Category: Accessories

Something Old for Something New

3/21/2018

Sometimes I like to find a way to use old things in new ways. Awhile back when I bought a Celestron 8se, I also purchased a nice foam carrying case for the combined mount and telescope. As I was trying to figure out how I might transport my new Evolution mount with my Celestron 8” telescope, I remembered the 8se foam carrying case and wondered if I could adapt it for a new use.

So I got the case out and laid the new Evolution mount with 8” telescope attached into the foam case. On each side of the old 8se carrying case I used a black felt pen to outline what I would have to cut out for it the new mount to fit.

Then I got my trusty Kobalt cutting knife out and began cutting out small segments of foam from the right side that I could then remove by hand. I was careful to try to “stay within the lines”, and the right side looked OK when done.

I then proceeded to tackle the left side. When finished I placed the Evolution mount with the 8” telescope attached into the left side and it was a nice snug fit!

I was then able to close it and zip it up, ready for carrying wherever I might want to go!

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Bahtinov Focus Mask tips

3/15/2018

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I highly recommend using a Bahtinov Focus mask to assist focusing your video astronomy setup. It is a very inexpensive way to take the guesswork out of focusing.

Slew to a fairly bright star and get it fairly close to focus. Then place the Bahtinov Focus mask on your telescope to produce interference lines that aid in focusing. Continue to adjust the focus until the center line is evenly spaced between the other two lines.

I have discovered that your exposure time or gain is important to getting the best possible focus. You can start with a fairly bright image to center the middle line.

To get an even better focus, reduce the gain or exposure time to where you just see the lines. You may find that you were not as close to focus as you thought. Here is what the above image changed to after reducing the gain, showing that the focus can still be improved slightly

After a little more adjusting, you will see a very nicely focused image.

Now remove the Bahtinov Focus mask and you are ready for viewing.

You can later quickly recheck the focus during the evening by slewing to a nearby bright star. Place the Bahtinov Focus mask on your telescope again to see if any slight focus adjustments are needed.

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Measuring Focal Reduction

11/28/2017

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The actual amount of reduction provided by a Focal Reducer can vary depending upon your specific setup (telescope, placement of focal reducer, sensor depth, etc.). To determine the actual amount of reduction for my equipment, I capture an image with no reducer and another image with the reducer in place, and compare the change in field of view to determine the actual amount of reduction (and thus effective f/value).

Here are my latest results with a Celestron 8" f/10 SCT, my Mallincam DS2.3+ video astronomy camera using MFR-8 and Celestron f/6.3 reducers.

In the past I would capture images, invert the colors to get black stars on a white background and print them. I would then pick two stars that appear in each of the images and use a digital caliper to measure the distance between them on the paper. Dividing the distance between the two stars when using a reducer by the larger distance measured on the image between the same two stars with no reducer gave me the amount of reduction.

Based on suggestions from others, I have started using nova.astronomy.net/upload to simplify this process and improve the accuracy of my process (Google "Nova Astronomy.net upload" to get the link). By clicking on the Browse button you can select one of your captured images and click the Upload button to process it. It will upload and process the file, and after a couple of minutes display "Success" once it has finished processing. You can click on "Go to results page" to view the results. On the right side of the screen under "Calibration", it will show the Size (width x height) in arcmin of your camera's image FOV (and a Radius in degrees). The width & height Size values vary proportionally with the amount of reduction used. I typically use the first FOV Size number (width) for my calculations.

Since the reduced image has a wider Field of View (FOV), to get the amount of reduction you divide the FOV with no reduction by the FOV of the reduced image: FOV Width of Image with no Reduction / FOV Width of Reduced Image = Amount of Reduction. I then compute the effective f value by multiplying the original f/10 value of my telescope by the amount of reduction. Let's say the computed amount of reduction is 0.8, then for my f/10 telescope my setup becomes f/10 x 0.8 = f/8.

I used M103 as my target since it has a good recognizable star field and it is easy to visually see the reduction. Below are the results of various focal reduction combinations with the camera in line with the telescope (no Diagonal in place).

