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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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Hilmi
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Imaging train, F ratio and clear aperture etc...
      #5695925 - 02/23/13 12:26 AM

I have been imaging with my 8" Meade SCT with an Optec Lepus reducer and a Moonlite focuser. First of all, I am finding that plate solve calculates my focal length to be somewhere around 1618 mm. Starting from 2000 mm that is not what I expected out of an F6.2 reducer. So I suspect it's a spacing issue. The Optec Lepus has a custom made adapter for my camera, so spacing on the camera side is perfect down to the mm. So I figure my Moonlite focuser has pushed the reducer too far back from the OTA. I also find that my scope sort of fails to illuminate the full size of my chip with this configuration. The light drop off is just too much at the sides and I need to take ridiculously long exposure get date on the edges of the frame even on very bright targets like M42. I and I am talking about a considerable part of the edges of frame.

So now I am thinking I need to change my OTA to something that is natively faster so that I don't need the reducer and one with enough back focus so that I can fit a rotator and AO unit in the future (room for growth). Should I buy this OTA, I will not be able to afford upgrading to a larger mount, so it has to be something that will be within the capabilities of my G11 once you add the weight of all that extra gear. If I save hard I see my budget stretching to around $5000 MAX. But I would rather spend less if possible. That amount includes all mounting hardware. I don't want to buy a scope to find out I need to spend more money again to fit it on my mount and again I need to spend money on a FF/FR so that my stars don't look funny etc.. etc..

I have no clue what sort of scope to go for.


P.S. Please don't suggest hyperstar. My camera and filter wheel profile is as big as a netbook, I can just about imagine how much clear aperture I would be loosing not to mention the funky diffraction spikes that would generate.


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jrcrillyAdministrator
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5695932 - 02/23/13 12:36 AM

I had one of these and liked it. Dunno what kind of wait time there is. I had one of these and liked it even better but at 34 pounds it might be heavy for your mount.

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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: jrcrilly]
      #5695937 - 02/23/13 12:45 AM

I read somewhere that these scopes have mirror tilt issues because any weight you put on the back goes directly to the mirror. I would think that an STT-8300 with a self guiding filter wheel, plus a Moonlite focuser and the AO and rotator I see in my future would qualify as a lot or weight. What's you'r experience with the way the scope handles weight hanging off the back?

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jrcrillyAdministrator
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5695948 - 02/23/13 01:02 AM

I never had issues as you describe with either model and a variety of cameras/filter wheels. My buddy who uses a heavy 2" filter wheel on his 8" RC hasn't, either. I think the collimation screws would have to be loose for anything to move.

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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: jrcrilly]
      #5695955 - 02/23/13 01:21 AM

And how does it do with an APS-C size sensor? My current combo including the reducer vignets way too much.

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Mike7Mak
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696057 - 02/23/13 04:42 AM

As far as I know distance from the visual back should not effect the amount of reduction but it might have everything to do with the vignetting. With my Mak I get vignetting if I mount almost any reducer at the end of the microfocuser, but screwed directly on the visual back or inserted into the focuser on a 2" nosepiece the vignetting goes away.

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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Mike7Mak]
      #5696069 - 02/23/13 05:02 AM

My spacing from reducer to camera is exactly correct. The guys at OPT did the calculation for me and then the
guys at Optec checked it again. My reducer to imaging chip distance is within 1 mm accuracy.

I cant mount the reducer closer to the OTA because then I will not be able to install my focuser. Plate solve tells me I am at F 8.something. I'm thinking that the AT10RC will give me the same focal length with one less element in the imaging train. I think it is well within the weight limits of the G11, especially if you consider that that weight includes the weight of the two D plates and a focuser. With 10" of backfocus, I have space to add my an Espresso Machine to my imaging train!


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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696081 - 02/23/13 05:30 AM

Will I need to move to the CSL Moonlite focuser if I get an AT10RC or will my CS Moonlite do?

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Mike7Mak
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696084 - 02/23/13 05:40 AM

Quote:

My spacing from reducer to camera is exactly correct. The guys at OPT did the calculation for me and then the
guys at Optec checked it again. My reducer to imaging chip distance is within 1 mm accuracy.



Then the platesolve is incorrect or the reducer isn't supposed to give you .62 reduction, which would be 1200mm not 1600mm. Sure you didn't get a .8 reducer?


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jrcrillyAdministrator
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696089 - 02/23/13 05:50 AM

Quote:

Will I need to move to the CSL Moonlite focuser if I get an AT10RC or will my CS Moonlite do?




They made me a flange so I could transfer my existing CS focuser to my 10" (and that flange also permitted me to later transfer it to my 12"). With the loads you describe, the CSL would be a very good idea, but optically either would work unless you move up to a huge chip.


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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Mike7Mak]
      #5696094 - 02/23/13 06:02 AM

Quote:

Quote:

My spacing from reducer to camera is exactly correct. The guys at OPT did the calculation for me and then the
guys at Optec checked it again. My reducer to imaging chip distance is within 1 mm accuracy.



Then the platesolve is incorrect or the reducer isn't supposed to give you .62 reduction, which would be 1200mm not 1600mm. Sure you didn't get a .8 reducer?




Anyway to validate this? I have my images on Astrobin with reducer if that helps.


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jrcrillyAdministrator
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696096 - 02/23/13 06:06 AM

Quote:

And how does it do with an APS-C size sensor? My current combo including the reducer vignets way too much.




I sold the STL-11000 just about the time I started fiddling with the Astrotech RC models and I don't remember whether I ever tried to light that camera with any of them. I've used up to Kodak 8300 chips with all of them, so that's the largest chip about which I can make definite statements. Illumination is good; I never got around to shooting flats. Curvature is there, but less than in an SCT due to the slower primary. I like the fact that I can shoot without any add-on refractive optics in the way.


