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Old 15-04-2019, 08:36 AM
glend (Glen)
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RASA 8", Can it do narrowband?

Thinking of ordering the 8" RASA but reading through the Bintel information on this scope made me realise it may have narrowband filter problems. Running at f2 is a big plus for narrowband, but the restrictions placed on the little RASA may make it impossible. From what I can gather in my reading the 8" RASA has a 46mm optical window ahead of the camera, and this optical window can be replaced by the special light pollution (CLS) type filter that Celestron is offering for this scope. I am not sure if narrowband filters can be used, in addition or in place of, the optical window, due to the sensitivity of the focal length of the RASA. Obviously, adding a narrowband filter between the optical window, say to the front of a CMOS camera is going to alter the focal length slightly, but is it that touchy? Further, the restrictions on cameras that can be used, mean that backspace to the sensor is very tight. My ASI1600MM-C, without the filter wheel obviously, might be ok, but i need to either be able to place filters ahead of it, or switch to OSC.

https://www.bintel.com.au/product/ce...v=6cc98ba2045f

Edit: I have found the Starzonia Filter Slider for the RASA 8,

https://starizona.com/store/starizon...a-8-zwo-cooled

However, the spacing and addition of another bit of glass still exist. Baader makes a 2" f2 set of narrowband filters, but allow another $1000 USD for those. That factor alone may push it out of my reach financially.

Is anyone using a RASA 8" for narrowband? Or have any comment of filter use beyond the CLS special?

Last edited by glend; 15-04-2019 at 08:52 AM.
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Old 15-04-2019, 09:05 AM
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Have a look at Dylan O'Donnel's yotube channel. They would be a bit of a pain but it can be done if your backspacing on the cam is tight enough. He was using a filter drawer with a custom made plate to mount the camera closer. It means swapping filters manually but when you can image at F2 it might be worthwhile.

I was a bit tempted by the 8" RASA idea as they just about look like they are designed with a camera just like mine in mind, but the budget won't stretch to it for a couple of years.

Edit of my own: That Starizona filter drawer is basically what Dylan was using. Looks like they have put into production the part he had custom made to tighten up the spacing, they have also added the knob to pull the drawer out, without that it is a job for your fingernail as the normal groove for the job is not accessible on this setup.
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Old 15-04-2019, 09:32 AM
glend (Glen)
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Thanks Paul, I will check out what Dylan is doing. I just realised that the RASA 8 is almost exactly the same focal length as my new TS71SDQ, which already is doing narrowband with existing equipment. So sure the RASA is faster but probably not enough to justify the buy. Time I have. I do have a tendency to get excited about something and then step back when the light of day hits it, so to speak.
So I guess it has just become a general knowledge question then, for other potential RASA 8 buyers.
Must put a note on the fridge: "Do not buy another telescope"!
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Old 15-04-2019, 09:41 AM
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I have been looking at this set up and note that apparently an electric focuser is planned and I have a feeling that out of any set up the RASA would find the greatest benefit.
I am guessing but I expect ease of focusing may be the most important issue assuming that one can change a selection of filters with relative ease manually.

Another thing I wonder about...
It will need a dew tube and it will extend past the camera and therefore perhaps turbulance from the (presumably hot) air from the camera may be an issue.
I would really like one of these but I will wait and see how things go..I would like to read reports of one using interchangable filters, an electric focuser and a suitable dew tube...
This set up would also benefit from a curved sensor ..pity that the curved sensors curve the wrong way...maybe in time☺
Alex
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Old 15-04-2019, 09:42 AM
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Camelopardalis (Dunk)
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Glen, you’ve got a case of guinea-pig-itis?

I’m curious as to how that would work out, f/2 would rip through the sky in no time!

An alternative to avoiding the band shift problem is to use wider band pass filters. This doesn’t consider any special coatings, but I’m not sure they would be any more or less necessary
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Old 15-04-2019, 09:43 AM
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Quote:
Originally Posted by glend View Post
Must put a note on the fridge: "Do not buy another telescope"!
You beat me to it

Interesting all the same
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Old 15-04-2019, 09:45 AM
glend (Glen)
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Quote:
Originally Posted by xelasnave View Post
I have been looking at this set up and note that apparently an electric focuser is planned and I have a feeling that out of any set up the RASA would find the greatest benefit.
I am guessing but I expect ease of focusing may be the most important issue assuming that one can change a selection of filters with relative ease manually.

Another thing I wonder about...
It will need a dew tube and it will extend past the camera and therefore perhaps turbulance from the (presumably hot) air from the camera may be an issue.
I would really like one of these but I will wait and see how things go..I would like to read reports of one using interchangable filters, an electric focuser and a suitable dew tube...
This set up would also benefit from a curved sensor ..pity that the curved sensors curve the wrong way...maybe in time☺
Alex
Alex I believe Celestron have already released an electric focuser for their SCTs. The RASA focuser looks to be the same focuser. Yet again something else to spend money on.

Here is the CN thread on the RASA 8, note that one guy is running the Celestron Motor focuser.

https://www.cloudynights.com/topic/6...ges-yet/page-5
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Old 15-04-2019, 10:57 AM
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Yep, the focus motor I have put on my SCT is suited to the RASA 8" as well as the 11" with a retrofit kit to put a standard focus assembly in it, the 11" are so twitchy to focus that they come fitted with a two speed feathertouch off the shelf now.
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Old 15-04-2019, 01:55 PM
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I thought the RASA focuser was a custom design by Celestron, BICBW. I just seem to remember one of the guys on CN, who puts out some great images taken with his C14 btw, had been talking about consulting with them on it.

But for sure, the CFZ at f/2 would be pretty thin...

