Barlow vs. Televue Powermate

[mhminor]

Hi all,

In a previous post, Brent brought up a good point.  I use the SN10 to look at just about everything including planets and the moon.  Brent reminded me that because the SN10 is so fast, it really performs better when viewing and taking pictures of the deep sky.  He recomended using a barlow, which I do, a Zuhmell 2X.  I have been thinking of stepping up to a more powerfull barlow, (or better quality) or going to a Televue Powermate.

Given the optics of the SN10, which would be a better fit, and which would be better for astrophotography?

Many thanks!

Mike

[DSOMAN]

Hi Mike,

You are starting to get into an interesting area of your telescope optics and performance when you start using a Barlow. When you use a X2 Barlow you have turned your scope into an f/8, 2000mm FL scope.
If you use a X3 Barlow then the focal length becomes even longer and the speed decreases even more.

Planetary images will be bigger and also dimmer and depending on the eyepiece, more blurry.

This is fine for visual observation but unless you have excellent seeing conditions you probably won't see any advantage to using a Barlow beyond X2.

If it were me I would get some really good higher power 2" eyepieces rather than a Barlow but that is a matter of choice.

The SN10 can support a 4mm eyepiece [with good seeing] with no problem [250x]. Get too much past that [with a Barlow] and you start running into diffraction limitations of the scope.

Astrophotography is a whole different story. When you use a X2 Barlow you have doubled your FL thus the scope will show up minor tracking errors more noticeably. Your guiding system needs to be able to handle this additional magnification or you might see elongated stars that were not apparent with the shorter FL.

In my system [scope and camera] the system resolution is about 1.5 arc sec per pixel, even though the scope itself has a resolution of 0.45 arc seconds [for visual work ]. With a X2 Barlow inserted before the camera it would be 0.77 arc seconds per pixel.

Sounds like I just doubled my resolution, not quite. Seeing in my area on a good night be around 2-3 arc seconds per pixel. That means that atmospheric conditions will in effect blur my images by that amount. I am not even getting the 1.5 arc sec resolution of my system. Using a Barlow in this case would not provide any benefit.

If you are going to buy a new Barlow I would recommend a 2" X2 of the highest quality you can afford. The two you listed are both good. I would probably go with the TeleVue.

Hope this helps.

Maybe someone else can jump in here with some comments.

Brent

[mhminor]

Thanks Brent for the detailed information.  I have to admit, after your reply I had to run to my astronomy library to research some of your terms in order to "keep up".  I did a lot of SLR photography in the 1970's but my grasp on the science of optics is very limited and my weakest area when it comes to astronomy.

I know the optics for the SN10 is not designed for lunar/planetary viewing, and I have toyed with the idea of purchasing the LXD75 SCT to focus on the solar system but I am still in the process of retro-fiting the SN10 along the lines of your recomendations and I don't have the spare cash to specialize.

You had mentioned the best barlow/lens combination would be 2".  All my lenses are 1.25". I don't mind purchasing 2" lenses, but I noticed the two inch lense seems to be limited to the higher field of views.  Would this be suitable for planterary observations?

You also bring up a good point about seeing conditions.  I'm a high school special ed teacher so most of my observing is limited to Friday nights.  I haven't developed much a track record for seeing conditions and expermentation, but I wil preservere.

[DSOMAN]

Hi Mike,

You can do just fine with a 1.25" Barlow. I mentioned the 2" only because they would be the ultimate match for the SN10.

Check out this link:  http://www.telescope.com/control/telescope-eyepieces

You could purchase a TeleVue or other 12.0mm eyepiece and a X2 Barlow so you would have both a 12.0 & 6.0mm combo. This combo should do quite well on planetary objects with the SN10.

You will get a feel for seeing conditions after awhile. The object is to keep increasing magnification until you start to see some shimmer, after that additional magnification won't help any. For me, my eyepiece limit seems to be around 8 - 10 mm on a good night. I have easily resolved the Cassini Division on Saturn's rings with a 12mm eyepiece.

Brent

[mhminor]

Hi Brent,

Thanks for the tips on measuring seeing conditions as well as the barlow/lens recomendations.  I will try to get some 2" lens's as money permits.

Now if the winter storms would just limit themselves to come during week (we need the water) I might get more viewing in...

 Mike

[firemouth]

I use a 2.5X Televue Powermate. It is awesome for lunar and planetary. I need a very clear night for DSO to push my 20mm plossl or my TMB 8mm planetary.

[mhminor]

Thanks for sharing your experience with the Televue Powermate.  That looks like my next purchase.

Mike