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C-14 Edge - f11 : too slow for fainter comets?

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#1 Thomas Ashcraft

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Posted 07 December 2017 - 03:24 PM

Just curious: Is a C-14 Edge recommended for faint comet photography? Or would this telescope be considered too slow to efficiently catch subtle detail? - Thomas



#2 Don W

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Posted 07 December 2017 - 03:35 PM

If that's a concern, and I really don't understand why it would be, then you can use a Celestron or Meade 6.3 Reducer to make it faster.


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#3 barbarosa

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Posted 07 December 2017 - 03:54 PM

The reducer for the Edge OTAs is much more expensive, $599, than the f/6.3 for  non-edge SCTs


Edited by barbarosa, 08 December 2017 - 11:22 AM.

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#4 Ron359

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Posted 07 December 2017 - 04:01 PM

If that's a concern, and I really don't understand why it would be, then you can use a Celestron or Meade 6.3 Reducer to make it faster.

The standard .63 reducer won't work with the Edge.  It has its own .7 reducer - which are expensive and hard to get, or so I read here often.  As long as the faint comet is mostly tiny coma/nucleus, capturing it with slow f/ratio scopes is done with tracking at the objects rate.  You could use add a hyperstar and shoot at f/2 for those big bright comets with tails.   I'd also look at getting a standard XLT C14 which could use the .63 reducer/corrector or the better Starizona type II R/C, which saves money and gets a bit faster f/ratio at the cass. end for imaging.  Like planets, for those small faint comets, you don't really need the big, wide flat field the Edge provides.  The XLTs are also hyperstar compatible.  


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#5 carolinaskies

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Posted 07 December 2017 - 08:10 PM

Just curious: Is a C-14 Edge recommended for faint comet photography? Or would this telescope be considered too slow to efficiently catch subtle detail? - Thomas

Imaging a dim comet with this telescope won't be an issue.  You can opt for a Fastar to make it F/2 for imaging.  You can spend $$$ for simply a focal reducer which turns it into F7 but unless you plan to use it for visually observing the comet as well going with a Fastar setup is the way to go.  A 14" will help gather more light of diffuse comets.  Most comet hunters who are visual observer opt for F4-F6 for both the wider view and easier to pick out diffuse objects like comets.  


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#6 Eddgie

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Posted 07 December 2017 - 09:13 PM

I have meant to ask this question many times before.

 

If the EdgeHD is corrected for coma and the field only has 1/3rd of the curvature of the standard SCTs, why do you need to use the Celestron reducer?  The field of my EdgeHD 8" was flat enough that even with the 31mm Nalger, stars were super sharp at the edge.

Why wouldn't a regular 2" .7x reducer work well on the EdgeHD scopes when using something like an APS-C sized chip?



#7 carolinaskies

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Posted 07 December 2017 - 09:36 PM

I have meant to ask this question many times before.

 

If the EdgeHD is corrected for coma and the field only has 1/3rd of the curvature of the standard SCTs, why do you need to use the Celestron reducer?  The field of my EdgeHD 8" was flat enough that even with the 31mm Nalger, stars were super sharp at the edge.

Why wouldn't a regular 2" .7x reducer work well on the EdgeHD scopes when using something like an APS-C sized chip?

The issue I believe is in regards to maintaining the same backfocus as it has without the reducer.  If the backfocus of a reducer is different then OAGs may not work.  The Celestron version guarantees no change in the backfocus when using their reducer.  This was one reason many people preferred the Meade 6.3 R/C over Celestron R/C years ago.  



#8 WadeH237

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Posted 07 December 2017 - 09:42 PM

Why wouldn't a regular 2" .7x reducer work well on the EdgeHD scopes when using something like an APS-C sized chip?

I think that it comes down to spacing.

 

The EdgeHD scopes have a back focus point where they are optimally corrected.  The Celestron reducers specific to the EdgeHD maintains the same spacing and back focus (except for the 8").

 

I have an Optec Lepus reducer that I have used with my EdgeHD 8" scope.  The reducer needs to be 105mm from the imaging sensor.  I've got the spacing all correct - but I don't get the expected reduction.  The reducer is .62x, so I would expect the resulting focal length to be 1260mm.  What I actually get is 1455mm.  The reason for this is that, to reach focus, I need to turn the Edge's focuser counter-clockwise significantly past the optimal point.  That results in a longer than expected focal length, and it also results in some aberrations in the corners, so that the stars are not round.  And that's on my 4/3 sized chip.  If I were using a DSLR full frame, I would expect to need to crop quite a bit.

 

I also have the Celestron EdgeHD reducer for the 8".  It also requires 105mm spacing between the reducer and the sensor.  I am hoping, though, that the SCT focuser will be at the place where I get both optimal correction and the expected reduction.  Since the Celestron reducer is .7x, I expect to get 1422mm, and with pinpoint stars across the entire field.

 

I've not had a chance to test the Celestron reducer yet,  I have heard that some people are finding that they need slightly different spacing than the 105mm between the reducer and the sensor.  So I am really curious to see how it works out.  But I've got a bunch of mount work to do, plus I need to get a few more adapters to the the spacing correct to do the testing.



#9 jhayes_tucson

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Posted 07 December 2017 - 10:24 PM

I have meant to ask this question many times before.

 

If the EdgeHD is corrected for coma and the field only has 1/3rd of the curvature of the standard SCTs, why do you need to use the Celestron reducer?  The field of my EdgeHD 8" was flat enough that even with the 31mm Nalger, stars were super sharp at the edge.

