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RASA 8" filters and spacing

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#1 RRod

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Posted 17 September 2019 - 04:13 PM

Hello

I am not sure if this topic has been discussed somewhere in here, if yes, please direct me.

 

I have been wondering if the RASA spacing needs to be adjusted depending on the filters we are using. Celestron indicates that if we use filters the clear window needs to be removed. So this clear glass window is 2mm thick, so it means that if we use a filter for examples, an Astronomik filter which is 1mm thick, we need to increase the spacing 0.5mm or if we use a 3mm thick filter we need to reduce the back focus spacing by 0.5mm. Would this be correct? I think the RASA is very touchy and spacing is critical for perfect star field.

So far, I have the telescope which I send for a repair due to mirror tilt and I got it back, but the weather looks bad for the next week, so I am wondering if anyone have look into that.

 

Thank you

 

Rodrigo



#2 caseyfinn

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Posted 18 September 2019 - 12:32 AM

Your logic is sound and some of those thin shims Zwo send with their cameras might help but I haven’t had to fuss with that while changing filters. When Using osc, put in the light pollution filter instead of the clear glass and refocus. Later in the month when the Moons out and it’s time to change to mono, no problem just take that filter out, switch cameras and add a ha filter all the while leaving the filter slide in place.

 

here are some tips I’ve learned from those same oblong stars you mentioned earlier: your mount needs to be solid as these are fairly big, long and heavy-compared to a refractor. Don’t be shy firmly attaching the camera rotator, use both hands so theirs no play. A 3D printed plastic cable guide helps with flexure in the imaging train and reduces those funky diffraction spikes. I got a Starizona filter slide to change filters, it was designed for Zwo cameras with 17.5mm back distance. The glass insert that comes with the scope is to be left in place until you change it to one of their /Astrodon light pollution filters which I really like when the moon isn’t too strong. If you plan on mono then you’ll need a Baader or Starizona filter slide. Have fun



#3 RRod

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Posted 19 September 2019 - 01:32 PM

Thank you for your message. Just a question, what cameras  are you using? I use the ASI 294 in combination with IR filter which I mount into the filter slider, but I remove the clear glass window. When I do narrow band, I switch to ASI 1600 and mount the Ha filter, with the clear glass removed. Do you leave the clear glass when you use the narrow band filters? if you don't use LP filters, do you add IR filter? I can't find anywhere if the clear glass window has IR/UV coatings

 

 

Rodrigo



#4 caseyfinn

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Posted 19 September 2019 - 04:47 PM

All Zwo: Asi183mm Pro with Baader highspeed F2 filters (cleanest shots, besides halos around bright stars with O3)  and Asi183mc pro and Asi294 with Celestron Light pollution filter (works ok from my bortle 7 skys). I've tried the Triad and Omega NBP with little success.  I forgot about the IR-UV cut being necessary. Celestron says the RASA is optimized for wavelengths between 390-800nm.

 

If you leave the clear glass in place and use extra glass you will have donuts around the edges, or other fun things to chase around. I used my light pollution filter with narrowband by mistake for a while.

 

I'd like to try the Asi071mc and see how it looks. Maybe later.

 

Here are some of my photos, mainly with the RASA: https://telescopius....file/Casey_Finn

 

Cheers



#5 RRod

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Posted 19 September 2019 - 09:54 PM

Very nice pictures!! How long are your Ha subs? One thing I noticed with the RASA 8”, is after focusing any star in the center, the stars in the corner look out of focus, if then I move the focuser to focus those stars everything looks good. I was expecting the stars in the rest of the field will go out of focus. This behavior does not make sense to me. I already sent the scope to Celestron for that and now I got it back with the same behavior. I wonder if this is a design issue


Edited by RRod, 19 September 2019 - 09:54 PM.


#6 caseyfinn

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Posted 20 September 2019 - 12:09 AM

I start with 60 sec subs in narrowband. Check the mean, looking for around 1200. Then adjust time or gain from there.  

 

The skyx has focus3 that works really well. Start of by getting  rid of the donuts so you’re close to focus. Take a 4 second shot, zoom into a corner with small but not tiny stars and select that as your focus area. Hit autofocus and it’ll take a bunch of short shots, plot them out and find a nice curve. I usually get between 1-2.2 fwhm (depending on seeing) and that focus will be great for at least an hour while the temperature outside drops like a rock. Desert weather. I’m using the Celestron focus motor btw.

 

The smaller chip of the 183 is always in focus, evenly, but the 294 not entirely. It’s got a larger sensor. Your 1600 is bigger still. At the end of the day you can always just crop out whatever you don’t like.

 

Not all optical systems are designed the same. Celestron says the 8” has an image circle of 22mm while the 11 has a 44mm imagine circle. They say something about a larger usable field for each, but at F2 who knows. You might try to make really good flats also. That’s what I’m gonna work on to use more fov.

 

Cheers



#7 RRod

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Posted 20 September 2019 - 07:42 AM

Thank you for the information. I think my problem is the focus routine since I am using APT and  it does not have a good one. Most of the focusing programs are not stand alone applications so it becomes a higher investment Motor focuser helps a lot. I have Rigel systems

Rodrigo




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