Refractor imaging scope quiver...

[Jannerland]

I'm enjoying the ease of using a WO Redcat as I start learning about imaging but  about a I'm curious how what's considered a sensible choice for a future second scope to give me a bit more choice of targets that don't just end up as wee dot in the center of the frame. 

I've played around with CDC to frame up a few targets with my camera (2600MC) and am thinking of something which would give some when towards a range 750mm - 450mm (with reducer) as potential next option .

I wondered how people have chosen their next step up the aperture /  FL ladder?

My considerations

1)  limited number of clear nights we get in UK  - I don't see myself with more than 2 possibly 3 scopes as I have no desire to end up with a load of glass sat in a case not getting used

2) I have easy access to relatively good skies (B4 when no clouds) at home  but like the idea of portability

3) would rather buy quality than quantity (see above)

4)  the ability to get flat fields at a couple of FL's is more or less essential.

Would love to know if others who've been here before feel there's there logic to that thinking?

Ta..Rob

 

 

[fifeskies]

William Optics GT81 Mk4  , a good step up from your Redcat

Needs the matched reducer (0.8) and this reduces native 478mm to 382mm, just outside the range you mentioned but its a top class performer for widefield.

Very portable too.

Use the astronomy.tools visualiser to see how it works for you.

 

I have the 533mc camera and have added an example of my imaging with the GT81, your camera will show more.

Elephants Trunk Nebula  , and the North America Nebula

 

 

 

Edited November 26, 2021 by fifeskies

[vlaiv]

Depends on your mount.

I'd say that you should aim for 1.5-1.8"/px and with ASI2600 that would mean ~420 - 500mm of FL.

Say that you are using x0.8 FF/FR and you have F/7 refractor - that would mean around 500 - 600mm native FL.

And the winner is

https://www.firstlightoptics.com/stellamira-telescopes/stellamira-90mm-ed-triplet-refractor-telescope.html

+ suitable FF/FR

https://www.firstlightoptics.com/stellamira-telescopes/stellamira-2-08x-reducer-flattener-for-90mm-ed-triplet.html

[Jannerland]

thanks both!

So the mount is an AZ-EQ6 GT which should give me some headroom weight capacity wise. FYI I'm getting just under 1" RMS from the PHD logs  and acceptably round stars (to me) until the corner extremes

Can I ask how you arrive at the  1.5-1.8"/px   values?

I did previously take a  look at the ccd calculator tool and saw how deep into undersampling the current scope /  camera combo is .

I was taking the tool as meaning that the range 0.67 - 2 would be "acceptable" for ok seeing?

For either of those scopes above do you feel I could also expect a flat field at the native FL and not just with the respective flattener/reducers? 

 

[ollypenrice]

1 hour ago, Jannerland said:

thanks both!

So the mount is an AZ-EQ6 GT which should give me some headroom weight capacity wise. FYI I'm getting just under 1" RMS from the PHD logs  and acceptably round stars (to me) until the corner extremes

Can I ask how you arrive at the  1.5-1.8"/px   values?

I did previously take a  look at the ccd calculator tool and saw how deep into undersampling the current scope /  camera combo is .

I was taking the tool as meaning that the range 0.67 - 2 would be "acceptable" for ok seeing?

For either of those scopes above do you feel I could also expect a flat field at the native FL and not just with the respective flattener/reducers? 

 

You'd need supernatural seeing to image at 0.67"PP and a guide RMS of half that, so about 0.3 arcsecs. This is possible with premium mounts but a good EQ6 runs around 0.5. Again, that's a good one.

I would use the guide trace and not the round stars test since equally bad tracking on both axes will give round stars!

Pixels are becoming so small these days that many of us are finding that binning them is the only way to get a sane sampling rate. With CMOS cameras that means software binning after capture.

I image small targets with a TEC 140 refractor and Atik 460 CCD giving about 0.9"PP, but Vlaiv has given a convincing demonstration (on my data) that this is still over sampled. I like the images but could get away with less than the TEC's 1 metre FL. Examples:

https://www.astrobin.com/full/419975/0/

https://www.astrobin.com/full/342334/0/

Olly

[vlaiv]

3 hours ago, Jannerland said:

Can I ask how you arrive at the  1.5-1.8"/px   values?