Why wouldn't a regular 2" .7x reducer work well on the EdgeHD scopes when using something like an APS-C sized chip?

If the sensor is small enough and you don't care about back focus or precise color correction, you can use almost any reducer and it will work just fine.  Of course you won't know how big of a chip you can get away with without doing some experimenting.  The Celestron reducer is optimized to work specifically with the Edge design to produce the same back focus distance as the scope and to produce a flat well corrected field over a large sensor.  The Celestron reducers are not simply designed to relay one image plane to another.  They are optimized to produce a well corrected, flat field over a large area as a part of the whole telescope design.  In principle, this approach will be far superior to simply designing a simple focal reducer system as a relay.  How well it works in practice depends on how well the system is toleranced, fabricated, and assembled--and that's where Celestron has experienced some real-world difficulties.

 

John



#10 555aaa

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Posted 08 December 2017 - 02:20 AM

Well, faint comets tend to be pretty small to really tiny. So you'd want to be at an image scale that's suitable for objects which are more like fuzzy stars. Seiichi Yoshida's web site below is a great source for current info on comets.  Sorry to be bucking the trend here but I'd be careful about going too far into the fast/wide angle configuration.  I think it depends a lot of the camera you have available.

 

http://www.aerith.ne...ly/current.html

 

The image below is at f/10 when the comet was at about 16th magnitude so my short answer is no, not too "slow". That doesn't really matter. What matters is the pixel scale. You want the largest aperture that illuminate the pixels in your camera  at the desired or available pixel scale (arc seconds per pixel). I think the better question to start with is what camera are you thinking of, then the optics can be matched to the camera. I think the new cooled CMOS one shot color cameras would be ideal here. Cameras and optics at this level are similarly priced, right?

 

comet2016a8_stack_crop.jpg


Edited by 555aaa, 08 December 2017 - 02:42 AM.


#11 freestar8n

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Posted 08 December 2017 - 06:24 AM

Slow vs fast has a tangible impact in terms of exposure time to make a nice picture - but in terms of whether or not it works well for an object it depends on how much additional detail you get.

If a comet is moving quickly and you can’t track it tightly then you won’t get a benefit of more detail at long focal length and you might as well use a faster system. But if you can track it tightly and there is detail at the arc second level then slow and high res has benefit.

In general if you want detail in the coma I would use long fl and slow - but if you want to capture the full extent of the tail I would use hyperstar.

Examples of both can be found on the web. I have done both hyperstar and f/10 with edgehd11.

As for the custom reducer - the edgehd reducer doesn’t just correct field curvature or something. It has to correct the full mess of aberrations that appear when you shift the primary way forward for use with the reducer. That pushes the imageplane way back from design and increases the f ratio. Each sct will react in different ways to this shift and a custom reducer is needed for good results across the field.

There are many good examples of unmodified edgehd’s with reducer producing tight images across the sensor. But it is sensitive to backfocus and alignment.

Frank

#12 P26

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Posted 08 December 2017 - 09:49 AM

Hi Tom,

 

Responses are shooting in all directions as your question lacks a number of specifics.

 

What do you consider a "dim" comet? Do you own the Celestron already or looking to buy one?  Do you own a camera or looking to buy one?  And if you own one, what is it? 

 

10 - 16th magnitude comets are usually quite small and benefit from a focal length of at least 2000mm.  But tracking becomes super critical at long focal lengths, and achieving pinpoint star images at 3500mm probably won't work out without adaptive optics.  For dim stuff you can use track and stack software to center on the comet.  Chip sensitivity is important.  Also critical is pixel coverage.  Based upon the Nyquest theorm 2 arc-seconds/pixel is optimum for this kind of work.

 

Regards,

 

Pete


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#13 Thomas Ashcraft

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Posted 08 December 2017 - 12:03 PM

Thanks to all of you for the varied replies. Very helpful. Here is a little more info.

 

I have been mostly doing planetary video work with Jupiter with a C-14 on a CGE Pro using a ZWO ASI 120. I have put in many hundreds of hours trying to catch a fireball striking Jupiter. No luck thus far but my system is pretty good I think for this specific purpose. The mount is unguided but tracks steady enough with Firecapture for useful imaging.

 

This year, while Jupiter has been out of range, I started imaging comets with the C14 using a Canon 6D and Backyard eos software. It all works ok for detecting fuzzy comas of dim comets but the unguided C14 is unwieldy and now needs to go back to Jupiter work. Hence I am thinking of a separate dedicated scope and system for comets using a spare CGEM-DX I have. I don't have much money to spend but I now have the comet bug and am beginning to explore the possibilities.

 

I can do average imaging but I know there is twice the detail to be had and would like to get ion tails and disruption events and time-lapses of motion and other subtle cometary phenomena.

 

Again I really appreciate the info and am studying every response.

 

 

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#14 Ron359

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Posted 08 December 2017 - 12:22 PM

If you're looking to contribute scientific useful images, you might look at the imaging requirements from this P.I. with the PSI that is studying activity on faint comets.   When I contacted him about imaging with a DSLR he replied they have never worked with DSLR images before.   Note, that they do all the image processing for details so that does save you a lot of work.   

 

got interrupted mid-post and forgot to add this link:

 

http://www.psi.edu/41P45P46P#useful


Edited by Ron359, 08 December 2017 - 05:44 PM.



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