JerryWise
Postmaster
Reged: 12/26/03
Posts: 7839
Loc: Lexington, SC
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As everyone's plans for New Years Eve celebrations became clear, so did the sky. The choice; a New Year's celebration or observing under a clear sky? (Nobody even missed me.)
I'm testing a recent production Takahashi TOA-150 triplet APO and the last new Takahashi FS-152 APO doublet side by side on an AP-1200 mount. Cameras are Canon 20Da (TOA) and Canon (H Griffin modified) 350d (FS).
Both cameras were at ISO 800 for 3 minutes. I carefully focused both scopes with a Stiletto focuser and confirmed with the Canon 20Da live focus. This between 8:00pm and 8:30. The temperature showed 28 outside the observatory and 41 when I began imaging. The inside thermometer read 31 at the end of the session. No measurements were made of the OTA or lens elements so the time and images are not equated to a degree drop. I'll repeat with this information later. However, the results make the point very well without these readings.
The two images below are stacks of 17 stacked and processed with ImagePro DDP. The insets are clips from the original Canon Raw Images with the times as time stamped in the raw image file.
I frankly had no idea focus could go off this much in under an hour. Even 20 minutes change is enough to make the difference in a stunning image and a so-so image. (CN member Mike an I had discussed focus shift in TOAs recently so I took another look at it.) This isn't breaking news to the RCOS crowd with temperature compensating focus routines but the extent of movement in even refractors is of interest to some of us. It may be beneficial to take 3 images, remove the camera and refocus. While that is being done cap the camera and shoot a dark frame of two.
Takahashi FS-152
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JerryWise
Postmaster
Reged: 12/26/03
Posts: 7839
Loc: Lexington, SC
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TOA 150:
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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Wow, guess I will get my sn6 out at least a couple hours before I focus with the stiletto! An occasional refocus looks like it is in order.
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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JerryWise
Postmaster
Reged: 12/26/03
Posts: 7839
Loc: Lexington, SC
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That's another part of the problem justabob. I had the observatory open all day and the scopes were at ambient. This occurred as part of the normal evening cooldown. Looks like if there is any temperature change going on constant refocus is required.
I wonder how those like Dean that setup and image all night (producing world class results) deal with it. Dean?????
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Jerry
Celestron C-11 Hyperstar
(and some other nick-knacks)
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cvedeler
Carpal Tunnel
Reged: 12/20/05
Posts: 2164
Loc: Scottsdale, AZ
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I have a digital thermometer on my mount. Any change of 2 - 3 degrees C means I need to check focus. My problem is using the stiletto on a bright enough star and then slewing back to exactly the same place I was before. My Atlas mount just can't slew accurately enough.
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Chris Vedeler
Astro-Physics 160EDF
Astro-Physics 900GTO
Q453HR / QHY8 CCD camera
Canon 450XSi
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www.aznightsky.com
Scottsdale, AZ
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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Thanks Jerry nice work. I do not have a large refractor and have never noticed this in subs taken with my little wo 66. But it is quite evident with the larger 6" Smidt Newt.
Bob
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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stefsaber
Post Laureate
Reged: 06/24/06
Posts: 4721
Loc: Rainy Florida
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Wow, interesting to note. Nice looking shots 
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-Stefan
"A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools." -Douglas Adams
Current Scopes: "Gator" William Optics 66SD---Black Swan William Optics Megrez 80 II ED Triplet---Zhumell 10" Dob
-Sirius Mount---Canon Rebel XT-
Past Scopes: ETX 90---Vixen ED80Sf
Fort Myers, FL
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lawrie
Carpal Tunnel
Reged: 01/31/06
Posts: 1744
Loc: Okanagan Valley
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Interesting stuff, I wonder if there is a relationship between size of the scope and the amount of contraction.
You might think so.
Then as Chris says, after re-focusing, the issue becomes framing the image again.
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Clear Skies
Lawrie
Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D
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Mike D
scholastic sledgehammer
Reged: 08/15/07
Posts: 864
Loc: South GA
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These are excellent shots BTW, and thank for the refocus tip!
I wonder why the star colors look so different in the doublet v. triplet?
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Vixen SXD
Vixen ED80Sf
Hutech 350D
STI Stiletto
Meade 2080
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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For what it's worth the TOA 150 image seems sharper and shows better color correction even with the thermal focusing issues.
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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PGW Steve
professor emeritus
Reged: 10/03/06
Posts: 632
Loc: Winnipeg, Canada
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Jerry, did you correct the focus after you noticed that it was off and take any more shots? I'd like to see how close it came back to the first shot or if seeing played into it somehow.
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A-P 1200GTO - On the list for a Mach1GTO
14" LX200R OTA
FSQ106ED Tak .75 reducer +1.6 extender
WO Megrez 80mm 600mm triplet APO
WO ZenithStar 66 Triplet ED APO
EQ 6 PRO, HEQ-5 PRO
Astrotrac TT320, Manfrotto 475&488
DSI PRO II W/filters, Orion SSAG
Hutech 40D and Unmodded EOS 40D
16-35mm f2.8L II, 24-70mm f2.8L,70-200mm f2.8L IS 300mmf4.L, 400mmf5.6L
31T5,22T4,13E,9T6,8E,5T6,3.5T6
5’X8’ Wells Cargo trailer to haul it in!!!
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JerryWise
Postmaster
Reged: 12/26/03
Posts: 7839
Loc: Lexington, SC
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Mike, the FS had the modified 350d giving it more redish tint.
