Re: Close up of M81 without CCDT67


Roland Christen
 


Can you start making some smaller RC’s😀
RCs have two issues that prevent me from ever making them. Both primary and secondary have heavy aspheric surfaces that are extremely difficult to make smooth, and can potentially produce a lot of scattered light. The second issue is collimation which has to be very exacting.

The whole point of an RC was that it used only two reflective surfaces and did not require any refractive elements to eliminate coma, whereas a normal Classical Cassegrain and a basic Newtonian both have off-axis coma that require refractive coma correcting elements. Therefore the RC in a professional observatory could be used to provide a larger coma-free field and be able to pass all wavelengths from deep UV to far IR. The Classical Cass and Newtonian both need refractive elements which restricts the useful wavelength range.

Few people know that just because an RC eliminates coma, it does not inherently produce a sharp flat field. It still has field curvature and off-axis astigmatism, so in order to cover a wide field with pinpoint stars, it needs a field flattener with refractive elements. If that's the case, then it is actually easier to make a Classical Cass or even easier a Dall-Kirkham and do the coma-correction and field flattening with a simple refractive element near the focal plane. The D-K with its spherical secondary is miles less sensitive to mis-collimation and actually can produce a sharper flat field than an RC. The primary mirror is 4 times easier to make (and thus make well), and can be made with very smooth surfaces because of the very mild aspheric that is needed for full correction.

Rolando

-----Original Message-----
From: Robert Chozick via Groups.Io <rchozick@...>
To: main <main@ap-gto.groups.io>
Sent: Fri, Feb 14, 2020 9:40 pm
Subject: Re: [ap-gto] Close up of M81 without CCDT67

Can you start making some smaller RC’s😀Thanks for the suggestion. One of the problems is how sensitive the CMOS cameras are. It is very easy to blow out stars. With my FSQ at f/5 I can’t shoot over 2 1/2 -3 minutes before bright stars are blown. 

I worked hard on my collimation and it seems to have paid off.  I bought a focuser collimation ring and used a laser to collimate.   

Robert 


On Feb 14, 2020, at 4:56 PM, uncarollo2 <chris1011@...> via Groups.Io <chris1011@...> wrote:


I like the resolution on that Running Man image. Very nice.

Try masking down the outer 1/8 inch of the mirror, see if that reduces the spray of light around the brightest stars. If that works, think about adding a disc in front of the mirror with a diameter perhaps 1/4 inch smaller than the mirror.

I have seen this on all Russian RCs (RC Optical scopes) where a disc of slightly smaller aperture really helped. Most of the RC mirrors from Lomo had turned edge and were only spec'd to 95% of the full aperture. The very outer part produced a heavy spray of light around bright stars.

Rolando



-----Original Message-----
From: Robert Chozick via Groups.Io <rchozick@...>
To: main <main@ap-gto.groups.io>
Sent: Fri, Feb 14, 2020 3:47 pm
Subject: Re: [ap-gto] Close up of M81 without CCDT67

So, would you say the new advantages of the small pixel CMOS cameras is that they bring high sensitivity to small pixels?

I also got a shot of the Running Man with the 1600mm setup:


I got M101 at 1600 but the seeing was worse and guiding not as good. 


Robert



On Feb 14, 2020, at 3:10 PM, uncarollo2 via Groups.Io <chris1011@...> wrote:

You are running 0.6 arc sec per pixel, which to me is not oversampling for high resolution imaging. In fact, for galaxies i prefer 0.3 to 0.4 arc sec per pixel which really brings out fine detail. My 17"F8 astrograph and the QSI 683 has such pixel scale and really does a superb job on small faint galaxies, even here in Northern Illinois. You have to have good tracking, of course, and on the best nights I can get below 0.15 arc sec RMS with the 1600 encoder mount.

I see a lot of images that are way undersampled (3 to 4 arc sec per pixel) with poor guiding that produces thick stars and very little resolution. To me these images resemble Brownie camera snapshots versus images taken with an 8x10 view camera. (shows my age, doesn't it)  :^))

Rolando



-----Original Message-----
From: Robert Chozick via Groups.Io <rchozick@...>
To: main <main@ap-gto.groups.io>
Sent: Fri, Feb 14, 2020 2:14 pm
Subject: Re: [ap-gto] Close up of M81 without CCDT67

Thanks Roland.  My last dark sky outing was my first use of this scope and camera.   I am really confused on the whole image scale question.  I bought this camera because it has the largest pixels of any of the CMOS cameras. The image scale is .6 with this camera and 1600mm.   My scale is about 1.8 with my FSQ 106 - 530mm.  If the recommended guidance of a scale of 1-2 for image scale is used the 1600mm should be too small an image scale.  Most CMOS cameras have only 2.5-3.5 micron pixels vs 4.63 on my ASI294.  Is oversampling bad?  The .6 scale in my image sure looks ok.    What about .4 or .3?  I intentionally did not get larger than a 1600mm focal length because of this issue (and yes, guiding issues are not as bad vs 2000 and over).  Everyone asks why I got an 8 RC instead of a 10 or 12 inch RC.  The above reason is why.   Each new CMOS camera that comes out still has really small pixels.   How good would 2.5 micron pixels look on a 2500 mm scope?  

Robert

On Feb 14, 2020, at 1:18 PM, uncarollo2 via Groups.Io <chris1011@...> wrote:

That's really nice. Sharp and great color.

A question: do you think that 1600mm is a sweet spot for all kinds of deep sky imaging, especially for high resolution work? Especially since the newer Cmos cameras have such small pixels and would be able to take advantage of a high resolution optic for small faint galaxies.

Rolando



-----Original Message-----
From: Robert Chozick via Groups.Io <rchozick@...>
To: main <main@ap-gto.groups.io>
Sent: Fri, Feb 14, 2020 12:06 am
Subject: [ap-gto] Close up of M81 without CCDT67

I got another shot of M 81 on the same trip as the M81-82 image, this time at f/8 1620mm.


Robert Chozick




Robert Chozick




Robert Chozick



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