So close to ordering, but then...

[Datalord]

I almost pressed the button on a combination of QSI690, WO132 and celestron CGX. But then...

The Celestron RASA has always been such a magnet to me and as I read through everything I got excited about it again. Woe is me. Combo: CGX+RASA+QHY367, which obviously takes me out of the narrowband game.

Questions: 

1. Is narrowband so much better, considering how extremely fast the RASA is?

2. If 1=yes, should I look at some way of doing mono + filter?

3. Should I forget about the RASA and just do what I originally thought to do?

[wxsatuser]

If you have LP narrowband is very useful, it's helped me no end.

[steppenwolf]

Broadband and narrowband imaging are two very different beasties - one produces 'natural colours' in all objects, the other produces 'false' colours in emission nebulae. To be able to do both types of astro-photography opens up many more opportunities to you so think very carefully before you go down the RASA and one shot colour route, not so much for the camera - I still like OSC imaging in the right circumstances - but you are limiting yourself to only this type of imaging with the RASA.

[swag72]

I think that narrowband imaging is very versatile and also increases your imaging time considerably....... I borrowed a friends OSC for a couple of months expecting to get one and use it..... I found out that very quickly as the moon rises you can in effect wave goodbye to almost 2 weeks of imaging time a month. At least with narrowband (Ha in particular especially if you get the narrowest 3nm filters) you can quite effectively image around a full moon if you pick your targets well. 

The ability to do narrowband AND broadband would affect my choice of scopes..... I wouldn't have a scope that could only do LRGB imaging unless I had another system that could also do mono and narrowband  

[pete_l]

The thing about short focal length imaging is the wide FoV. Now, a lot of people like having some "context" around the object of their image - it acts like a frame around a picture. However that wide FoV also limits the exposure time of your subframes when the Moon is high or if you have a lot of LP. And to people who live in light polluted areas; the Moon is just another streetlight.

Long focal lengths give you a darker sky background. A point known to visual astronomers who will increase the magnification to ease the view of low surface brightness targets. In imaging, a small FoV allows you to image your target, but without getting too much surrounding sky in the shot. Something that may be good or bad, depending on your personal tastes. Remember, however, that few Messier objects are over 30 arc-minutes in diameter.

The solution might just be as simple as choosing your nightly targets to suit the conditions. When there is no Moon, go for the deep, extended objects. And when the Moon is up, try for planets and clusters instead. Maybe even image the Moon, itself?

[Datalord]

Thanks for the input, guys. I'm super keen on both the rasa and the narrowband. Not a good combo.

Hmm, maybe I should do the rasa with my 6D and get a qsi to do narrowband on my WO? Pete, your point is quite good, but that puts me more into the large refractor path... 

It has taken me months to figure out what to do... 

[geordie85]

Flip a coin and let the universe decide.

Heads = RASA 

Tails = William optics.

This is how I decide all my difficult decisions. 

[pete_l]

How about an 11-inch Celestron and use it with Hyperstar?

Ultra-short FL for wide fields, F/10 for small targets and the possibility of a FR for stuff in between!

 

Since that would be the more expensive option, you know it's probably the right choice

[AKB]

Sorry, I'm being really thick here... WHY does the RASA preclude NB?

There's a RASA adapter to the Baader UFC system, which has a filter slide for most formats.  Is it a back focus issue with the camera??

———

PS: of course, it is Manual only.

[steppenwolf]

28 minutes ago, AKB said:

Sorry, I'm being really thick here... WHY does the RASA preclude NB?

Certainly not being thick - you are quite right, a slim filter drawer will allow narrowband imaging (by that, I mean the capability of changing filters in front of a mono camera) but filter changing will be a purely manual affair which many imagers would find clumsy when there are sequence automation programs like SG Pro around.

[AKB]

Fair enough, although given the insanely long total exposure times that many NB imagers use, I would have thought that changing once per night was more than enough!

Glad I wasn't missing anything basic.  Hope to be doing some NB with a Baader UFC and Hyperstar myself, if we ever get to see the sky here.

 

[pete_l]

1 hour ago, AKB said:

Sorry, I'm being really thick here... WHY does the RASA preclude NB?

It doesn't preclude it. But at F/2 the angle of convergence of the light cone is very steep (which is also why the point of best focus is so absolutely tiny). Some NB filters might not be designed for such small F-ratios. It would be worth talking to one of the technically competent vendors, telling them what you intend to do and asking for advice on the best filter choice.

[AKB]

Baader, possibly others, make 'fast' (F2) narrow-band filters. 

[Datalord]

4 hours ago, AKB said:

Sorry, I'm being really thick here... WHY does the RASA preclude NB?