Lawrie, I'm thinking about a way to test that. I would have thought the FS-152 would be worse. It has a longer tube which provides more expansion length.
Steve, seeing was getting better as the evening progressed. I don't have any images later on though. Fireworks began to interfere so I closed up early. I'll be playing with it more as the week goes on. Seems we have a run of clear nights.
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nighthound
scholastic sledgehammer
Reged: 08/30/06
Posts: 960
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I have to tweek focus with the Sky 90 a couple times a night when temps drop during winter months. With the Vixen R200SS/Moonlight I tweak focus and collimation twice during a full winter night. The TAK holds focus quite a bit better.
NH
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Lord_Vader
Vendor - Aries
Reged: 07/25/06
Posts: 140
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Jerry,
The simpliest way to fix focus shift problem is to install a BORG's helicoid focuser with a movement scale. It is quite easy to determinate what is a focus shift per degree F.
Then you will need only to monitor the temperature change and perform focus adjustment by simple turn of a focuser on a necessary number of scale marks.
You don't need a 4" focuser. 2" is enough for DSLR weight.
For larger CCD's may be a 4" will be needed.
I use a 4" BORG model on my 180mm APO. And focus adjustment is very easy to do with it.
BTW
"I'm thinking about a way to test that. I would have thought the FS-152 would be worse. It has a longer tube which provides more expansion length."
it seems, that focus degrades more in TOA. This is because TOA is a faster scope and therefore it is more sensitve to a focus shift. So, all is exactly opposite!
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JerryWise
Postmaster
Reged: 12/26/03
Posts: 7839
Loc: Lexington, SC
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Quote:
Jerry,
The simpliest way to fix focus shift problem is to install a BORG's helicoid focuser with a movement scale. It is quite easy to determinate what is a focus shift per degree F.
Then you will need only to monitor the temperature change and perform focus adjustment by simple turn of a focuser on a necessary number of scale marks.
You don't need a 4" focuser. 2" is enough for DSLR weight.
For larger CCD's may be a 4" will be needed.
I use a 4" BORG model on my 180mm APO. And focus adjustment is very easy to do with it.
BTW
I'll take a look at that. I'm also thinking about moving the FeatherTouch Digital focus drive over to the TOA. It has a very precise readout that could also work for that. Problem is, there would be two focusers on the TOA.
Quote:
it seems, that focus degrades more in TOA. This is because TOA is a faster scope and therefore it is more sensitve to a focus shift. So, all is exactly opposite!
Excellent. I hadn't thought of that. Much more sensitivity. Thanks.
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ChazK
Pooh-Bah
Reged: 11/23/05
Posts: 1453
Loc: Melbourne, Florida
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Thank you Jerry......I had no idea!
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Tak TOA 130mm/NJP Temma2
Tak FS 60CB
Canon 40D (unmodded)
Canon EF 28-135mm f/3.5-5.6
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Dean
Post Laureate
Reged: 12/31/04
Posts: 4912
Loc: Bailey Co Elev 8780 feet
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Quote:
Jerry,
...
"I'm thinking about a way to test that. I would have thought the FS-152 would be worse. It has a longer tube which provides more expansion length."
it seems, that focus degrades more in TOA. This is because TOA is a faster scope and therefore it is more sensitve to a focus shift. So, all is exactly opposite!
My understanding is that most focus shift with temperature change is due to the different material of each lens element contracting at different rates and that ideally the tube should shrink at a rate that compensates for len's focus change. I don't know how true that is, but if it is it could mean that the FS152's tube does a better job compensating for the len's focus shift.
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"Don't sweat the petty things and don't pet the sweaty things" - George Carlin
deanrowe.net/astro
Whats with that avatar?
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cvedeler
Carpal Tunnel
Reged: 12/20/05
Posts: 2164
Loc: Scottsdale, AZ
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The aluminum tube ideally should shrink at the same rate that the focus shifts from the lens cooling. It is pretty much impossible to get these two to happen at the same rate. Roland Christen discussed this on the AP Yahoo group some months ago.
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Chris Vedeler
Astro-Physics 160EDF
Astro-Physics 900GTO
Q453HR / QHY8 CCD camera
Canon 450XSi
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www.aznightsky.com
Scottsdale, AZ
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D. Perry
member
Reged: 09/28/07
Posts: 50
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I quickly became aware of the thermal coefficient issues when I started imaging with my FSQ-106ED. It seems very susceptible because of its 4 lenses, relatively fast focal ratio (f/5), and all metal tube. After training the TCF-S to compensate for the tube's changes, I no longer need to worry about it. The TCF-S constantly refocuses as needed.
All scopes are prone to this issue. Refractors are no exception, especially large refractors with long metal tubes! The symptoms are less noticeable in physically small scopes, scopes using tubes of composite materials, and high focal ratio scopes (generally speaking, of course).
- Danny
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Daniel Perry
www.californiastars.net
Ontario, California [ 34N | 117W ]
• Astro-Physics 900GTO & ATS Portable Pier
• Meade 12" LX200-ACF OTA
• Celestron 9.25" SCT
• Takahashi FSQ-106ED
• Stellarvue SV80S (Lomo super APO triplet)
• SBIG ST-10XME
• Canon EOS 5D MII (unmodified)
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lawrie
Carpal Tunnel
Reged: 01/31/06
Posts: 1744
Loc: Okanagan Valley
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Would having a dew heater on the scope affect this?
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Clear Skies
Lawrie
Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D
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