Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

C11 Edge HD and Celestion OAG performance

astrophotography Celestron SCT
  • Please log in to reply
5 replies to this topic

#1 Dovakun

Dovakun

    Explorer 1

  • -----
  • topic starter
  • Posts: 50
  • Joined: 16 Aug 2018

Posted 26 March 2020 - 10:03 AM

I've been imaging deep sky objects for about 2 years with an ES ED102. Last month I decided to pull the trigger and upgrade to a C11 Edge HD. With some clear skies and all the accessories needed to get the scope running I was able to set it up and start trying to image at 2800mm... Needless to say it took some time to align the finder scope, check collimation, focus and guide. I was able to take some 5 min subs on M1 and M51 and while guide was great for what I was using with the ed 102, I'm thinking it may need to be better on the C11. My average RMS error was about .4. RA was as low as .19 but the Dec would spike every once in a while causing the ASIair to lose the star briefly. Has anyone experienced this? There are a few things I want to try such as improving the spacing for the OAG to get better focus and replacing the dc adapter for the dew heater (the 15v adapter is too high for the Thousand Oaks controller with the 11" strip). Below is a cropped image of M51 autostretched in PI taken during first light. The dark rings of course are dew which is easy enough to fix, but the star trails are going to be a bit trickier. Does anyone have tips for improving guiding or have the acceptable RMS error for a scope at this focal length?
 
Right now I'm running the scope into the adapter to the celestron OAG, into a zwo asi120mm-s guide camera without extension tube, then I have the 16.5mm spacer, 11mm spacer and the ZWO asi294mc pro. Does anyone have an example of the spacing for this setup? I know the asi294 requires 55mm of back focus but with an OAG I'm not sure where that should be measured since it isn't a lens like the reducer for refractors.

 

The mount in use is a CEM60 with 2 21lb counterweights.

 

M51test.jpg



#2 jdupton

jdupton

    Mercury-Atlas

  • *****
  • Posts: 2,600
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 26 March 2020 - 10:35 AM

Dovakun,

 

   I see you are using the CEM-60 mount in your signature. Given average seeing in most places, your RMS guiding of 0.4 doesn't look bad at all regardless of whether this is in arc-seconds or pixels. (in fact, it looks pretty darned good to me.) At 2800 mm focal length with your ASI294MC-Pro camera, you are shooting at an image scale of about 0.341 "/px. Compared to your guiding, you are doing great in my opinion. If your local seeing is around 2", then even the smallest stars in your images will be roughly 6 pixels across. A 1/2 arc-second guiding error will be barely noticeable on all but the very tiniest of stars.

 

   You may be able to help the occasional momentary loss of guide star by expanding the guide box in PHD2. I use 25 to 29 pixels for my guide box most of the time to help with recovery after wind gusts or other things that cause a quick star movement. I think that your occasional momentary star losses are pretty normal at this image scale. I see them with my 11" EdgeHD which is why I run with a larger than default guide star search box in PHD2.

 

   Regarding star trails, these may be due to polar alignment and the distance off-axis of the guider. The combination leads to field rotation that can become visible at this image scale. You should at least try to do a very careful polar alignment (using a drift method preferably) for one session to see if trailing diminishes. I set up for several days at a time and drift to a point where I get less than a few arc-seconds of drift per hour unguided. Using a modified version of the DARV Method, it takes about 20 minutes to refine my polar alignment to that point.

 

   Do not worry about any "55 mm back-focus requirement." It is meaningless for the camera when used with the 11" EdgeHD. (The 55 mm only applies to non-EdgeHD focal reducers or field flatteners.) The only specification that needs to be maintained as closely as possible is the distance from the last glass surface on the EdgeHD correction lenses inside the baffle tube to the sensor in the camera. That distance should be set to as close to 146 mm as possible using whatever extension rings you have available.

 

 

John



#3 Dovakun

Dovakun

    Explorer 1

  • -----
  • topic starter
  • Posts: 50
  • Joined: 16 Aug 2018

Posted 26 March 2020 - 11:41 AM

Thanks for the response! Good to know about the 55mm of back focus. I was reading that spacing may affect the guide focus though since it's using the same focus as the imaging camera. I definitely want to get the .7 reducer at some point, so maybe that will help as well. Regarding the polar alignment, I'm using a polemaster and the RA being so low makes me thing that polar alignment is pretty spot on. I'm wondering if there is something like balance or wind that is causing the DEC error. Good to know that .4 is good guiding though. It's the best I've ever seen it coming from using a guide scope.



#4 jdupton

jdupton

    Mercury-Atlas

  • *****
  • Posts: 2,600
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 26 March 2020 - 12:01 PM

Dovakun,

 

   When you get the EdgeHD 0.7x focal reducer, nothing will have to change in your set-up. That is the real joy of the matched EdgeHD reducer. For the 11" EdgeHD scope, adding the matched focal reducer results in exactly the same 146 mm back-focus requirement. I can use my scope with or without the EdgeHD reducer and nothing in my imaging train has to change after removing or adding it.

 

   Once you get a full imaging session in with your set-up, you can objectively determine your polar alignment quality. Just stack all of your sub-exposures without aligning them first. Any drift will result in star trails of course because they are not aligned with one another before stacking. Measure the total length of the star trails in pixels. Divide by the time difference in minutes between the first image of the session and that of the last. You now have your drift rate in pixels per minute. Multiply that result by the length of you sub-exposures in minutes. If the resulting number is greater than 1.0 (pixels) then you could benefit from improving your polar alignment even more.

 

 

John


  • Dovakun likes this

#5 lucutes

lucutes

    Messenger

  • *****
  • Posts: 417
  • Joined: 17 Oct 2016
  • Loc: Western Canada

Posted 26 July 2020 - 05:00 PM

Currently looking for a picture of a C9.25 or C11 setup with the Celestron OAG and ASI-Pro camera. Having issues with backfocus behind the OAG. I am using the Focal Reducer and the Large SCT Adapter but having issues with the spacers behind the OAG. I hear its preferable to add space ahead of the OAG using a Baader Clicklock adapter (which I have) but I am not sure how to connect it to the OAG after.  Any help would be appreciated.



#6 jdupton

jdupton

    Mercury-Atlas

  • *****
  • Posts: 2,600
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 26 July 2020 - 05:57 PM

lucutes,

 

   This may not help you much. I use a Moonlight CHL focuser which eats up almost exactly what is needed in backfocus when paired with the Celestron OAG and an ASI294MC-Pro camera. The following link to another post I made here on CN shows the setup and lists the various adapters I use.

 

https://www.cloudynights.com/topic/707312-asi294-and-celestron-oag/#entry10187833

 

   You would need to use one or more adapters between the visual back and the Female M42 T-thread Adapter (part of the OAG kit). That would take up the same space ahead of the OAG as is used by the focuser in my setup.

 

 

John




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: astrophotography, Celestron, SCT



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics