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Recat 51 on the way - any suggestions or ideas for EAA

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#1 GaryShaw

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Posted 17 October 2021 - 12:51 PM

Hi

I finally succumbed and ordered a wider field scope for viewing, and perhaps imaging, larger celestial structures. This is also my first non-newtonian experience… I may even drift a bit into some AP lite. So, my question is whether any of my EAA colleagues have tried this little F4.9 scope for EAA and how that experience has worked out. If anyone hasn’t heard of this little scope, here’s a link:  https://williamoptic...t51_Reprint.pdf

 

Thanks for your thoughts or ideas on this scope.

Cheers

Gary

 

ps: I made arrangements to ensure this purchase will not bring cloudy skies to any of you…


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#2 Sacred Heart

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Posted 17 October 2021 - 06:27 PM

I do not have the scope, but have fun with it.  I have a 480mm refractor and a 550MM refractor and I'm having a ball.   With a small scope like that you may want to use your largest pixel size camera you have.  I would think a 3.76 micron would be better than a 2.4.   See if you can make a Bahtinov mask for focusing.  I live and die on mine.

 

My advice..ENJOY!!!              Joe



#3 arbit

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Posted 17 October 2021 - 10:07 PM

I started with an 80mm F6 APO and unlike most, worked my way down to a *smaller* aperture with the RC51 :-)

I use it for both EAA and AP. Livestack in SC4 and save the stacked frames for later processing.

Its been an excellent experience so far. Initially I wondered if its a "designer" scope, but nope. Excellent build, no need for flattener due to the optical design (at least for the 533MC), and built in tilter and bahtinov mask. The shoe for guidescope has multiple screws, keeping it very rigid (if you guide).

Just 2 points.

1. Its a helical focuser, so good idea to spend a few minutes during day time getting used to it. I find it holds focus extremely well.

2. The pixel scale with a typical 3.75um pixel is 3.1 as. Thats okay for widefield, but will obviously limit fine resolution You may want to go for a smaller pixel if you want finer resolution. If and when you get to AP can also drizzle while processing. Astrobin has many examples with the RC51.

Its been great so far. And widefield has a majesty all its own.



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#4 GaryShaw

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Posted 18 October 2021 - 06:35 AM

Thank you Arbit…..

 

I’ll test the Redcat with both my current cameras as well as the cooled mono 183 I’ll be ordering for photometry. Both my 178 mm and the 183 have 2.4 micron pixels so I’m looking forward to the wide field views with a narrow band Ha filter - hope the 1.98” image scale will yield some nice detail.

 

cheers

Gary



#5 alphatripleplus

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Posted 18 October 2021 - 07:33 AM

I think you'll have a lot of fun with that scope at f/4.9 with a small pixel camera, especially if want to dabble in wide field H-alpha shots. I have been using my AT72EDII reduced to f/4.4 for H-alpha and it works well. You'll probably find you need longer subs though in H-alpha, but still very doable for EAA.



#6 GaryShaw

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Posted 18 October 2021 - 08:20 AM

I’m hoping it will be ok but the Ha filter I have is pretty narrow so I expect some longer than normal exposures. Perhaps I’ll be able to just run shorter exposures and resign myself to longer overall integration times….anyway, I’m looking forward to learning about refractors and enjoying all of the Cygnus Loop in a single frame!

Gary 



#7 alphatripleplus

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Posted 18 October 2021 - 08:56 AM

You'll probably find that with shorter subs read noise becomes noticeable, since a narrowband H-alpha filter will dramatically reduce shot noise. My experience with two refractors in the f/4 to f/6 range, a 7nm H-alpha filter and the 2.9 micron pixels of the ASI290MM is to stick to at least 15sec subs and high gain, and I usually prefer to go longer than 15secs. However, you can use shorter subs, but you may start to notice the read noise contribution, even at a high gain value that minimizes read noise.



#8 arbit

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Posted 18 October 2021 - 08:58 AM

Before I used guiding, because of mount limitations, could do only half the minimum sub time recommended by SC for the LXtreme. Output was reasonably okay.

