Sunday, April 23, 2017

Meade Focal Reducer Tests

I purchased and just received a Meade f/6.3 focal reducer/field flattener for my Meade LX200GPS 8" telescope.

The purpose of this optic (which is mounted between the telescope and the camera/eyepiece) is to provide a wider field of view which is much brighter than without. The telescope is build at f/10, which is quite dim, but that is made up in the fact that it gathers a lot of light because the mirror collector is 8 inched in diameter.
However, for astrophotography purposes, f/10 makes for longer exposures compared to a f/6.3 optic setup... in other words, the time of exposure is reduce by almost 40%. For example, if (without the focal reducer) it took 10 minutes to create an acceptable image of a galaxy, then with the focal reducer that time would be shortened to just over 6 minutes.
The disadvantage is that the field of view is much wider, thus making the object (in this case some galaxy) appear nearly 40% smaller than without the focal reducer. So it is a trade-off: either a short exposure (which is desirable) or a more magnified view.

In my tests, I took an image without the focal reducer. Here is the setup:
DSLR + T ring + camera adapter + telescope
And here is the "base line image":

Then I inserted the Meade Focal Reducer:
DSLR + T ring + camera adapter + Focal Reducer + Telescope 
And the resulting image:

Then I inserted the electric focuser for the telescope:
DSLR + T ring + camera adapter + electric focuser + focal reducer + telescope
And the result:

I was surprised to find that by adding more distance AFTER the focal reducer in this manner that the field of view increased and the exposure time was also shorter... and a bonus, it appears that there was less vignetting!

Again, here are the resulting images:
No focal reducer
Focal reducer with                      Focal reducer with
minimum distance                       maximum distance

Thursday, April 6, 2017

What celestial objects will be visible during the August 2017 Solar Eclipse?

Using a freeware star program called Stellarium, I was able to set the time and date to the Great American Solar Eclipse (August 21, 2017) and removing the "atmosphere" found out that there will be at least three planets visible (Mercury, Mars, and Venus) and as a bonus, the Orion constellation and the Orion Nebula should be visible as well !  Here is a screen capture (click on the link below the photo it to get a larger view)

Here is a close up of the area around the Sun at that time: (Note the bright star of Regulus is just to the left of the Sun/Moon)

Saturday, April 1, 2017

Comparison of the Celestron C5 telephoto and the Bushnell Northstar 90mm telescope

As I look forward to the solar eclipse in August, 2017 which passes through central Oregon I have been wondering how each scope's view will make the eclipse appear. 

Having just purchased an old school Celestron C5 (previous blog entry) 

and the Moon being visible last night, I mounted it and my Bushnell Northstar 90mm telescope

on tripods and using the same camera (Canon t3i) at prime focus I imaged the Moon for comparison shots. Here are the results:

                           Bushnell Northstar 90mm                             Celestron C5

Not only does the Celestron C5 have a wider view but the exposure difference was significant.

At ISO 100 the Bushnell required 1/5 sec exposure due to the fact that it is a f/14 lens
whereas the Celestron C5 only needed 1/20 sec because it is an f/6 lens.

That means that under low light the C5 will be easier to focus and can use a shorter exposure time than the Bushnell. However, the Bushnell gives a closer view of the subject, nearly twice the magnification of the C5.


As there is no such thing as a telescope that can be "all things to all people" it appears that, if possible, I will use both scopes during totality (if I have sufficient help from others).

The Bushnell might work well for a closer view of any prominences that may appear on the edges of the Sun during the eclipse and for imaging the Baily's Beads.

The red around the edges are prominences
and Baily's Beads (on the right hand side)
result from the Sun peeking through the
mountain peaks of the Moon during a solar eclipse.

The Celestron C5 will capture more of the surrounding corona due to a larger field of view. The fact that the corona is so dim will also play to the C5's ability to let more light come to the sensor in a shorter period of time.