Preparing my Setup

Following the good advice from this blog, I ordered the Mylar emergency blankets which arrived today. I had prepared some cardboard fixtures  but as it turned out they did not seem too steady.   I finally  decided to go ahead affix the two sheets of  Mylar  with some office tape and a rubber band to a  500mm mirror lens. Prior to this I focused on the moon last night and locked the lens at infinity (hope this works).

Besides this 500mm lens, I also prepared a filter for a 250mm lens, but I am not sure that that is the correct lens for this project. The sun would not look as big, imagine Venus!  The only reason why  I am considering this is because the lens glass (for the 250mm) is of better quality. I don’t know which of the lenses to use, though I am leaning towards the 500mm lens regardless of quality.

I am also concerned about the correctness of the focusing procedure I used, I may acquire a very blurry image if I acquire anything viewable at all.  Here is the second filter for the 250mm lens

The next available camera is a Nikon D70, but the highest zoom lens I have for it is only 200mm. Is it worth using? Maybe not a good idea, I would  probably be wasting my time with this setup.

Finally, the 7″ monitor I will most likely  use to  “monitor” the acquisition of images and the fine tuning of the focus.

The camera will be activated using one of these:

  •  intervalometer
  •  wireless remote control
  •  iPhone App to control  camera

PRE-FLIGHT

As part of the preparations I ran a test today, Jun 4th. My 500mm lens is a “soft lens” of the type called  “mirror lens”.  I did not spend much on it, just wanted to   photograph occasional birds and  egrets, so I do not have great expectations.

At best this  outcome can serve to depict what not to use in terms of  lenses.  Following is today’s test, that came out, as expected,  too soft. Imagine Venus. I used as adviced, two sheets of Mylar as described in the paragraphs above. I wanted to perform the test at the same time of the Venus transit tomorrow but  I forgot, so it was done 30 minutes past 6PM, the time at which the event  will happen at my location.

If possible, I would like to hear what changes can be done to setting to improve this , I am thinking, maybe move to ISO 10o? The f stop of lens is fixed at 6.3.

3 comments

  1. Hi Anangelu,
    The apparatus looks super-cool! I am feeling jealous now 🙂

    Regarding the Venus and Sun image sizes for your lens-camera combination:
    (note that these are only estimates for a “perfectly sharp” image, to give you an idea about how big the sizes are expected to be around!)

    Canon T2i (pixel pitch 4.17 microns) with 500 mm lens
    Pixels in Solar disk diameter = 1099 pixels
    Pixels in Venus diameter = 34 pixels.

    Canon T2i (pixel pitch 4.17 microns) with 250 mm lens
    Pixels in Solar disk diameter = 550 pixels
    Pixels in Venus diameter = 17 pixels.

    Nikon D70 (pixel pitch 8.4 microns) with 200 mm lens
    Pixels in Solar disk diameter = 218 pixels
    Pixels in Venus diameter = 7 pixels.

    See this post.

    Regarding the use of mirror lens:
    I think that it is perfectly all-right to use the mirror lens for this type of photography (provided it is properly focused) for the following reasons:
    1. Since there will not be any “out-of-focus” bright object sources (nothing else can compete when the Sun in in the frame), you will not see any of the out-of-focus donuts shaped bokeh.
    2. The atmospheric blur will act like the “equalizer” and you may not perceive much of a difference in sharpness between the images taken with you normal lens (refractive optics) and the mirror (catadioptric) lens. [Again, you will have to make sure that the focus is right on]

    Since focusing is the main concern, and I would assume that you will have to do manual focus through the mirror lens, I would recommend that you try to setup up your apparatus such that you can view (live) through the monitor while you focus manually.

    Regarding your shot of the sun today:
    I am not terribly disappointed with the outcome of your experiment with the 500mm lens. Also, I don’t think that it is “soft.” The problem is that it is overexposed. I guess that I forgot to mention in the recommendations (I shall add it now) that you try to set the exposure yourself. Since the sun will cover only a very small portion of the sensor (because of the focal length being used), a huge portion of the image will be “dark”, and if you leave the auto-exposure on, then it will overexpose the image (like this one). So, please set the exposure mode to manual and set your exposure using the histogram and/or by looking at few of trial photographs. Also, note that the exact exposure will depend on the time of the day and weather conditions. If you use the histogram, make sure that the second peak (corresponding to the sun’s bright image) of the bi-modal (two-peaked) histogram is about 80% to 90% towards the right, but still “inside” the histogram plot. If I have to take a wild guess at the proper exposure that would have been “right” for the above photograph, it would be ISO 100 at 1/3000 sec. [Remember, tomorrow the situation may be a little different, so please use manual exposure and practice chimping!]

    Well, all the best and lets hope for good weather tomorrow.

    Regards,
    Indranil.

  2. Anangelu · · Reply

    Thanks for the comments. I hope today i will correct the problems. Yes, I have to focus manually, which is the problem with this catadioptric lens; it is very difficult. I did set it up to manual mode, (selecting the aperture since the f stop is fixed )and ISO. I will run several tests at 5 pm prior to the event.

    I can see what the lens sees through the monitor attached, I don’t have to look at the camera scrmeen or viewfinder. But this lens is particularly difficult to focus, I have to admit that.

    So, based on the chart, is the 250mm a viable solution? Is it worth not using the catadioptric? I can focus better manually with the 250. BTW; I am watching from Florida.

    1. Well, based on the “perfect image” calculations you should get about 17 pixels of Venus (the projected image on to the detector in millimeters is really very small) if you use the 250 mm lens, whereas, you can expect to get about 34 pixels using the 500mm lens. Based on these facts I would tend towards using the 500 mm lens. I am going to be using a 300mm lens on my Canon RebelXT (that is because it is the max telephoto that I have) and I am expecting to get about 15 pixels of Venus. But remember, for me it is not a matter of choice. In your case, I think, if you are not comfortable with the 500 mm mirror lens, then, use the 250mm lens. Take some experimental shots during the afternoon and see how big a solar disk you can get. The disk of Venus will be 33 times smaller, so you can get an estimate. Between, I had already guessed that you are in probably in Florida, based on your timing (of the start of Venus transit :-)).

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