But there was a very visible difference in the detail once the guiding was on and I could do the right exposure time.

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#9 GaryShaw

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Posted 18 October 2021 - 09:58 PM

Hi Enance42

I received an email with your post ( no images though) but so far your post hasn’t shown up here in the thread. Perhaps I am the only one not able to see it.

Gary



#10 Larry Mc

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Posted 18 October 2021 - 10:33 PM

A friend of mine has a SpaceCat 51mm on a Skywatcher mount that he uses a DSLR on. That is one sweet scope!

He's been using a wide-field list that I put together a year ago. (went thru a period of using my ASI294MC camera on my 60mm guidescope to image with instead of guiding, LOL)

Anyway, here's my wide-field objects webpage, http://www.stellar-j...eField-tour.htm 

And the spreadsheet that I put together for it:  http://www.stellar-j...bjects list.pdf


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#11 GaryShaw

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Posted 18 October 2021 - 10:35 PM

Thank you Larry…!



#12 GaryShaw

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Posted 19 October 2021 - 11:13 PM

Do any of you have suggestions on a guides cope to work with the ASI290mm mini that I already own? I assumed I'd get the Williams Optics UniGuide scope but I see the threads are 42mm for that and the 290 mini has 28mm threads.



#13 YossiZ

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Posted 19 October 2021 - 11:26 PM

Do any of you have suggestions on a guides cope to work with the ASI290mm mini that I already own? I assumed I'd get the Williams Optics UniGuide scope but I see the threads are 42mm for that and the 290 mini has 28mm threads.

I use the ASI290MM-Mini with the ZWO mini guidescope. It works well for me.
The guide scope can't be aligned with the main scope so it can't be used as an electronic finder, so I added a red dot finder for initial alignment of the mount.

Edited by YossiZ, 20 October 2021 - 12:29 AM.


#14 arbit

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Posted 20 October 2021 - 01:01 AM

Do any of you have suggestions on a guides cope to work with the ASI290mm mini that I already own? I assumed I'd get the Williams Optics UniGuide scope but I see the threads are 42mm for that and the 290 mini has 28mm threads.

This is for the RedCat?

I have the ASI120 mini on the WO 32mm guidescope.

The GS has a 42mm thread as well as a 1.25 inch adaptor. You can directly screw on a planetary camera like the 224MC using the threads or use the adaptor and slide in the guide camera like the 120mini (290 mini is the same size as far as I know).

For the RedCat, I'd also recommend the Redcat saddle for very rigid guidescope mounting, unless of course you have that already.


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#15 alphatripleplus

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Posted 20 October 2021 - 07:46 AM

With the Redcat's 250mm focal length, for EAA you can most probably get away without guiding it. Of course, that will depend on how long your sub-exposures are, and your mount. 



#16 GaryShaw

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Posted 20 October 2021 - 08:29 AM

I’m inclined towards guiding in part since I like to take my time and often sketch while the image is stacking and while the detail is coming into its full bloom in Sharpcap. Others have indicated guiding is helpful for longer observations with AZ mount’s which is what I have. I’ve never tried guiding with the AZ Mount Pro so it’s all part of the great adventure with a new refractor.

 

My other mount is a CEM 70G which may not work well with such a light weight scope. I’d probably need to find a pretty small counterweight to get close on balance but even that may not work.

Gary



#17 alphatripleplus

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Posted 20 October 2021 - 09:36 AM

One thing to keep in mind is that if you have an Alt-Az mount, guiding will not have any impact on field rotation during a sub-exposure -  The star at the centre of your guidescope that you are guiding on won't move, but the rest of the field will rotate around it. So your maximum sub-exposure is likely to be determined by field rotation, which will depend on where the scope is pointing when using an alt-az mount. That's probably the main reason that I think most people only guide with equatorial set-ups. Just my 2 cents.


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#18 GaryShaw

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Posted 20 October 2021 - 01:27 PM

Thanks for confirming what I suspected Errol. I’m assuming Sharpcap will help with the rotation - yes?

#19 alphatripleplus

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Posted 20 October 2021 - 02:18 PM

Thanks for confirming what I suspected Errol. I’m assuming Sharpcap will help with the rotation - yes?