There's a RASA adapter to the Baader UFC system, which has a filter slide for most formats.  Is it a back focus issue with the camera??

———

PS: of course, it is Manual only.

Yeah, the manual part was the one stopping me, but if that is actually a sound possibility, I'll read up on it. It's not a remote observatory after all. 

3 hours ago, pete_l said:

It doesn't preclude it. But at F/2 the angle of convergence of the light cone is very steep (which is also why the point of best focus is so absolutely tiny). Some NB filters might not be designed for such small F-ratios. It would be worth talking to one of the technically competent vendors, telling them what you intend to do and asking for advice on the best filter choice.

There's no end to the complications with this astro stuff. 

2 hours ago, AKB said:

Baader, possibly others, make 'fast' (F2) narrow-band filters. 

And no end to solutions, as long as the money well doesn't dry out. :-) 

[Waldemar]

The HyperStar is no comparrison for a RASA. Imaging circle of a HyperStar is 25 mm at it's best. The RASA has no problem with 44mm.

Mono imaging is absolutely no problem either, as very slim drawer systems are available, that can hold even 50,4 mm frameless filters,
and what is the problem with changing filters by hand? 
You will have to have a dedicated spacer made though, for the spacers that come with the RASA are M42 and M48, while for a full frame camera you will need M54 or so.
On top of that they are made with a backfocus for DSLR which is a total mystery to me...
if you decide to go for the RASA I can send you technical drawings for it. I will have to know the backfocus of your camera, though
My TRIUS 36 has 17,5 mm backfocus, so I have that drawing at hand, but I can make you one for any camera if you wish.

When choosing a camera for the RASA, I would go for the smallest pixels you can get, because of the very fine resolution of the RASA
NB filters:  Baader has a specialty set for fast focal telescopes, but Atronomik says theirs will just do the job...
Focusing is indeed a pita, you will need the 'direct' worm focusing motor from Starizona's MicroTouch system and FocusMax IV to get going.
focusing by hand is not ideal, I would say it is merely impossible.

 

[Datalord]

Excellent, Waldemar. Although, this thing about backfocus gives me pause. I'm not even the slightest interested in messing about with crafting anything.

I was looking at qsi690, but with an interchangeable filter, I might look at a round camera of sorts. Maybe an atik? Waldemar, do you have suggestions that would actually fit on the rasa out of the box? 

[Waldemar]

27 minutes ago, Datalord said:

Excellent, Waldemar. Although, this thing about backfocus gives me pause. I'm not even the slightest interested in messing about with crafting anything.

I was looking at qsi690, but with an interchangeable filter, I might look at a round camera of sorts. Maybe an atik? Waldemar, do you have suggestions that would actually fit on the rasa out of the box? 

It all depends on whether you want to go OSC or mono and what size of sensor you want to use.

Astrodon makes a great dedicated UHC filter for the RASA, that will do great on an OSC and takes no backfocus, with some spacers the ATIK will be fine.
Going mono, you will need a filterdrawer system, size of filters depending on the sensor.
if you just tell me what your plan is camera wise, I will do the arithmetic for you, but it is not hard, I just have the advantage of experience with the RASA.

QHY got very nice camera's that will do great with the RASA. But there are more choices of course. A round camera, not exceeding 114 mm diameter, would be great, cause it will prevent extra obstruction. ATIK, Starlight Xpress, many brands got them. So... your choice...

I will tell you what you need, as soon as I know what camera you choose, or even tell me to find a suited one for you, but mono or OSC and sensor size are the minimum choices you will have to make 

[Datalord]

I always wanted a mono and do NB. But I was looking at qsi and Moravian, but both of those are now out. I'm back to square one. 

I'd like a high resolution camera, so the 16200 chip,  as in atik16200, is interesting, but that is not round. Atik 490EX looks like the best bet, coupled with the baader ufc system. What's your take on that? 

[Datalord]

Interesting take on the RASA: http://www.astronomyforum.net/astronomy-beginners-forum/227412-ccd-celestron-rasa.html

Basically he argues that the 6D is a good option as a colour ccd, so I have that covered.

Also, I read that there are no RGB filters to use with a RASA. Is that true? (http://www.harrysastroshed.com/Rasa.html)

[Waldemar]

I would like to bring this to your attention: http://qhyccd.com/QHY183.html

Very nice cold MOS camera and very suited fro the RASA and @ an incredible price...
Yes, it is only in colour but very small pixelsize, which will compensate for the loss of definition with a Bayer matrtix.
But you got a huge flat imaging circle of 44mm (not 70 like Harry said, Celestron came back from that fantastic figure), so to take full advantage of the super wide field, a full frame sensor is kind of an obligation: http://qhyccd.com/QHY367.html the pixel size is about 4x of the 183, though, so you miss out on the definition advantage

I do not understand why RGB could not ne used in a fast system, because the bandwith of RGB filters is so wide that the shift of a fast system will not be noticeable.
NB  is another story, but like Harry said Baader has a nice set suited for f2  systems. (Astrodon does not)

Putting a DSLR camera in front of the RASA is kind of a sin... you would like advantage of full aperture not an obstruction from a big DSRL camera...

