Whats the best lens size for planets? 12"dob

[12inchDoB]

Hi,

im getting my mirrors redone soon and im wanting to purchase some more lenses to view planets & nebulas..

i have 25mm & 7.5mm plossl sirius lenses.

is it worth purchasing a 2X barlow?

what size lens should i get for optimal closest viewing with my 12" Dob

thanks

Stu

[John]

Assuming that your 12" dob is F/5 (1500mm focal length) then I'd say your 7.5mm, which gives 200x, would be very useful. Occasionally you may be able to use more power so a 6mm might be useful then (250x) or even a 5mm for 300x although that might not get used too much.

Other eyepieces you might like to consider would be a 12.5mm (120x) and a 32mm 2" eyepiece to give you low power, wide field viewing for deep sky objects.

[12inchDoB]

Thanks John.

How do i work out my focal length?

I see pictures of planets with other similar 12" scopes and the planets seem a lot bigger/closer..

I have hunted my house/garage for my UV filters and i know i had a 5 & 12.5..

ill have to look on ebay for a cheap/decent 5mm/6mm & 12.5mm, but 3X barlow is a no no ?

thanks alot for your help

Stuart

[michael.h.f.wilkinson]

Images of planets are usually taken at F/20 to F/30 to match the resolution of the scope with the physical pixel size of the CCD. Therefore, a 4x to 5x barlow would be suitable for imaging. For planetary viewing I tend to use 200x to 250x with my 8". Very rarely do I go beyond that, although a 12" could be pushed beyond in good conditions. A 5mm or at a stretch 4 mm would be a good maximum magnification EP. At the cheap end, the "planetary" eyepieces of various brands (TMB and clones) are very good value for money. More expensive, but better are the Vixen NLVs. If you can live with the poor eye relief (I cannot) Plossls are fine and Orthoscopics are excellent.

[12inchDoB]

The distance from my 2 mirrors is exactly 4 foot or 48 inchs or 1219mm

Is it the distance from my eye to main mirror? or mirror to mirror?

but then how do i work out my Focal? if distance between the 2 mirrors is 48" and my main mirror is 12 inch in diametre?

Sorry for being a noob lol but not yet got my head around it..

i need to see drawings or formulae lol

trying to work out if i have F3,4, 4.5 or 5 etc

thanks

[John]

The focal length is the distance from the front surface (centre) of your main mirror to the point were light comes to focus - which is where your focuser is. If its 48" beween the mirrors then you will need to add on, say, 8" to allow the light cone to exit the side of the tube so thats a focal length of 56" - so somewhere around F/4.7 as a focal ratio (FR = focal length divided by primary diameter).

F/4.5 - F/5 is a very common focal ratio for newtonians.

Best convert it to mm to do the magnification calculations as eyepiece focal lengths are specified in metric these days !

[12inchDoB]

thanks John!

I feel so stupid! just got my tape measure out and although my tube is 12", my main mirror is actually 11inch !! DOHhhhhhhh

which takes it bang on F5 1400mm focal / 279mm mirror = 5.01

Good news is the recoat price has dropped slightly but would have preferred a 12" as my name suggests LOL

[michael.h.f.wilkinson]

Do you not have the specification of the scope somewhere? This should list either the aperture and focal length, or aperture and focal ratio.

If you have the focal ratio it is easy to get the EP focal length in mm that gives the "optimal" magnification (which is equal to the diameter of the objective in mm, 305 in your case): it is equal to the focal ratio. Your F/4.7 should be usable with a 4.7mm to achieve this optimal magnification. On my F/10 I regularly use a 10mm Radian. Theoretically, double the optimal magnification is the maximum for the scope, but 600x is not something to try under our skies. On my scope (203mm aperture), I have pushed it to 288x in good conditions using a 7mm LV, or more recently a 14mm UWA with 2x Barlow.

[michael.h.f.wilkinson]

thanks John!

I feel so stupid! just got my tape measure out and although my tube is 12", my main mirror is actually 11inch !! DOHhhhhhhh

which takes it bang on F5 1400mm focal / 279mm mirror = 5.01

Good news is the recoat price has dropped slightly but would have preferred a 12" as my name suggests LOL

I see you have solved it as I was typing my post. A 5mm would be a good option then, or perhaps a 4 mm for good skies.

[12inchDoB]

so although a 5mm lens is the ideal size (focal length ÷ aperture), i could get a similar but not so good result with a (9mm with 2X barlow)?

This is where it throws me,, 20mm lens using 4 X barlow (20 ÷ 4) isnt as good as a 5mm..

I dont know whether to get a 5mm lens & 2,3,4XBarlow this month then next month recoat my mirrors. or visa versa.. cant afford both lol

[12inchDoB]

What do you think

5mm 1.25 PLOSSL HD for £37

or i can get a 4 or 4.5mm TMB Planetary II for £50

Vixens look lovely but out my range

[Trident]

Hi 12inchDob

I have very recently got a skywatcher 300p flextube that i`m getting used to. the other night i took my first look at saturn with the 2x economy barlow from skywatcher and the 10mm super plossl that the scope came with. this works out at 300x being 1500/5. the combination is comfortable to use compared to smaller plossl EP and very cheap to buy, 10mm i have seen for £18 and barlow is £12.

The Image was very bright indeed with clearly defined seperation of the rings to the planet and resonably large (wife said "it looks like you`ve stuck a photo on the end of the scope ". Overall the image was sharp bright and large however..

there wasn`t much detail on the rings themselves or the planet, everything was so bright it seem detail was washed out (almost over exposed). I`m not sure if this is really down to EP`s setup, scope or what?

I`m currently thinking of a 4 or 5mm TMB EP at around £45 ish as i think they will increase contrast.

