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Category: Reaching F/30
Reaching F/30 with a Refractor
Posted on Apr 23, 2011, 12:25 am Author: Jen Winter

Reaching F/30 on your Refractor is actually quite simple

Telling some customers that reaching F/30 is as easy as third grade math may sound simple to some, but it can strike fear in the hearts of others.    It's ok.  We let you cheat and use a calculator.

F/30 is just a ratio.  It doesn't bite.   
It is only a ratio of the relationship between the aperture of your telescope objective to the distance at which it focuses. 
We all know F/5 scopes and F/6 scopes have "fast glass" and have their own issues, so F/ratio isn't ancient mysticism.
Also, we don't need to get EXACTLY to F/30.   In general, F/27 to F/33 are all good, viable options.

Most modern refractors sold today are typically F/5 to F/9.   This is a far cry from the F/30 light we need with a DayStar Filter.  But changing a telescope to a different ratio is actually quite easy.  What we want is a nice, parallel light cone reaching the filter. 

We can either use a barlow (or negative lens) to straiten out the angles of the light just before the DayStar or we can reduce the aperture of the telescope and block light that is at a hard angle by aperture reduction.

Here is where our 3rd grade math comes in: 

Focal ratio changes are strait multiplication.
A 2X barlow doubles the focal ratio.
A 4X barlow quadruples it.
so an F/8 telescope with a 4x becomse F/32.

Likewise, with aperture reduction: 
Reducing a 150mm aperture to 75mm is dividing by 2
This doubles the focal ratio.

Sometimes people with very large apertures of 150mm or greater just reduce the aperture down by using an aperture mask and yield a nice smooth F/30 beam which focuses in the exact same spot it did before. 

The math again, is very simple. 
to find your barlow needed, divide F/30 by your telescope's F/ratio.
Tele Vue Powermate barlow lenses come in 2X, 2.5X, 4X and 5X.

We recommend a Tele Vue Powermate Barlow above any other barlow lens.   This is because the 2X, 2.5X and 4X are truly telecentric.   That means the light bends eactly strait and doesn't budge from strait and parallel.  This is really very important with a DayStar.   Light that hits the filter at an angle will shift in wavelength.  If the barlow is sending some light strait, but some at an angle, your image will have vignetting and areas that are darker, lighter and on/off band for Halpha.    This is a pretty severe effect that can be easily avoided with the right barlow lens.

Some observers get creative.  Some observers stop down the aperture AND use a powermate.  Perhaps  you own a 150mm F/8 and already own a 2X powermate.   If it is stopped down to 75mm the 2X powermate is added, then one has a nice combination without buying a new powermate.

The position of the barlow is, however, something you should pay attention to.  The farther UP from focus you insert the barlow, the farther BACK it pushes the resulting focus (multiplied by the X number).  So if you have a 4X powermate that you insert 1 inch before focus normally would happen, then your new point of focus is now 4 inches behind the powermate.   Not so terrible, but a 2" diagonal can occupy almost 4".  That means adding 16" behind the 4X.  
So we always rec ommend (if using a diagonal) to be sure that powermate gets inserted after the diagonal!

Tele Vue makes a great accessory called the PMT adapter with a Male T-thread end.  With this adapter, you unscrew the back barrel section of your Powermate.  You thread on this short 1/2" long PMT ring instead - and it now threads onto any T-Thread female device... namely the front of our DayStar Quantum and ION series filters.  

Tagged Keywords: Observing, Solar, telecentric, 2X, DayStar, Powermate, ERF, Aperture, Reduction, Stop down, mask, barlow, 4X

Next »  Why do I need F/30?

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