Yes, SharpCap will rotate one frame relative to the next in a live stack, but it can't compensate for field rotation during a particular frame's sub-exposure. That's why we often hear of people trying to estimate how long they can make their subs with an alt-az mount before field rotation is noticeable in individual subs. 


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#20 GaryShaw

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Posted 21 October 2021 - 07:39 AM

I’ve seen several versions to sky charts that indicate allowable exposure times based on sky locations. Does anyone have a sense of how accurate these charts are likely to be? In some locations, exposures were pretty short but most areas of the skies allowed for much longer exposures than I would have thought. It’s just this wide variance in exposure times that makes me wonder about the accuracy of these charts. If folks have used them with success, it would be interesting to know that.

thank you,

Gary 


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#21 alphatripleplus

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Posted 21 October 2021 - 08:04 AM

Gary, yes there is a wide variation in maximum exposure time with alt-az tracking  due to field rotation based on sky direction and the observer's latitude. For example, for someone at the earth's equator, field rotation near the celestial pole is very large, while there would be zero field rotation for the same observer when looking at a target on the celestial equator. 



#22 davidparks

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Posted 21 October 2021 - 10:43 AM

The RC51 is a great and yes fun scope to use for EAA, and for AP.  Extremely sharp, flat field, with a full frame image circle.  I use the APS-C sized ASI2600 with mine.  I've even grown to prefer the Helical Focuser.  While certainly preference for the focuser type is personal, I've found that having a ZWO EAF focus motor in-line mounted makes a more balanced, and easier to store in case setup than having the focus motor (if you use one) sticking out at a right angle.  Not a critical argument or a deal breaker in any case, but just something else I enjoy about the Redcat and its helical focuser.   Urban city sky glow / light pollution performance is much like any other scope, no better or no worse... I use mine fine for EAA in the city with a street lamp right in front of the house.  WO's built in focus mask is very convenient.

 

The RC51 works with the standard ~55mm backfocus, so you can use an OAG, Filterwheel/drawer, spacers as you normally would with your ASI Cams.

 

Your ASI290mm mini will work fine in either the ZWO mini guidescope, any of the WO guidescopes, or an OAG.  I use the ZWO OAG with the 290mm mini on the Redcat, along with a Filter Drawer with my OSC ASI2600.  EAA is fantastic, and the displays always draw people at my local club's public events.

 

RST-135 / Redcat 51 / ASIAIR Pro / ASI2600MC
 
RST-135 / Redcat 51 / ASIAIR Pro / ASI2600MC
 
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#23 GaryShaw

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Posted 22 October 2021 - 01:20 PM

David: I'd be interested to know what the case is and how tailored it was to your gear. Looks a bit like a Pelican Case with customizable foam.

Also, I see what looks like a Williams Optics 'saddle' on the RST. Was that an add-on you needed to accommodate the RC on the RST?

Thank you,

Gary



#24 davidparks

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Posted 22 October 2021 - 09:55 PM

The case is a Nanuk 935, a real performer.

Correct, William Optics Saddle Plate.


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#25 Ambart3561

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Posted 28 October 2021 - 04:46 PM

I’ve seen several versions to sky charts that indicate allowable exposure times based on sky locations. Does anyone have a sense of how accurate these charts are likely to be? In some locations, exposures were pretty short but most areas of the skies allowed for much longer exposures than I would have thought. It’s just this wide variance in exposure times that makes me wonder about the accuracy of these charts. If folks have used them with success, it would be interesting to know that.

thank you,

Gary 

Hi Gary

 

See post #37 here:

 

https://www.cloudyni...rotation/page-2

 

There are two references to equations used in these calculations. There is a graph for my Latitude based on calculations from a spreadsheet. I use it as an exposure guide to limit field rotation at whatever observing AltAz I'm on. Simple use is to pick an exposure kind of on the long side, take an image, check for rotation, use that exposure, or, go shorter or longer as needed. After using this for a while you get a feel for "sensitivity," I would not really call it "accuracy."

 

Est AltAz Exposure.jpg


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