How do you find the Celestron motor focuser?
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Old 15-04-2019, 02:22 PM
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that_guy (Tony)
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also note that for an f2 system you'll need specialised filters since the fast system due to wavelength shift. thats what dylan used with his f2 hyperstar/rasa narrowband images.
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Old 15-04-2019, 02:22 PM
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multiweb (Marc)
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With the C11/Hyperstar combo I have standard baader NB filters. The filter is countersunk in the spacer so it doesn't modify the FL/camera spacing. I heard about standard NB filters not working because of the steep light cone and the angle of incidence but TBH I didn't have issues imaging with them. It could become a problem maybe if you try to separate very close emission lines such as Sii,Nii,Ha but overall it seems good enough.
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Old 15-04-2019, 02:31 PM
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The 8" RASA apparently has a different focuser mechanism to the rest of the range (If it works, I would reckon it might be the first of many. I suspect it replaces what has amounted to a simple sleeve sliding on grease up and down the central baffle tube with a linear bearing to eliminate the mirror flop endemic in the rest of them) but the visible bits of it still amount to the same thing, a threaded shaft that protrudes through the rear cell and the focus knob is actually just a brass nut, secured in bearings that keep it in a fixed position in the rear cell so turning the nut moves the shaft (And mirror) up and down the baffle, if you can call it a baffle in the RASA given they have no secondary mirror or visual back on the rear cell, mirror support tube?

So far it is making focus a lot easier, for manual focus you can just pop on a Bahtinov mask, run the mirror towards the rear cell a few turns to go past focus then with looping exposures step it to focus progressively. The step positions seem to be repeatable all the way down to single steps, which by my workings are mirror movements of under 1 micron at a time. It is probably down near the limit of accuracy of the focus shaft thread. Edited to add: The focus motor is 1000 steps per revolution of the focus knob.

In short, it looks like money well spent. The only thing that would be handy for it would be a little hand controller box if the scope was being used visually as I am not using a Celestron mount. If it comes to that I might see if I can cobble something together that will work with an Raspberry Pi or similar but for imaging it is a non issue. There were some issues with the ASCOM driver which seem to be resolved now, when I first tried it, big focuser moves would crash SGP and I did have some issues with APT as well, though a USB cable change seemed to sort that. I might have to try the other cable again and see if the driver update has fixed that.

Last edited by The_bluester; 15-04-2019 at 02:43 PM.
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Old 17-04-2019, 01:44 PM
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Does the light come from the mirror direct to the sensor or pass thru a lens prior to? ??
I am thinking a convex curved sensor would move this set up ahead.
The sensors being developed are concave but a mirror needs opposite..alex
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Old 17-04-2019, 01:48 PM
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With an appropriately curved sensor I wonder how much you could reduce fl...mmm wouldnt an f1 be nice and I am not talikng about a race car☺
Alex
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Old 17-04-2019, 02:35 PM
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The RASA has a set of correcting lenses inside the OTA, they are advertised as producing a "Flat" field and to be honest, the few images I have seen from one seem pretty impressive.

This is under two hours of one minute subs! I would not say that the stars are perfect in the corners but it looks more like sensor tilt than field curvature.

https://www.astrobin.com/full/399238/0/

If I had some cash to splash I would be pretty tempted by the idea. I have seen an image of the same area with the RASA 8" and an OSC camera and it was pretty flash looking, to me anyway. But I would not go there without a focus motor!
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Old 17-04-2019, 02:45 PM
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Mmmm now I am thinking a dragon fly set up with one scope for each filter. ... thats what I will spend my Noble prize money on☺

I have sent a couple of crazy emails to folk calling for a astro camera with a concave sensor...

But back to reality by the time you buy the filters etc it wont be cheap..not like all the other scopes ...what am I saying.
Still for me when I am in one spot and dont have to travel and can get my observatory placed etc..I think get rid of everything and go for a RASA.
Alex
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Old 17-04-2019, 02:55 PM
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I am still tempted, but for once I am not tempted to be an early adopter.
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Old 25-04-2019, 03:38 PM
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I have been thinking about its advantages for me particularly in the city where objects only appear for an hour....also I find it hard getting a good polar alignment if I need to take it in and out I cant use polemaster as I cant see the CSP..I can get close enough with a guess these days close enough to do short exposures.. If I was doing 30 sec captures..plus maybe an hour even add an hour from the following night conditions permitting and get an image that may just not be practically possible with an hour a night and you need five..for example.. .as with my current scopes. Up North I have the time but fast would mean more data in less time.
So it looks good to solve my problems if it delivers the dream.
I am thinking a rig with no guiding which may get by on the HEQ5 an great travelling set up.
I am getting back to not imaging in Sydney if I can keep up going North for a dark spell.
But I would have to get rid of the current scopes.. I couldnt... so they would sit and rot probably☺.
But its nice to have the wanties and come up with the reasons you must have whatever to make your world complete.
Bintel say none until July so plenty of time to think about the prospect.
I shudder to think what the filters would cost...must find out...and if my current set could be used at all..you may suffer but it may be tolerable...It could be pretty tollerable I expect the filters could be a shock.
Must look.
Guess $700 maybe more..have no idea..did a price get mentioned above? And changing filters would not worry me if via a draw as I sit out there..I am not into set it running and check in the morning types that is beyond my ability and desire ..change a filter even every 20 minutes would be ok with me...What I would like is the RASA the eight and the 80mm on one mount all with cameras that would enable me to merge the data from all three☺and a puppy.
Alex
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Old 25-04-2019, 03:55 PM
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Six hundred and fifty quid at first look...I shudder to do the currency conversion...
Alex
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Old 25-04-2019, 03:57 PM
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So I am working down from $1400 approx just for filters.
Alex
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