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Recommend a telescope with FL>>600mm for moon, terrestrial imaging, maybe astro...

reflector imaging
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#1 bokemon

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Posted 06 August 2020 - 07:20 PM

Hello folks,

I recently got a CEM25P mount to do some astrophotography.  The equipment I already have is a Nikon full frame camera, Micro 4/3 camera, 180mm prime lens, and Sigma 60-600mm lens, which BTW is at least "pretty good" for regular photography.  It seems most of the refractor telescopes under $1000 are around 400mm FL and F6, so it overlaps too much with a lens I already have.  I want to get a reflector telescope with focal length much more than that for the following things:

Get a very zoomed in picture of the moon.

View and photograph terrestrial objects.

Maybe do some astrophotography if a reducer can somehow work to make the focal length "manageable on this class of mount" like 900mm or so.

 

There's a few things I am not really clear on.  One of them is the notion of "image circle".  I get that sometimes the image circle will be smaller than the sensor size of my camera, so then I may as well use the M43 camera, or buy one of those custom astrophotography cameras. Let's say hypothetically there's a 6" reflector with 1500mm focal length.  But if I use my M43 camera on it, which has 3.75" pixels, that gives approx 200 x 3.75um / 1500mm = 0.5 arcsec/pixel.  I read the "seeing limit" is somewhere around 1-2 arcsec, so would this setup be pointless?  Should I just use the full frame camera with 6 um pixels in DX crop mode?

 

The other question is regarding focal length reducers.  My understanding is that they literally take the image circle and shrink it down in size while making it brighter.  Let's say originally I was using the FF sensor and it had a little vignetting.  I use a focal reducer, and now the image circle got a lot smaller, so I have to use the M43 camera with smaller pixels.  But I end up taking (almost) the same picture, and the increased aperture (F stop?) gets cancelled out by the smaller pixels.  So how was this useful?

 

I guess for photographic purposes it has to have large image circle, flat field, low coma, etc?



#2 cst4

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Posted 06 August 2020 - 08:07 PM

My suggestion would be a C6. With a 0.63 focal reducer the focal length is near 950mm which seems to be what you are looking for. And those focal reducers make for a nice flat field. The native focal length of 1500mm is great for moon shots and telephoto wildlife shots. Plus the image on the camera is right side up correct image... I don’t think that is the case with Newtonian reflectors. Plus a C6 is a very nice, portable, and relatively cheap telescope.



#3 17.5Dob

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Posted 06 August 2020 - 09:27 PM

Hello folks,

I recently got a CEM25P mount to do some astrophotography.  The equipment I already have is a Nikon full frame camera, Micro 4/3 camera, 180mm prime lens, and Sigma 60-600mm lens, which BTW is at least "pretty good" for regular photography.  It seems most of the refractor telescopes under $1000 are around 400mm FL and F6, so it overlaps too much with a lens I already have.  I want to get a reflector telescope with focal length much more than that for the following things:

Get a very zoomed in picture of the moon.

View and photograph terrestrial objects.

Maybe do some astrophotography if a reducer can somehow work to make the focal length "manageable on this class of mount" like 900mm or so.

 

The moon is small...as in really SMALL !!!

You need 2,200mm just to fill the frame using a FF camera...More if you want a "close up" without cropping....You need 1,400mm to fill the frame of a M4/3...and more if you want a "close up" without cropping

The big problem with trying to use a long fl Newt/  standard SCT during the daylight is the extreme field curvature..You might not notice it when you crop down to just a few moon craters in the center of an image, but it will be glaringly obvious when used for daylight photos.....you'll also need some way of adapting the dovetail to work on a photo tripod...and another a side note...

 

How good/quick are you at manually focusing a telescope, while looking through your cameras view finder, and setting manual shutter speed , because no camera can "auto anything" when uncoupled from a lens....It can easily take you several minutes to get perfect focus and set a shutter speed. Hopefully your intended targets are inanimate...

 


Edited by 17.5Dob, 06 August 2020 - 09:32 PM.


#4 Stelios

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Posted 06 August 2020 - 10:23 PM

There's a few things I am not really clear on.  One of them is the notion of "image circle".  I get that sometimes the image circle will be smaller than the sensor size of my camera, so then I may as well use the M43 camera, or buy one of those custom astrophotography cameras. Let's say hypothetically there's a 6" reflector with 1500mm focal length.  But if I use my M43 camera on it, which has 3.75" pixels, that gives approx 200 x 3.75um / 1500mm = 0.5 arcsec/pixel.  I read the "seeing limit" is somewhere around 1-2 arcsec, so would this setup be pointless?  Should I just use the full frame camera with 6 um pixels in DX crop mode?

 

The other question is regarding focal length reducers.  My understanding is that they literally take the image circle and shrink it down in size while making it brighter.  Let's say originally I was using the FF sensor and it had a little vignetting.  I use a focal reducer, and now the image circle got a lot smaller, so I have to use the M43 camera with smaller pixels.  But I end up taking (almost) the same picture, and the increased aperture (F stop?) gets cancelled out by the smaller pixels.  So how was this useful?

