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What camera for the RASA 8"?

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#1 Monsy

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Posted 16 September 2019 - 06:14 PM

Hey!

 

Got a new RASA 8" telescope and would like to find a proper camera for it. I'm not planning to use the scope with filters, for the ease of use I would like to use an OSC camera. So I'm not sure if a Zwo Asi 183MC Pro or the Zwo Asi 294MC Pro is the better choise.

 

I started with some theoretical thoughts:

 

While the 294 camera has almost the ideal sensors size for the RASA 8 (perfectly illuminated field) it has a pixelsize of 4,63µm. The 183 camera has a quite smaler sensor (diagonal ~16mm) but has a pixel size of 2,4µm.

 

Using the formula 206*pixelsize/focal lenght to get the arcsec/pixel for the RASA 8-camera-combination, I get for the 294 camera 2.4 arcsec/pixel and for the 183 camera 1.3 arcsec/pixel. Let's assume a not that perfect seeing and some tracking issues and so on and assume the resolution I will get on a good night is in the range of 3 or 4 or 5 arcsec. Using a 183 camera with 1.3 arcsec/pixel the sensor would be capable of giving a resolutions of 3.3x it's arcsec/pixel value, that is for the 183 cmaera 4,29 arcsec, well in the range of my approximated seeing. Using the 294 camera with 2.4 arcsec/pixel, the sensor is only capable of giving a resolution of ~8arcsec - well below my average resolution due to seeing condiction etc.

 

So now I'm not sure what's better: larger field, but very much undersampling the average seeing with the 294 camera.... or a smaller field but a very good fit of the sensor resultion for my expected seeing with the 183 camera.

 

I read and saw the users got some nice pictures with the 294 camera (e.g. Trevor from Astrobackyard). What do you think? What would be your choise and why?

 

Looking forward to hear your opinion :-).

 

Regards,

Roman

 

P.S.: The Zwo Asi 1600 camera is no choise for me, cause it's a mono and I don't find a dealer still listing the color versions of this camera here. If available the 1600 MC color camera would be my first choise because of large sensors and a pixel size in between the 183 and 294 camera...


Edited by Monsy, 16 September 2019 - 06:16 PM.


#2 Michael Harris

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Posted 16 September 2019 - 06:51 PM

I have been imaging recently with a classic C8 and Fastar, which approximates (very roughly) the RASA. So far the ASI290 color and ASI178 mono cameras have worked well (color for quick DSOs, mono for asteroids). I have the ASI294MC arriving this week with the Hyperstar v4, which seems to be a popular combination. So although I cannot speak to the 294MC yet, it seemed the best choice for me.



#3 APshooter

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Posted 16 September 2019 - 11:41 PM

I had the same decision to make.  I already use the 1600mm with narrowband filters.  I wanted to add an OSC camera to the mix and Tolga suggested the 294 over the 183.  In my case the 294 chip is a better fit for my purposes, adding color to otherwise mono images.  Incidentally, the 294's pixels are almost identical in size to the Sony ICX694 chipsets 4.54um ones.

 

The 183 has a smaller chip size and wouldn't be as good a fit with the larger 1600mm chip.  Since I have not ordered one yet I can't comment on how well it works in the field.


Edited by APshooter, 16 September 2019 - 11:44 PM.


#4 kingjamez

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Posted 17 September 2019 - 12:23 AM

I use the 183mm, 183mc, and 1600mm on my Rasa 8. I love the larger field of the 1600mm but the larger pixels are very very noticeable. The stars are blocky and resolution is lost.

I’ve done considerable testing on the bubble nebula between the 3.8um and 2.4um pixels and the wisps of the nebula inside the bubble are far better defined on the 183. To that end, when the subject allows I always prefer the 183.

The other “advantage” of the 183 is that the smaller sensor makes tilt and collimation issues less noticeable. Of course you can do the same thing with a larger sensor by cropping.

Finally, the 183 is happy with 1.25” filters whereas you’d probably need larger filters for the 294.

-Jim
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#5 Monsy

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Posted 17 September 2019 - 05:55 AM

Thanks for your help ad suggestions! Trying out the 183MC Pro now!