The dedicated Astrodon filter is quite special and expensive ( ± €650) but Astrodon is famous for quality and it will work great with an OSC camera.

The combination QHY183 + Astrodon will put you back around € 1600, but that is about the price of a camera with 16200 sensor. (spacers not included)

Mirrorshift is practically non existent on the RASA, so autofocus can be used with little or no problem.

[Waldemar]

If you go for a high number of pixel sensor be aware you will need a fast PC or laptop as well... huge files...
I use an ASUS with i7 processor, 16Gb ram, 0,25 Tb SSD for programs and 1 Tb SSD for rom. It is dedicated to astro, runs everything without problems.

The investment for an OSC coldMOS with the Astrodon filter is a lot lower then for a mono with NB filters...

[Datalord]

So to be clear, you suggest I go the colour route instead of NB?

The qhy looks great, but I don't understand what astrodon filter you are talking about? 

The computer I have is plenty fast. :-) 

[Waldemar]

I am talking about this filter: https://www.astronomics.com/lpr-imaging-filter-for-celestron-rasa_p20326.aspx
OSC camera's nowadays are better then they used to be because of the incredible small pixel size and the back illumination techniques.

an OSC camera with 2,4 µm pixels has the same definition as a monocamera with 4,8 µm  pixels, which was one of the main reasons to choose for mono.
The difference of course is that NB imaging is not a real option.

I suggested this option, because the combination QHY and the LPF filter is a relative cheap way to get going with the RASA and get absolutely great results without
problems to have made dedicated spacers, just a 25 mm extra spacer and some adjustment rings for fine tuning.

Keep in mind that the 183 has a dynamic range of only 12 bit, while the 367 is 14 bit and any CCD camera is 16 bit. This makes quite a difference in dynamic range: 4096 x compared to 65536 x with a CCD... That does not mean that you can not make great pictures with a lower dynamic range, it is just more critical on integration times.

[ollypenrice]

For me F2 is too fast. If it were 'just going to work' I wouldn't feel this, but will it 'just work?' The light cone is so steep that the focal depth is measured in microns yet the focuser is of the moving mirror design. Not a good combination in my book. Another consequence of the shallow focal depth is the susceptibility to tilt. Any tilt will see one side of the chip in focus and the other not.

I'm in an atypical position as an imaging provider because going outside and finding that something at critical tolerance is no longer functional is simply not an option. I owe it to my guests to go outside so we can just get cracking.  Also, purely personally, I have more than enough kit to fiddle with around here without adding a new level of likely difficulty. If you enjoy a challenge, enjoy the tinkering and are up for it then you'll probably view F2 differently from the way I view it. 

The RASA is a widefield instrument. I would feel hamstrung by not being able to add NB data to natural colour given the nature of the targets. As Sara says, you lose at least a week a month if you don't have an Ha filter to use and no amount of signal will find the beautiful structures in broadband red which emerge in narrowband Ha. What matters with an Ha filter is not what it passes but what it blocks.

Olly

[Waldemar]

It is quite a challenge, absolutely thru. NB is of course possible, but to take full advantage of this scope, you need to use a full frame sensor.
The flat corrected area is big enough for that, but not as big as Celestron suggested in the beginning (70 mm)

The speed is a big advantage as well as a draw back at the same time. Integration times are incredibly short, but focusing is a problem.
Celestron found a way to deminish mirror shift and I do not understand why they did no do that on their SCT's...
the very narrow focusband is why I suggested the new MicroTouch directworm stepper with sub micron accuracy: http://starizona.com/acb/MicroTouch-Low-Profile-Stepper-Motor-P3360C745.aspx

The by Celestron supplied adapters are only M42 and M48, so not sufficient for full frame cameras. A dedicated adapter needs to be made for that and of course a filter drawer system with M54 thread and 50,4 mm naked filters should be included, which introduces another 'problem' : wavelength shifts because of the steep lightcone.
Like is said before, Baader makes NB filters especialy for such fast systems, which work well.

But like you said, loving a challenge and some tinkering is more or less a must. 
My personal interest in astro-imaging is the multitude of technical disciplines and problem solving... to get everything working together flawlessly.