read exactly what was written, as I am sure Mark did also.



That statement you made is not correct. The maximum magnification of that scope based on it's optical design parameters is no where near 50x per inch of aperture and 300X is way beyond it's upper limit. It's upper limit will be in the range of 150X to 200X. Even under perfect seeing the scope will not be capable of supporting 50X per inch of aperture.



With this telescope, the telescope will be the limiting factor on more occasions than the atmospheric conditions.

Why would you even mention what magnification a 4mm eyepiece will give him? The telescope will never be capable of supporting it, irrespective of atmospheric conditions

Cheers,
John B

[Varangian]

I would be buying it straight from a U.S supplier like astronomics where you know you're getting the real product. There are so many copied examples of this eyepiece and you really can't be sure you're getting the same quality (maybe you are, maybe you aren't). At least through an authorised U.S retailer you're getting an authorised TMB product. Two differences to tell the fakes from the real deal are the real deal has mm on the barrel and the fakes have MM and the real deals have a gloss black barrel while the fakes have a matte barrel.

[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.

[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.

[Varangian]

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]

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]

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]

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.




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]

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]

:) , 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]

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]

:D 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]

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]

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

[ausastronomer]

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]

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