#6 Monsy

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Posted 21 September 2019 - 10:27 AM

I use the 183mm, 183mc, and 1600mm on my Rasa 8. I love the larger field of the 1600mm but the larger pixels are very very noticeable. The stars are blocky and resolution is lost.

I’ve done considerable testing on the bubble nebula between the 3.8um and 2.4um pixels and the wisps of the nebula inside the bubble are far better defined on the 183. To that end, when the subject allows I always prefer the 183.

The other “advantage” of the 183 is that the smaller sensor makes tilt and collimation issues less noticeable. Of course you can do the same thing with a larger sensor by cropping.

Finally, the 183 is happy with 1.25” filters whereas you’d probably need larger filters for the 294.

-Jim

Hey Jim,

 

another question: do you have a flat field with the 183mc pro? I have bad problems to get the whole field in focus with my RASA 8. At least some corners do always show little donuts while the center of the image is in focus. Tried two (new) cams now, same issue. Fear that I have a tilted part in the RASA like written elsewhere here. Just to make sure, with the 183 mc pro, do you get pinpoint stats in every corner (assuming no tracking issues). Think I have to send in my RASA...



#7 kingjamez

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Posted 22 September 2019 - 01:01 PM

Hey Jim,

 

another question: do you have a flat field with the 183mc pro? I have bad problems to get the whole field in focus with my RASA 8. At least some corners do always show little donuts while the center of the image is in focus. Tried two (new) cams now, same issue. Fear that I have a tilted part in the RASA like written elsewhere here. Just to make sure, with the 183 mc pro, do you get pinpoint stats in every corner (assuming no tracking issues). Think I have to send in my RASA...

Well, yesterday I would have said yes but I went back and looked at sub from last night, it's not perfect. I had it quite good last week.

 

When I went to investigate, I found one pair of my push/pull collimation screws was loose. I must have not tightened it enough when I collimated it last.

 

I highly recommend not sending it to Celestron as I don't trust the shipping process to not knock it out of collimation. It is possible to collimate it yourself, the instructions are correct in the manual. It may take some time to dial in since I think it's probably best to do fine collimation by examining the edges of your actual imaging camera, not just by having a well collimated star in the center. 

 

It's a lengthy, but not difficult process, and certainly takes a lot less time than shipping it to celestron. 

 

Here is today's abberation inspector... I'll recollimate tonight under the stars now that I know my screws were loose. 

Attached Thumbnails

  • NGC281__15Cc_Ha_Focusfpos_10866_120secs_22_023637_frame1_mosaic01.jpg

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#8 RRod

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Posted 27 September 2019 - 09:11 AM

Hey Jim,

 

another question: do you have a flat field with the 183mc pro? I have bad problems to get the whole field in focus with my RASA 8. At least some corners do always show little donuts while the center of the image is in focus. Tried two (new) cams now, same issue. Fear that I have a tilted part in the RASA like written elsewhere here. Just to make sure, with the 183 mc pro, do you get pinpoint stats in every corner (assuming no tracking issues). Think I have to send in my RASA...

Hello

I think your problem is not a collimation issue. I had the same issue and I sent the scope to Celestron. It came back with the same problem. Once I reach good focus using a star in the center of the field, the left corner shows doughnuts. So try to focus only in one direction, try to do it pushing the mirror forward up the tube.

It is easier if you have a motor focuser because you can calculate how many steps you move the focuser and you can go inwards pass the focus point for 200 steps for example and then go forward. It seems like the mirror tends to tilt a bit and falls square when moved in a particular direction. Celestron also commented that the focusing mechanism has some backlash and can be affected by the direction you approach focus from. I think that could explain why some people do not see this problems and others do, since the backlash could vary from telescope to telescope.

I also observed something interesting, after making sure the stars in the center were in focus, I moved my focuser enough to focus the out of focus stars in my corner (took about 400 steps), it is crazy all looked good after that. I was expecting that the stars in the rest of field  will go out of focus but they there were fine.

So my take from this is the focus tolerances are very small in this telescope and the smallest mechanical errors are a big deal. 

Hope this helps 

Rodrigo


Edited by RRod, 27 September 2019 - 09:19 AM.

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