1. C8 SCT with DS2.3+ (camera attached directly to back of telescope with no focal reduction)

FOV Size: 20 x 12.5 arcmin (#1 Width = 20)
f/10 (no reduction)

2. C8 SCT with DS2.3+ and MFR-8 (reducer attached to the DS2.3+ with no spacers)

   FOV Size: 30.4 x 19 arcmin
Reduction = 20/30.4 = 0.66   (#1 Width/#2 Width)
f/10 x 0.66 = f/6.6

3. C8 SCT with DS2.3+ and Celestron f/6.3 Reducer (reducer attached to the back of the telescope)

FOV Size: 25.9 x 16.2 arcmin
Reduction = 20/25.9 = 0.77 (#1 Width/#3 Width)
f/10 x 0.77 = f/7.7

4. C8 SCT with DS2.3+ and MFR-8 (attached to camera) plus Celestron f/6.3 (on the back of the telescope)

    FOV Size: 43.2 x 27 arcmin
Reduction = 20/43.2 = 0.46   (#1 Width/#4 Width)
   f/10 x 0.46 = f/4.6

I have found that this method for accurately computing the amount of focal reduction works well. Computing the precise f/ratio is another matter.

The computed f values vary from what you might have expected. The actual reduction can vary depending upon the setup (e.g. the distance between the focal reducer and the sensor). In #3, the Celestron f/6.3 Reducer produced f/7.7. If I repeat the same tests with a Diagonal between the telescope and the camera, the f values will be different.

Finding the exact f/ratio may not be that critical, and in general an estimate is good enough. It is a indication of how fast your setup is since the lower the f/ratio, the brighter the image and thus less exposure time is needed (If you halve your f/ratio with a reducer you will go from a f/10 system to f/5, and your images can update in 1/4 the time). Through my testing I did find that using a Diagonal changes my f/10 system to about f/11.6 (the FOV width went from 20 arcmin to 17.2 arcmin when inserting a Diagonal into the light path between the telescope and the DS2.3+ camera).

5. C8 SCT with a Diagonal and DS2.3+ (and no focal reducer)

FOV Size: 17.2 x 10.8 arcmin
Change = 20/17.2 = 1.16 (#1 Width/#5 Width)

So, if I want to get the max amount of reduction for the brightest image, I learned it is better to use the Camera in line with the telescope (as close to the telescope as possible) rather than on a Diagonal if you can. The image is slightly larger when a diagonal is used. It is not a major difference though, and you may need to use a diagonal if you have an Alt-Az mount and you are trying to avoid hitting the mount.

Generally speaking, if you move the Camera further away from the telescope the image will get bigger, the FOV smaller and the image not quite as bright. When you compare using the DS2.3+ with the MFR-8 without any spacers to using it with spacers, you will see more reduction, larger FOV, and a brighter image. This confused me a little at first since I was thinking the Camera was moving further back in the light path when adding spacers. Then I realized that adding spacers just moves the reducer forward in the light path closer to the telescope - the Camera distance from the telescope does not change. The primary reason there is more reduction is the increased distance between the reducer and the sensor produces a greater amount of reduction.

When I began these tests, it was really more from an interest in quantifying the amount of reduction and the effect on the Field of View in order to have accurate FOV rectangles in SkySafari. Having an engineering background, I'm a numbers kind of guy. It bugs me if I put in parameters in SkySafari and the FOV rectangles don't match what I actually see through the telescope. Now that I know precisely what the reduction ratio is for different setups, I can better use the SkySafari FOV rectangles for various combinations of telescopes, reducers and cameras.

I did find that using both the MFR-8 and the Celestron f/6.3 reducers together begin showing some vignetting effects (darkening around the edges of the image). To get even more reduction without vignetting, I have ordered a HyperStar reducer for my C8 to convert it from f/10 to about f/2.1 (more on this later).

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Using USB3 Extension Cables to be Cool

8/6/2017

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I have found two very good reasons for using Remote Video Astronomy (RVA) for capturing images of the Sun in the summer:

1. It is hot outside and that makes humans uncomfortable
2. It is hot outside and that makes laptops really uncomfortable

As I was testing my setup for the upcoming eclipse, I found my laptop would start to overheat if it was outside next to my telescope. The fact that the laptop is black didn’t help any. I tried putting it under a covering and aiming a fan at it, but it still would overheat and declare it was shutting off. This is the last thing I needed to occur at high noon (i.e. 1pm Daylight Savings time) on the day of the eclipse when totality would start 12 minutes later.

