90 MilliMeters

Posts Tagged ‘telescopes’

Ursa Major holds a few deep sky objects for observers with small telescopes that are visible even under moderately light polluted skies. Bode’s Nebulae, better known as M81 and M82, are two such objects and both are readily visible in a low power eyepiece. Messier 81 was first discovered by Johann Elert Bode in 1774. Consequently, the pair of galaxies (M81 & M82) is sometimes referred to as “Bode’s Nebulae”. In 1779, Pierre Méchain and Charles Messier re-identified Bode’s object, which was subsequently listed in the Messier Catalogue. M81 & M82 are located approximately 10° northwest of Alpha Ursae Majoris making this an easy target to locate. You can download a Telrad finder chart here or a more detailed finder chart here.

As with any telescope at low power, the 90mm at 40x shows these two galaxies at either edge of the field of view. M81 looks like an oval of defused light radiating out from the bright center while M82 appears as a thin, elongated, evenly distributed silver of light. Increasing the power to 71x improves the contrast and I can detect a dark dust lane in M81 about half way from the center to the faint outer edge. Stepping up to the 150mm at 54x the spiral nature of M81 becomes evident. Since this object can handle higher magnifications, I dropped in my 20mm eyepiece coupled with a 2x Barlow for 76x. Now I can see greater detail in the spiral arms of this grand galaxy. Amazing how just a small amount of magnification can improve the view.

In the 150mm at 38x, M82 is an elongated glow that is brighter toward the western edge. I bumped the power up to 76x for a better view but was unable to detect any detail. The light is more evenly distributed across the surface at this power and the galaxy fills the eyepiece.

If you observe from an urban location where light pollution is an issue, such as here at the 90mm observatory, these objects will benefit from using a narrow-band filter. Narrow-band filters pass the critical hydrogen-beta and ionized oxygen wavelengths, while blocking out the remainder of the visual spectrum. They also increase the contrast of emission and planetary nebulae, revealing detail that is often washed out by background sky glow.

When observing M81 & M82, as with any deep sky object, try using different eyepieces and eyepiece/Barlow combinations to see which gives the best view. Take time to study the object for a few minutes, you’ll be surprised at the amount of detail you’ll start to see. Don’t forget about averted vision. A good amount of detail can be seen with this observing technique.

Until next time, clear skies.
-Dan

[Disclosure]

The Celestron Omni XLT 150R arrived Wednseday, November 18th. Of course I didn’t get to take it out of the box until Friday and start to putting it together. After a quick read through the manual, I unpacked the mount and started assembling. The process went very smooth. The mount, Celestron’s CG-4, which is a EQ mount, sits atop of a heavy-duty stainless steel tripod with 1¾” legs. Once I had the unit assembled, I attached the optical tube. The dovetail mounting system is a refreshing change from some of the older OTA attaching methods I’ve used in the past. This package came with a couple of features I didn’t expect like the azimuth adjustment and provisions for adding a polar alignment scope.

Time to balance the telescope. Balancing the mount in RA was a straight forward process. Celestron includes two counter weights (7lbs. and 4lbs.) with the CG-4 mount that make RA balance easy. Balancing in DEC is quite another matter. In the manual, Celestron states, “The Omni XLT150R is the most difficult to balance depending on the latitude, the accessories used, and which part of the sky the telescope is pointing to. Balance this telescope as best you can.” That’s a heads up. Since the telescope is a 6″ rich field (focal length =  750 mm), it goes without saying the the OTA is going to be heavy toward the front. Balancing in DEC is a little time consuming and at the time I’m writing this I still haven’t achived good balance in DEC. It’s my guess that it can’t really be achived. Like Celestron says, “balance this telescope as best you can”.

If the skies hold out for tonight, I take it out for final adjustments and “First Light”. Stay tuned.

Clear Skies,

- Dan

Disclosure

Messier 52, an open cluster located in a rich Milky Way field in Cassiopeia.

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