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Skydetector
member
Reged: 08/21/06
Posts: 66
Loc: Albuquerque NM
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I was just wondering what I migh gain if I put the PSM40 on my 10" dob. You know they make the white light solar filters that are not a full filter. They have that little offset that actually does the filtering. I could put the PSM40 in that spot...
I made a full aperture white light solar filter for it with a five gallon bucket lid and some solar film. Works fairly well. Could I do the same with the SM40 and put the blocking filter at the eyepeice, or put a piece of solar film over PSM40??
Just thinking out loud wondering if that might be worth pursuing as a winter project 
Safety is paramount, of course.
Mike
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I guess I'm a looker, cause I'm looking in the ground, I'm looking in the night sky and I'm looking at the sun.
Orion XTi10 Reflector
Coronado PST/SM40/Coronado DSM
Fisher Metal Detectors
http://www.charliebates.org/
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darkstar528
Postmaster
Reged: 03/06/07
Posts: 6784
Loc: Hodgenville, Kentucky, USA
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Like the PST, can you tune the SM40 to the wings and pull out spots?..
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Blue skies,
Stephen "Darkstar" Ames
PST(#96038), VIXEN 8-24mm,CEMAX 2x Barlow, Thousand Oaks White Light Filter and a Meade Elec EP
CFI, CFII, MEI, working on EIEIO!
BAA Member
My solar site:
http://seemysunspot.com
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Skydetector
member
Reged: 08/21/06
Posts: 66
Loc: Albuquerque NM
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Darkstar, yeah, the PSM40 has a tuner wheel to tune with. May not be worth fooling with. Like I said, just thinking outloud 
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I guess I'm a looker, cause I'm looking in the ground, I'm looking in the night sky and I'm looking at the sun.
Orion XTi10 Reflector
Coronado PST/SM40/Coronado DSM
Fisher Metal Detectors
http://www.charliebates.org/
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marktownley
Carpal Tunnel
Reged: 08/19/08
Posts: 2257
Loc: West Midlands, UK
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You would need the blocking filter - aka the coronado diagonal too...
Then i'm not sure it would work cos it's a reflector...
*whistles and walks away quietly*
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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Quote:
You would need the blocking filter - aka the coronado diagonal too...
Then i'm not sure it would work cos it's a reflector...
That won't matter (if it's a perfect reflector). You'd have the convenience of being able to reach that tuning knob even while you are looking down the eyepiece, something you cannot do with a refractor of equal focal length. Mirrors generally scatter a bit more light than do lenses, so you might pay a contrast penalty.
I see three potential difficulties.
The first is the size blocking filter you will need to accomodate the full solar image from a focal length that long.
The second is your need to have the blocking filter in a straight-through configuration. Most of the Coronado blocking filters are built into the diagonal. You can't use a blocking filter diagonal as your secondary mirror becuase you couldn't afford a blocking filter as big asyour secondary needs to be.
Third is tha need to have your front filter mounted ABSOLUTELY square to the tube. Otherwise, the small amount of tilt available via the tuning knob may not be enough to bring you on band.
Gene Baraff
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ragebot
Carpal Tunnel
Reged: 08/26/05
Posts: 1672
Loc: Tallahassee, FL, USA
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I am no expert and these are just random thoughts.
In general narrow band filters, like HA, tend to be associated with what most folks consider "slow" OTAs. It some cases the focal ratio is in the f30 range. I have a DS SM40. I removed the front and back elements, got an adapter from OPT, and put the whole mess on an ED80. This is a very common mod and adapters are available for a wide range of OTAs; but to my knowledge only refractors.
My first concern would be that an off set might mess things up. A standard "tuning" method is to tilt components and the offset would seem to "tune" in and of itself; perhaps "tune" so much that you could not get surface structure or proms to pop. On the other hand mounting the SM40 with no offset would mean having to deal with the central obstruction of a reflector.
Another consideration is that you really don't need a super dooper OTA for narrow band observing since only a narrow band of light is not filtered out. My suggestion would be to get a 80-100 mm acromatic OTA and make or buy an adaptor for you SM40 front end.
One thing I really like about my SD SM40 and ED80 setup is I can mount the whole mess on an iOptron mount, leave it set up next to the sliding glass door to my patio, carry it out side with one hand, and be observing in less than 5 minutes; while it takes much longer to get my 8 in newt ready.
Another general consideration is that day time seeing often does not support a lot of magnification, so not only is a bigger OTA more effort to set up but often there is not really an improvement in what you see in the EP.
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Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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I would like to respectfully agree with some, and respectfully disagee with some others - of the points ragebot has made.
1, Association with "slow" OTA's: That's an associatioon which is fully valid for filters placed behind the objective. There is no optical reason for this association to be valid for filters placed ahead of the main objecdtive.
2. Offset messing up tuning: That will not happen because light, from the center of the field of view coming through the offset filter and continuing down to the primary mirror, is still travelling parallel to the optical axis. If the filter has been mounted perpendicular to the scope as I stressed in my previous posting, your tuning will be fine.
3. That you don't need a super OTA: Agree fully, for the reasons ragebot gives.
4. Considerations about daytime seeing: Agree fully, for the reasons ragebot gives.
Gene Baraff
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Andy Lunt
Vendor-Lunt Solar Systems
Reged: 10/31/07
Posts: 7
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This is a good question.
It raises the some questions regarding the use of a Dob, and given some of the responses I have read, there may now be some confusion.
There are no issues with placing a filter on the front of a Dob. Whether that filter is offset or not will not effect the performance. As long as the light passes thru the etalon correctly (properly mounted on the front) what happens after that is standard to any scope.
The Dob does have a mirror. The issue as I see it have been the amount of heat that the mirror sees, and the amount of heat that is potentially passed back by the secondary should the scope drift. When using the Dob in white light mode, the front film prevents any heat entering the system. This makes the system safe to look thru and pass by (no re-focused beam).
I have many 8" dobs  and the white light views are great.
Placing a 40mm filter on the front will reduce the heat load to a negligible level.
However, due to the focal length (the only real issue) the image will be very dim.
A 40mm filter has probably been designed for a faster system. (Filter transmissions are designed for specific focal lengths) Typically 400mm. This provides an image size of 3.63mm. The Dob is probably closer to 1200mm. This provides an image size of 10.9mm. The difference in area being 6.75mm vs 94mm. What you are basically faced with is an image that only has 7% of the light per square mm when compared to the 400mm focal length.
For imaging purposed this may not be an issue, but in my experience, you will not be able to see the image well at all.
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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Quote:
It raises the some questions regarding the use of a Dob, and given some of the responses I have read, there may now be some confusion.
Thank you for chiming in. Your response speaks to an aspect I had overlooked completely. I'm happy to have learned something useful.
Gene Baraff
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