So I have opted for the RVA solution I have used many times before – using active USB extension cables to run inside to my laptop so it can stay cool and record the eclipse. I use SkyFi at the telescope and SkySafari on my iPhone to control the mount from inside.

I can go inside to cool down my body, and then go outside again to visually look and see how the eclipse is going (wearing certified eclipse glasses of course – never ever look at the Sun directly without proper certified protection - do not use sunglasses).

I am going to be using a MallinCam DS2.3+ for the eclipse, which uses USB3. My laptop has a USB3 port that I can plug the MallinCam USB3 cable into. The following 10M active USB3 extension cable has worked well for me when I need more cable length. It is currently available on Amazon for about $35 and a compatible power adapter for about $7.50.

Due to the higher speed and specs of USB3, these cables have a power connector on the female end of the cable. I plug its power adapter in at the telescope and insert the DS2.3+ USB3 cable into the end of the active USB3 extension cable. I plug the other end of the USB3 extension cable directly into my laptop inside.

If more distance is needed, I use a second USB3 extension cable to the laptop. If you use two cables, you may only need to plug power into the cable at the telescope end. Since the DS2.3+ draws its power from the USB3 cable, powering that end of the cable gives plenty of voltage to run the DS2.3+. If you do have communications issues when using two cables, you probably need to supply power to the end of the other cable as well where the two USB3 extension cables are connected.

This RVA solution has worked well for me and my laptop. Keep Cool!

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StarSense AutoAlign Target Centering

10/28/2016

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The Celestron StarSense AutoAlign (SSAA) accessory simplifies the alignment process and enables you to start your viewing quicker. This accessory was the subject of my first blog (see 11/16/2014). It has a feature that is particularly beneficial to Video Astronomy since the ability to center your target on the camera chip is very important.

Note that there is a difference between GoTo accuracy and tracking accuracy. For an equatorial mount, the better the mount is polar aligned toward the celestial pole, the better it will track. But that is a different SSAA topic. The better the star alignment, the better your GoTo’s will center your target. That’s what this topic is about.

The standard AutoAlign process captures images of 4 alignment areas of the sky and uses up to 100 stars in each area for its alignment calculations. If you slew to an object and find it is not centered very well, you can add a calibration reference for that region of the sky and then re-slew to the object. I sometimes find this is particularly helpful when your target object is in a sky region quite different from any of the prior image reference areas. Once you add a calibration reference, objects in that sky region will now be better centered after a GoTo selection. The StarSense can use up to 10 calibration reference areas, so I mainly use this feature when I have not added a calibration reference in a particular area of the sky.

Here is an example of an object to the South that is not near any of my initial alignment areas. After the initial GoTo slew finished, the target was near the top of my field of view.

I then used the StarSense Hand Control to initiate adding a reference calibration which did not take very long, and then re-selected the same target. After the GoTo slew finished, the target was now near the center of my field of view.

The only difference in these two images is the addition of a reference calibration. I did not make any manual centering adjustments.

The SSAA manual has a section that describes how to add a calibration reference. Here is my mark up of that section if you have already slewed to a target and simply want to better center it by adding a calibration reference of its region of the sky without making any manual centering adjustments. Note that there is no need to try to manually center a target before adding the calibration point.

For clarity, here are my own set of steps for adding a calibration reference to better center a target after you have initially slewed to it.

1.       Press the Align button
2.       Scroll up/down until you see Add Cal Reference
3.       Press Enter
4.       Press Enter again. (There is no need to perform step 2 as described in the manual. You are already pointed at the sky area of interest. And if you have not performed any manual position adjustments there is no need to press the Up and Right direction buttons.)
5.       You will then see messages on the LCD screen as the SSAA acquires an image of that area of the sky, finds up to 100 stars in the image, computes a solution of what it is seeing and adds it as an additional calibration reference.
6.       When finished, press Back and use the hand controller to re-select your target and initiate a second GoTo.

When the new slew finishes, the target should now be better centered in your field of view.

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Something Old for Something New

Bahtinov Focus Mask tips

Measuring Focal Reduction

Using USB3 Extension Cables to be Cool

StarSense AutoAlign Target Centering

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