Galaxy Imaging

[Astroblagger]

Hello

Im wanting to start imaging Galaxy's close uo, iv got my trusty old ed80 but focal length isnt long enough, Iv got an HEQ5 and a C9.25, what are my options for galaxy imaging with a long focal length without spending too much? would i need a neq6 for the C9.25 or can i get something with long focal length that would sit on the HEQ5 happily, any advice appreciated. 

[vlaiv]

Maybe this:

https://www.rothervalleyoptics.co.uk/altair-astro-6-f9-ritchey-chretien-astrograph-telescope.html

[Astroblagger]

Thanks for the advice Vliav, i take it i need a suitable reducer corrector factored in to the price? Will this be a good match for my Atik 460ex.

[DaveS]

Or a similar clone from FLO.

Probably your best bet for long FL imaging on a HEQ5, as it's short and relatively light, keeping the moment arm small.

You won't need a flattener / reducer for a 460 / 694 camera as the RC is flat field natively.

[Astroblagger]

F9 is a bit slow, or will that be ok with a CCD? will i be ok with my finder guider for guiding or need an OAG?

Edited September 6, 2019 by Astroblagger

[vlaiv]

For smaller sensors you don't need flattener. You might want one for APS-C sized sensor (I'm not sure). RCs have rather flat field, but not fully flat - for large sensors they need flattener.

I use RC8" with ASI1600 without flattener and field is good on that size (21mm or so diagonal).

This scope has 1370mm FL and with 460ex this will give you 0.68"/px - and that is rather high sampling rate and you will be oversampling natively. There are two things that you can do - add FF/FR, or bin.

I think that you should bin x2 - that will give you 1.37"/px - and that should be rather good for HEQ5 and regular guiding results (something like 0.8" RMS).

When you bin x2 it's like having twice shorter focal length so you will effectively have "F/4.5" scope - which will be fast enough (F/ratio and "speed" of scope is not measure of how fast you will take images - it's aperture at resolution, and in this case you will be using 6" at 1.37"/px - which will be quite fast).

Here is an example for you - this is two hours of lum in rather heavy LP (mag 18.5 skies) - 8" scope (F/8 natively - also RC but 8") at ~1"/px:

I would not call that slow ...

You can "speed up your scope" with use of FF/FR, and a good one would be Riccardi x0.75 FF/FR - but that would give you 0.91"/px - and I think you will still oversample on this resolution. Binning with FF/FR will give you 1.82"/px - and that is going to be rather low resolution - comparable with 80ED - so you will gain nothing (apart from speed, because of almost x4 light gathering for same sampling rate).

[alacant]

Hi

Without a heavy mount available, we sometimes use a 6" f8 reflector. Cheap, cheerful, sharp  and flat corner to corner even on aps-c. This is the one we have. It's as sharp as you'll see anywhere, especially with an OAG. Here is 7331 and stephen's quintet using the same.

If you want fast and long, I think you're gonna need a heavier mount.

HTH

[Astroblagger]

brilliant advice everyone thanks gives me something to think about. 

[ollypenrice]

On 06/09/2019 at 13:28, Astroblagger said:

F9 is a bit slow, or will that be ok with a CCD? will i be ok with my finder guider for guiding or need an OAG?

If you're talking about an object which fits on the chip with or without reducer (as I guess you are with small galaxies) you can forget about F ratio and simply concentrate on aperture. You are in the dreaded 'F ratio myth' territory here and this has been done to death (largely by me!) on SGL and elsewhere. The key point is that reducers, in this scenario, don't bring in any new photons. A camera lens is faster at F5 than F10 because the lens is letting more light in. Using a focal reducer to go from F10 to F5 does not let any more light in. They are not equivalent.

The other concern is resolution in arcseconds per pixel. To image usefully at 1"PP (a nice scale for galaxies) your guide RMS needs to be no greater than 0.5 arcsecs. This is possible but not guaranteed with an HEQ5. And you also need seeing which is stable enough to make this possible. I find that sometimes we have it and sometimes we don't, in which case we shoot colour and leave luminance to a night of better seeing.

Olly

 

Edited September 15, 2019 by ollypenrice
Typo

[Astroblagger]

Thanks Olly, great advice as usual. Would I bin the colour 2x2 aswell or could I bin it 3x3? Hoping a finder guider will do the job, as I don’t want to shell out for an OAG. 

[jimjam11]

I just got an RC6 to try from FLO and it has impressed me. This is 55min of M33:

 

I am seeing star FWHM from 1.96" (in HA) to 2.5" for lum. This stack is 2.89" combined so I binned it x3 to get 1.69"/px.

I also have a 150p and for a while I have doubted the mirror. This proved it, the stars are significantly smaller with the RC6 compared to the 150p:

 

[Astroblagger]

Really need to read up on the subject of pixel size and resolution. Is there any easy to understand tutorials on the subject? 

[ollypenrice]

On 28/10/2019 at 11:34, Astroblagger said:

Really need to read up on the subject of pixel size and resolution. Is there any easy to understand tutorials on the subject? 

There's nothing much to it. This calculator will tell you the resolution in arcsecs per pixel. http://www.12dstring.me.uk/fovcalc.php To get a reasonable image scale you'll need about a metre of FL with the 460. Longer will certainly over-sample but you can bring it down by binning. (My galaxy imaging is done at 1 metre focal length with the same Atik 460 as yours, giving about 0.9 arcsecs per pixel. That gives a satisfying image scale and is usually achievable in the real world.)

You need to give PHD your guidescope FL and pixel size then it will give you the guide RMS (in effect the error) in arcseconds. The RMS needs to be no more than half your image scale if you are going to lose no detail to guiding error. 

I could bin colour (making 1.8"PP) but don't. I just don't like it and I often want to make my stars out of RGB only, so I want them small and tight. However, I'm not usually fighting endless cloud (right now being an exception!!!) so I'm not under pressure to bin.

The 6 inch RC looks promising but don't expect it to be easy to collimate.

Olly

[Adam J]

On 06/09/2019 at 11:46, Astroblagger said:

Hello

Im wanting to start imaging Galaxy's close uo, iv got my trusty old ed80 but focal length isnt long enough, Iv got an HEQ5 and a C9.25, what are my options for galaxy imaging with a long focal length without spending too much? would i need a neq6 for the C9.25 or can i get something with long focal length that would sit on the HEQ5 happily, any advice appreciated. 

You are not going to get into sub 1"/pix territory reliably with a HEQ5 pro and a heavier load irrespective of what you put onto it. The best I have ever managed is 0.6"/pix for a short period the normal RMS is about 0.8 - 1.1"/pix though. 

What I would do is get something like a 200mm F5 Newtonian and an IMX290 mono based guide camera and use the short exposure technique. That way the guiding wont matter, use <8 second exposures and 1000's of them at high gain, will give very good results. 

With your current camera you are a little stuck with longer exposures and so I would keep the focal length less than 800mm, something like a SW 150PDS might get you something ~1.2"/pix but nothing close to the detail you can get with the short exposure technique. 

I think its good to reflect on the fact that the best amature image of M82 I have ever seen was taken with a QHY5III 290m guide camera:

https://www.cloudynights.com/topic/568316-m82-full-power-with-short-exposure-t300-qhy5iii-290-m-qhy5iii-178-c/

Larger scope of course but the technique will still work with a 8 inch Newtonian just less detail. 

Adam

 

Edited November 7, 2019 by Adam J

[Adam J]

On 27/10/2019 at 17:13, jimjam11 said:

 

Getting separation on the double star in the neck of the trunk is a mark of good optics. 

Adam