 

I guess for photographic purposes it has to have large image circle, flat field, low coma, etc?

0.5"/px is on the oversampled side, but not terrible. I image frequently at 0.56"/px with my Edge 800" and a 0.7x reducer. The rule of thumb is that at about 1/3rd your seeing you are still doing OK. And if worse comes to worst, you can always bin or downsample after the fact. Check my Astrobin for some images with the Edge (mainly most recent ones).

 

*My* understanding of a focal reducer is that it increases the FOV and speeds up the F/ratio. What it does to the image circle (it will be brighter at same exposure of course--required exposure for same brightness goes by square of F/ratio) depends on the scope and the reducer. Most (but not all) reducers will produce an image circle with minimal vignetting on an APS-C sensor. A quality reducer will have precise specs available. 

 

For what you want to do, I recommend an Edge HD SCT, or, if this is over budget once you add reducer, one of the ONTC (best) or UNC Newts by Teleskop-Service. 

 

HOWEVER, note that none of these large scopes will be suitable for use with the CEM25P. You should get something like an EQ6R-Pro if you want to go big. 



#5 17.5Dob

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Posted 06 August 2020 - 10:45 PM

For what you want to do, I recommend an Edge HD SCT,

And try mounting that on a photo tripod for terrestrial use...
 

"I want to get a reflector telescope with focal length much more than that for the following things:

.

.

View and photograph terrestrial objects."
 

But I do agree, an Edge is probably 4X the capabilities of the CEM25 on the AP side

 


Edited by 17.5Dob, 06 August 2020 - 10:50 PM.


#6 17.5Dob

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Posted 06 August 2020 - 10:56 PM

To the OP.....I have yet to see any camera zoom lens, match the sharpness of a small refractor..While you will gain no additional additional  fl over your Sigma  zoom, you will gain a lot of sharpness, allowing you to crop more aggressively......



#7 bokemon

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Posted 07 August 2020 - 12:17 AM

 

*My* understanding of a focal reducer is that it increases the FOV and speeds up the F/ratio. What it does to the image circle (it will be brighter at same exposure of course--required exposure for same brightness goes by square of F/ratio) depends on the scope and the reducer. Most (but not all) reducers will produce an image circle with minimal vignetting on an APS-C sensor. A quality reducer will have precise specs available. 

 

For what you want to do, I recommend an Edge HD SCT...

Thanks for the reply.  What you said about the focal reducer is correct, if you start off with an APS-C sensor.  In the case of the Edge HD telescopes, it appears that the original image circle is "good enough for full frame sensor" (which is what I have), and using the focal reducer, it is "good enough for APS-C.  In my case, using the reducer will just create a lot of black edges in my image and I may as well use DX crop mode on my camera, thus dropping from 24 MP down to around 10 MP.  Of course, the center of the image will be brighter.  Now this is the part I am not 100% sure about, but I think if I just took the original 24 MP picture without the reducer, and down-sized it to APS-C size, it should have a similar signal to noise ratio and all that.

 

Well, in any case, the Edge HD series of telescopes you mentioned seem like a good idea because they claim to have flat field, low coma, etc etc.  Too bad there is not a 6" version of it.   I think Meade has a 6" version, which they call ACF.  Do you know of any others?



#8 bokemon

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Posted 10 August 2020 - 01:55 AM

Hello again,

So I've been looking thru different types of scopes, and this is what I found out so far:  (I could be wrong on any of these, so...)

 

For SCT type scopes like the C6, the "baffle size" is already 27mm, so that means the image circle will be even smaller than that, and if I use a reducer, even smaller again.   Plus the focal length is kind of long, and the aperture is kind of small, so that kind of rules it out for my purposes.

The 8" SCTs are kind of too heavy for my mount, plus they have 2000mm focal length, so too long.

 

6" RC seems ok, with slightly shorter focal length and F9, plus the image circle tends to be larger due to the larger secondary mirror.  According to this thread https://www.cloudyni...telecompressor/ I can get even 34mm image circle with 0.75x compression.  Apparently the problem is "low contrast"?  Can this be fixed with just a contrast adjustment, or is some information permanently lost?

 

Some "imaging Newtonian" like 6" F5 or even the 8" versions from TS if they are made from carbon fiber, they will be light enough.  But for the cheaper ones it is hard for me to find specs about image circle size.

 

Are there any other types I missed?  At this point, I am aiming for focal lengths >900 but <1500 and as large an image circle as possible.  (Maybe this is an unreasonable request since everybody else likes to use APS-C cameras or those specialty cameras with dinky little sensors.)

 

For all of these, it looks like I will have to spend at least $150 more to get field flattening or coma correction, etc.  BTW, is there such a thing as a 2" "tele-extender" that can increase the focal length while at the same time expanding the image circle the same amount?  This is to make more of the image fit across a full frame sensor.  And if something like this exists, wouldn't I be better off getting the F4 newtonian instead of F5 or 6?




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