These values are mix of empirical and theoretical.

Theoretical framework is used to relate FWHM of star in images to needed sampling rate to record all the detail in image having stars of said FWHM. Again theoretical framework is used to estimate what sort of FWHM one can expect given certain aperture, usual seeing values and guiding performance.

Empirical values of actual FWHM values measured given seeing, scope and mount performance confirm above.

You can for example measure your maximum sampling rate given certain scope - by simply measuring your average FWHM (ones you are most likely to get - on average, or even good night - depends on what you aim for - good sampling on average night or good sampling on best of nights). Optimum sampling rate is x1.6 lower than FWHM in arc seconds. If you for example measure 3.2" FWHM of stars in your image - you need to sample at 2"/px (3.2 / 1.6 = 2) to record almost all detail (difference is imperceptible to human eye).

This value is derived from Gaussian approximation to star profile, it's Fourier transformation and Nyquist sampling theorem.

Expected FWHM values can also be estimated - and are usually "right" - or rather under estimated as usually seeing is worse then we expect. If we estimate FWHM values on most nights with usual mounts - we get this sort of "spread" of working resolutions:

less than 80mm - needs sampling lower than 3-2"/px

80-100mm - is good for 2"/px to 1.8"/px

100mm - 150mm is good for 1.5-1.8"/px

you need 8" or at least 200mm to attempt going below 1.5" - down to 1.2". In very rare cases you can maybe try 1"/px with 8" aperture, but it needs perfect skies and very good mount. It is however best done with apertures above 8".

I don't think any amateur aperture can hope to get below 1"/px at least 99.99% of time - there could be that one night in several years that will allow for less than 1"/px - but not by much.

[Jannerland]

Again, thanks both!. My takeaways:

1. O - Spectacular pictures Olly 🙂
2. O - My guiding set up could do with some work to at least see if there is improvement potential
3. O - By premium I assume we're talking 10Micron, Paramount etc?
4. V - I should review some data good / average nights to see where FWHM sits for my location / skies
5. V - I should look at the spread working resolutions as guideline for what would be optimal ?
6. V - Maybe a second camera with a different pixel size would be a good future plan if I want to keep closer to the optimal ranges
7. V - I would like to understand the theoretical better just for my own education  - is there any reading you would recommend which discusses in the context of astro-photography?

 

[vlaiv]

3 hours ago, Jannerland said:

O - By premium I assume we're talking 10Micron, Paramount etc?

Yes, but there are mounts capable of 0.3" RMS or even less - that are not in price bracket of those you listed. Mesu 200 and GTD E-Fric come to mind as more affordable yet premium performance wise mounts.

3 hours ago, Jannerland said:

V - I would like to understand the theoretical better just for my own education  - is there any reading you would recommend which discusses in the context of astro-photography?

I'm not sure I can provide you with reference to a text that discusses that in context of astrophotography per se, but do have a look at following articles (basic info that you can sort of piece together):

https://en.wikipedia.org/wiki/Airy_disk

(pay attention to Gaussian approximation)

https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem

https://en.wikipedia.org/wiki/Spatial_cutoff_frequency

https://en.wikipedia.org/wiki/Fourier_transform

(note 2d gaussian shape and its transform)

https://en.wikipedia.org/wiki/Convolution_theorem

That is about all that is needed to determine optimum sampling rate, and here are few more pointers that help put above together:

- Seeing blur is given as Gaussian kernel of certain FWHM.

- Guiding error produces blur that is again given as Gaussian of certain sigma

- Telescope aperture produces airy disk pattern that can be approximated by Gaussian

- These three blurs convolve to produce final blur and their corresponding variances add up (standard deviations add in quadrature as square root of sum of squares).

- Fourier transform of Gaussian is Gaussian and Gaussian falls of to 0 at infinity. At some point you can take it to be cut off frequency (for rule of x1.6 above I used when frequency domain Gaussian falls below 10% as cut off frequency).

- For planetary imaging - you can use actual spatial cutoff frequency as Fourier transform of airy pattern is MTF and that hits 0 at cut off frequency (unlike Gaussian that only tends towards zero).

 

[Elp]

Coming from a Z61, my next was a 130pds, larger aperture, longer focal length, still compact enough to fit into a large rucksack. The light gathering capability from such a small increase is staggering.