In my opinion you would do well to get a barlow to go along with any other EP`s you get. Small Plossl EP`s i have read as being a "miserable experience" to use, i think due to the minimum dia in the EP.

From what i understand the eco barlow i have isn`t great however the TAL one skywatcher do is supposed to be good VFM at £30ish

hopes this helps

By the way interesting photos, it looks like the flocking material has all come off?

was thinking of adding some to my secondary as it apparently makes loads of improvement for planetary views improving contrast.

[12inchDoB]

Hi Trident..

Yea the coated layer is flaking off, the black bits are just where there is plain glass and reflects back as black.. I will get that sorted in the coming weeks..

I only have a 25mm PLOSSL Sirius and what i now know is a 6.5mm PLOSSL as the numbers had rubbed off and i just measured it..

I want to get some pics of planets and trying to work out the best way to do it,

not sure whether to get a 2 or 3 or 4 Barlow but i dont mind paying £30.

i think that 4.5mm TMB (£50) is where im heading which would give me around 311 X magnif (1400 ÷ 4.5).. but then it would be pointless using a barlow on that as i think 300X mag is max without losing too much. but with a 2Xbarlow it would go to 600..

Could you send me a link of the BArlow you are going for? There are so many makes/models to choose from im not sure which to get for around £30

Thanks

Stu

[michael.h.f.wilkinson]

A barlow is a cheap way of multiplying the effective number of eyepieces you have. However, good barlows, which do not degrade the image quality too much, are expensive. if you add a 2x barlow to your 25 and 7.5 mm, you effectively add a 12.5 and 3.75 mm to your collection. The latter is a bit too much magnification in most cases (370x approximately). However, a 1.5x barlow gives you 16.7 and 5mm EPs effectively, which is fine. This will not deliver the same quality as a specialist planetary EP, but if you buy a decent one, it should be a good investment.

This one looks very practical:

TS Barlow 2fach - 1,25' - achromatisch - kurze Bauweise

But I do not know how good it is.

Regarding the EP options: the 4.5 planetary has the edge over the Plossl in two ways: (i) it has a wider field of view, which is handy in a Dob, because you do not need to nudge it, and (ii) it has higher eye relief, which is more comfortable. Whether the latter is of importance to you is a personal matter, but I much prefer a long eye relief (I wear glasses).

[12inchDoB]

Thanks Michael,

I could get that one shipped over as im not sure of where to locate it in the UK?

But What about the adaptable ones? unscrew a section to go from X2 to X1.5? is this ANTARES a good brand/model?

Antares Antares Deluxe 1.25" 2x (and 1.5x) Barlow lens

Stu

*EDIT*

I just veiewed your link in English and i see it does the same thing 2X to 1.5X by unscrewing a section.. Also somehow possible to connect with camera T ring

choices choices lol

TMB 4mm or 4.5mm and or above the TSB21 barlow..

[michael.h.f.wilkinson]

Thanks Michael,

I could get that one shipped over as im not sure of where to locate it in the UK?

But What about the adaptable ones? unscrew a section to go from X2 to X1.5? is this ANTARES a good brand/model?

Antares Antares Deluxe 1.25" 2x (and 1.5x) Barlow lens

Stu

*EDIT*

I just veiewed yoru link in English and see it does the same thing 2X to 1.5X by unscrewing a section..

It may be the same piece of kit with a different brand stamped on it. I do not know how good these are, but they may be useful.

[Trident]

looks like FLO do them but need to give them a shout for price, as a SGL member you get 5% off but need to email them before placing the order.

Tal - Tal 2x and 3x Barlows

this is the same barlow

http://www.365astronomy.com/skywatcher-x2-“tal”-achromatic-barlow-lens-125”317mm-p-2442.html?zenid=3a622fcc4d7028a776de2d0e315a2c7a

by the way if your looking at images ("I want to get some pics of planets and trying to work out the best way to do it),

i`d look at a philips webcam for planets or a SDC-435 (SBC-2000 is the new badge model) for deep sky. Loads of free software to make it work.

[12inchDoB]

Im going for the 4 or 4.5mm TMB Planetary II with a half decent 2X convert 1.5X Barlow.

Imagery I have a nice 12.1MP digital camera but with the zoom it doesnt fit a T-ring.. i might try a camera bracket bolted to the scope to hold it.

[12inchDoB]

Would u say get a 9mm TMB 2 and a good 2 X barlow?

or a 4.5mm TMB 2 planetary how would they differ in quality?

PLus i want a 2X convert to 1½X Barlow..

[12inchDoB]

Sorry to repost,,

But I'm collecting my lense tomorrow,

Still not sure what to get and ask your advice..

Im ordering this 2X&1.5X Barlow (thanks michael).

TS Barlow Lens 2x - 1.25' - achromatic - short

Would you order a 4.5mm for use on its own OR the 9mm TMB Planetery 2 to use with & without the barlow?

i only currently have 25mm & 6.5mm plossl serius lenses..

:rolleyes:

[michael.h.f.wilkinson]

The 9mm would give you 4.5 as well, and as you have a barlow (or 2), that will give you a nice range of magnifications. In principle, a separate 4.5 will give better quality, but I really do not know how big the difference will be. Newer design barlows give a minimal difference in quality, some produce quite horrible effects (mainly the old-fashioned single lens affairs). The only way to be sure is to test them.

[12inchDoB]

yea i dont mind testing but by the time ive paid shipping to & back, £26! i may aswell save a bit more to get a very nice one though limited on barlows that i can convert to 2 different sizes...

Eventually id like to have a decent collection built up for the kids "1.5X,2X,3X,4X" Barlow

The one you suggested earlier is what il try for now...

TS Barlow Lens 2x - 1.25' - achromatic - short