Testing the AP-27TVPH Focal Reducer

I just acquired a 0.75x Astro-Physics 27TVPH focal reducer for use with my TMB130ss refractor.  While waiting for some better skies, I did some initial tests to see if field curvature or vignetting effects are noticeable and am summarizing those results here.

Imaging Train

The imaging train for these tests is the following:

• TMB-130ss refractor,  focal length 910mm at f/7
• Feather Touch focuser option [Starlight Instruments, FT-3035]
• Replacement focuser end cap with AP 2.7" threads [Starlight Instruments, EC35-505AP]
• 0.75x Focal Reducer [Astro-Physics, 27TVPH] 
• Adapter from 2.7" threads to SCT threads, 8mm insertion length [Astro-Physics, ADA204]
• Adapter from SCT threads to T threads, 18mm insertion length [Antares]
• T-thread extension barrels, various lengths
• Baader T2 Quick-changer, 15mm insertion length [Alpine Astro, T2-6,7]
• QSI-540ws with T-thread mounting plate,  21.4mm diagonal

The camera I am using is the QSI-540ws which has a KAI-04022 sensor with 7.4um pixels.  The sensor is square with a 21.4mm diagonal.  Although all tests are based on that camera, the popular KAF-8300 sensor included in many of the new affordable CCD cameras has nearly the same diagonal size, 22.5mm, though not square.  I would expect nearly the same geometry and curvature results.

Geometry Results

For each T2 extension tube length, I measured the distance from the back edge of the AP-27TVPH focal reducer to the "seam" of the QSI-540ws camera body.  This seam between the blue and black parts is approximately 1mm in front of the CCD sensor according to QSI Imaging technical drawings.  This distance is reported in the table below as CCD (mm).

For each extension length, after bringing the image into focus with a Bhatinov mask, I noted the position on the measurement scale of the FT-3035 focuser.  This is reported in the table as Focus (mm).  You can see that the set of tests span most of the focusing range of the FT-3035.

To get an accurate measure of the reduction factor, I used the PinPoint plate solving feature in MaximDL to compute the image scale of each star field I imaged, reported as Scale ("/px), and the corresponding focal length.   The magnification factor, reported in the table as Mag (x),  is the ratio of the focal length computed with the focal reducer to that computed without the focal reducer.

CCD (mm)	Focus (mm)	Mag (x)	F-Ratio	Scale ("/px)
-	-	1.000	f/7.0	1.68
132	22	0.743	f/5.2	2.26
124	37	0.754	f/5.3	2.22
117	51	0.765	f/5.4	2.19
102	75	0.786	f/5.5	2.13
81	109	0.815	f/5.7	2.06

The specifications of the focal reducer state that CCD=118mm would give the nominal 0.75x reduction factor assuming I am measuring from the correct point.  My experimental results give this reduction factor at CCD=127mm.  I don't have an explanation for this 9mm discrepancy.

Field Curvature Results

For each setting, I shot a 20 sec, unbinned image of the star field around the bright star Sadr in Cyngnus to test geometry and illumination.  The image below shows the full frame for each of the focal reducer positions.  All images were subjected to identical gamma stretching so relative brightness can be compared.

Sadr star-field at each focal-reducer spacing

For each of the star field images, I then magnified the lower-left corner of the frame to see the degree of radial "smear" in the image.  Similar visual results for the other three corners are not shown.  These tests also show the effect of some under-sampling resulting from the use of the focal reducer.  My skies are usually around 2" to 3".

Bottom-left corner with no focal reducer

Bottom-left corner with 81mm focal reducer spacing

Bottom-left corner with 102mm focal reducer spacing

Bottom-left corner with 117mm focal reducer spacing

Bottom-left corner with 124mm focal reducer spacing

Bottom-left corner with 132mm focal reducer spacing

As expected, the curvature effects are more noticeable at the larger reduction factors.  The image for 81mm has more distortion than I expected.  Looking over the full frame, the 132mm configuration was noticeably worse.  I am going to try to stick with something around 120mm though the reduction is 0.76x at that distance.  The field is not as flat as I had hoped but will probably be a reasonable compromise in order to shorten the required imaging times.

Vignetting Results

For each setting, I shot a 0.1 sec flat field using an Aurora electro-luminescent panel.  For the focal reducer tests, this placed the peak intensity near the 40% grey point and about half that for the non focal reducer test.

In MaximDL, I used the Line tool to graph the illumination intensity across the diagonal of the sensor.  I also set the white and black point of the images to the span the range of pixels.  Each of the images below shows the stretched image and the line graph.  I do not have any more sophisticated curvature analysis software but this gives the general idea.

Eyeballing the lowest and highest averaged values along the curve and taking the ratio, the vignetting gives a 5% decrease in the corners without the focal reducer and up to 8% with the focal reducer.  Both of these are easily corrected with a flat-field normalization, so not much of a concern with this sensor size.

Flat-field illumination with no focal reducer

Flat-field illumination with 81mm focal reducer spacing

Flat-field illumination with 102mm focal reducer spacing

Flat-field illumination with 117mm focal reducer spacing

Flat-field illumination with 124mm focal reducer spacing

Flat-field illumination with 132mm focal reducer spacing

Flip Mirror Options

When imaging, I like having the ability to quickly switch to an eyepiece to get a wider field of view to provide context, to try looking at a target visually, or simply to align the mount without removing the camera.   I had been using an imaging train that I put together for this purpose.  However, with 120 mm to work with now, that system is no longer usable. The Vixen flip-mirror insertion length is too large.