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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: jrcrilly]
      #5696101 - 02/23/13 06:17 AM

Sounds good. Time to do a garage sale so I can fund the new scope.

Mike, The plate solve on Astrobin agrees with the 1618 mm focal length that MaximDL calculates. The focal reducer I have is the Optec Lepus reducer and it only comes in F6.2 So Now I am wondering if there is something wrong with it.


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jrcrillyAdministrator
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5696103 - 02/23/13 06:25 AM

Quote:

Mike, The plate solve on Astrobin agrees with the 1618 mm focal length that MaximDL calculates. The focal reducer I have is the Optec Lepus reducer and it only comes in F6.2 So Now I am wondering if there is something wrong with it.




Bear in mind that you have a lot of stuff in there ahead of the reducer and you've necessarily shifted the primary location to achieve focus in that configuration. When you do that, the telescope's focal length increases fairly rapidly (several times more than the amount of rearward shift). The reducer isn't starting with an F/10 setup, but with something slower instead. If you are sufficiently far back to reach F/13, then the reducer is doing its job at X.6 .


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Mike7Mak
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Re: Imaging train, F ratio and clear aperture etc... new [Re: jrcrilly]
      #5696401 - 02/23/13 11:21 AM

Quote:

If you are sufficiently far back to reach F/13, then the reducer is doing its job at X.6 .



Well ok. I didn't think focal length varied 'that' much with backfocus but apparently it's possible. Seems odd the length of a sct crayford focuser would add that much. Afterall these scopes are intended for use with a diagonal which also adds to the backfocus. I guess it would be instructive to find out exactly what backfocus distance provides the rated 'F10'.

This writeup says, among other things, the proper placement of a reducer on a C-8 ends up a 1/2 inch inside the visual back. That certainly jives with my experience, reducers seem to work best as close to the visual back as possible.


http://www.morrell.ws/index.php?option=com_content&view=article&id=49...


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RedLionNJ
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Mike7Mak]
      #5696947 - 02/23/13 04:37 PM

A "prime focal ratio" of somewhere between f12 and f13 is fairly typical for an "F10" mass-market SCT. When you hang something off the back of such a scope, you typically change the focal length in order to focus. You're effectively increasing the focal length. With my JMI NGF-S as my only piece of hardware on the rear of my mirror cell, infinity focus generally leads to a focal ratio of f12.8 or thereabouts. You sound to be in a very similar situation.

Too many people don't consider how the focal length varies with mirror separation in the SCT design. Image measurement (or plate-solving) is the best way to measure this. Something like FireCapture will also tell you the effective focal length in its logs. I recall the first time I got a measurement of my nominal 2X Barlow on my nominal F10 SCT with a Crayford focuser and a filter wheel hanging off the back - it was something ridiculous (but accurate) like f35!


Grant


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Jared
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5697989 - 02/24/13 10:15 AM

My guess is that your reducer is working fine, but that you are working closer to f/13 than f/10 because of your Moonlight. This would also explain your severe vignetting. You could always try imaging without the Moonlight to confirm.

As far as the AT Ritcheys go... I have one of the 10" scopes and have been quite happy with it. I wouldn't choose it for visual use, but it is a good scope for astrophotography. My sample has reasonable optical quality (perhaps 1/5 wave) and Ina's fairly stable focus as temperatures fall. On a good night, I have been able to get sub 2" FWHM values across a 42mm imaging circle with a field flattener. The flattener is not necessary for the 10" if you are using chips of 4/3" size or smaller (such as the 8300 based cameras). Above that size chip, you would need a flattener. With the 8", you might even need a flattener for 4/3" chips.

Personally, I have had no problems with torque from the camera causing the scope to lose collimation. I suspended roughly 3 kilos off the back of my Ritchey. If you included an adaptive optics unit and a camera rotator as well as focuser and flattener you would be exceeding that weight. You would also likely be exceeding the total backfocus of the system, so you may want to rethink at least one of those items. Not many scopes out there below the $12K price point can accommodate that many accessories.

Ritcheys are very finicky about collimation, but once you get it right you should be fine. My sample did a good job holding collimation even over pretty rough roads.

Personally, in your situation I wouldn't be planning on an adaptive optics unit and a field rotator unless you intend to upgrade more than just your OTA. I might even try imaging without the external focuser to see just what is possible with the Meade and Lepus. You might be quite satisfied (or not).


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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Jared]
      #5698258 - 02/24/13 01:22 PM

Jared

Thanks for your input. I can probably live without the rotator but would have been nice to have since I am moving more and more towards automation. That way I can spend more time with my wife while still imaging regularly.


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Jared
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Hilmi]
      #5698291 - 02/24/13 01:41 PM

I certainly understand the desire for a rotator--especially if you are going to be imaging at 1,500mm type focal lengths with a self-guiding camera. Just be cautious that whatever accessories you end up with, your OTA has enough backfocus to handle them.

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Hilmi
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Re: Imaging train, F ratio and clear aperture etc... new [Re: Jared]
      #5699254 - 02/25/13 02:41 AM

I'm not sure, I will need to confirm if changing the flange to fit the moonlite focuser onto the AT10RC will change the backfocus, but it looks like it would all fit into the 10 inches of back focus with half an inch to spare. I figure half an inch is enough space to focus.

This is for AT10RC (or AT8RC, both have 10" backfocus I believe) -> Moonlite CS -> Pyxis rotator -> AO8 -> Self Guiding Filter wheel -> STT-8300

Edited by Hilmi (02/25/13 02:45 AM)


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