After much hunting,  I found a candidate solution using the TFlip mirror from Teleskop Service which has an 82 mm insertion length.  In the USA, I was please to find it offered by Optcorp.  I also found an on-line review by a Polish amateur astronomer.

The unit has female T2 threads on both sides of the imaging path and male T2 threads on the eyepiece port.  This is the threading standard I am already using and so no room is wasted working around other thread types or push-fits.   Back-of-the-envelope calculations verify that this will just fit into my system.  I will update on the results if I ever spring for the steep $250 purchase price.


Update 2012.09.02 - Last night, we had clear but slightly hazy skies under a full moon. I tested the focal reducer on the Pelican Nebula under H-alpha light.  The first processed image below is from a stack of 23 Ha images, each 15 min.  I chose the 117mm extension length giving 0.7657x according to MaximDL plate solve or a focal ratio of f/5.4

Update 2012.09.03 - The second H-alpha image below uses same configuration with 25 frames of 900s exposures. This is part of the Heart Nebula including the central star cluster and the "tip" of the heart formation. The stars in this second image were not nearly as smooth as the first one.  I had to do more clean-up work.

NGC-7331 Past and Present

Over the last two nights, I had a chance to image the galaxy NGC-7331 in Pegasus.  After preliminary processing of the data, I looked back over my imaging notebook and found that this was one my very early imaging targets.  I was interested to compare the two attempts.

The first was created from a series of approximately 20 unguided 1 minute color exposures taken back in October 2008 when I first started astrophotography.  Exposures were taken with an SPC-9000 webcam, which I had modified for long exposure, through a Celestron 9.25" SCT.  Image scale is 0.46 arcsec/px prior to de-Bayering.  Field of view is very small - this is an uncropped image!

The second was created from a series of approximately 20 guided 10 minute exposures through RBG filters.  Exposures were taken with a QSI-540ws CCD camera through a TMB-130SS refractor.  Image scale is 1.46 arcsec/px.

In looking back,  I am impressed what was possible with that little webcam.

In the same image frame as NGC-7331, I also captured the violently colliding group of galaxies called Stephan's Quintet.  The inset on the top left shows this group at the same image scale.  On the bottom left is an old image of the same group that I took in December of 2008, soon after replacing my wecam with a Canon DSLR.

Since the second night was the peak of the Perseid Meteor Shower, I looked through the frames to see if I had caught any meteors.  What I at first thought were tiny, short meteors, I soon decided were probably satellites:

This image is a superposition of three different 10 min exposures.  On the first night, at 2012.08.11 00h20 CDT,  there was a uniform solid trace, labelled above. A second dimmer trace occurred along a parallel path a few hours later at 2012.08.11 02h50 CDT.   I don't know if these are the same satellite.

Of more interest was the pair of traces in the frame from 2012.08.11 02h50 CDT  (one trace is below and to the left of NGC-7331 and the other to the left of Stephan's Quintet.)   Just 24h later, in the 2012.08.12 02h50 CDT frame, another identical pair of traces appeared (one is to the left of NGC-7331 the other is above and to the right.)  The similarities and timing seem too coincidental not to be the same satellite.  Since the traces are bright in the center and dim on the ends,  I assume this satellite is rotating uniformly.

Update 2012.08.16 - No success in identifying any satellites in this field of view within an hour of the observation in CalSky.com.

Curiosity Lands on Mars

Sunday 2012.08.05 - This evening we followed the landing of Mars Science Laboratory via NASA TV streamed from our iPad to the television.  At the same time, we followed the flight simulation on the NASA web application "Eyes on the Solar System."   I was most impressed with this simulation.

Picture perfect landing after the "7 Minutes of Terror" descent.  Odyssey successfully rotated and relayed telemetry and initial images from Curiosity before it went over the horizon.  We were thrilled to follow the nail-biting event live.  After watching the nightly Olympic coverage, this was like watching the engineering community win a gold medal in world record time.    I struck not only by the mind-blowing space-flight engineering but also the communications network that brought all of this live to my living room ...  brilliant!

Here is the flight simulation of the sky crane doing its thing in the last seconds of descent.  This is what we watched as NASA controllers verbally relayed confirmation of powered flight and sky-crane deployment with the rover dangling twenty feet below.

This is the simulation of the rover on the ground

As the seven minutes of entry proceeded the atmosphere in the control room was charged.  Each new bit of telemetry information confirming that this complex choreography was coming off as planned.  Once a safe touch down was confirmed, the room erupted in to applause, hugs, and tears.  This video captures the moment as we saw it live.   I tried to get a shot of the celebration but everyone was jumping around too much.  I finally got a shot of controllers watching in awe the downloaded images.

Here is the first high-res shot of Curiosity's wheel. We took a picture of our television broadcasting the NASA TV feed so its pretty fuzzy.  The previous shot had been the 64x64 thumbnail of this same image

This next high-res shot on the left is of the shadow of Curiosity on the Mars surface.  The dust covers are still on the camera lens. These were later popped off.

What an experience.  I anxiously look forward to the science results MSL will generated over the next couple of years.  Way to go NASA!

Monday 2012.08.06 - This XKCD cartoon summarizes what today is going to be like

XKCD 2012.08.06 - http://xkcd.com/1091

Today, NASA/JPL treated to some more detailed images. This first image is taken by the  High-Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter.  It shows the rover descending on the parachute several minutes before touch-down.

This next image is from the left rear hazard camera looking back towards the rim of Gale Crater.

This image is from the front hazard camera, dust cover off now. The rover is facing Mount Sharp,  about 6km distant, its main destination.