Telescope v eyepiece?

[M40]

On the basis that there are very limited items on the shelf at the moment, I have had plenty of time to consider things rather than just splash out, which gives me time to ask your thoughts.

I am not into imaging as such, but I do enjoy sticking the DSLR onto the telescope and taking a single shot of what I am looking at. I am trying to get the best grey smudge I can of M31 but I am now at a bit of a crossroads, do I change the telescope or do I get a different eyepiece. I am very much on a budget but do have some flexibility. I have a skymax 180, my options as I see it are to purchase a new eyepiece like the Baader Hyperion Aspheric 36mm with a 2" 0.5x reducer or get a different telescope such as the SW 150PDS and use the eyepieces I have. I like the Baader eyepieces simply because I can bolt my camera directly onto it.

Your thoughts would be most welcome. 

[vlaiv]

I'm not sure of what you are exactly asking, but I can tell you this:

Skymax 180 with Hyperion Aspheric 36mm and 2" x0.5 reducer will not work as you expect.

There is only so much field that Skymax 180 illuminates and there is only so much field that 2" eyepiece can capture. Throwing x0.5 reducer in the mix simply will not do the magic for you. I reckon that skymax 180 illuminates about 30-35mm due to having 30mm rear baffle and only 25mm exit hole at the back of the scope.

Take that 35mm illuminated field and add in x0.5 reducer and it will turn it into 17.5mm illuminated circle. Baader aspheric has 45mm of field stop. You'll end up illuminating only 1/3 of what eyepiece can show you - you'll have massive tunnel effect when observing in that combination with only central 1/3 of field showing something and else being black.

With SW 150PDS - you have much better chance of doing afocal snaps, but you'll suffer coma in your snaps. You'll need coma corrector for this. In the end, I guess you'll get much better snaps by using prime focus imaging.

[M40]

Many thanks vlaiv, so I now understand that the eyepiece route is not the way to go 👍

[Louis D]

Afocal projection always has the negative aspect of capturing any image artifacts introduced by the eyepiece and the camera's taking lens.  For the 36mm HA, that would include pincushion distortion and chromatic smearing at the edge:

If you can live with these imperfections, it's not a terrible way to quickly capture images.  I captured the following moon image afocally through a 14mm Pentax XL with an Olympus C4000 camera 16 years ago:

[M40]

Thanks for that Louis, I think I need to get a grip of these terms. As I understand it, afocal projection is placing a camera with lens fitted against the eyepiece of the telescope whereas eyepiece projection is removing the lens from the dslr, fitting the camera to the eyepiece and prime focus is fitting the dslr direct to the telescope with no eyepiece or camera lens but possibly a reducer or barlow. 

I also need to understand the numbers that vlaiv has posted. 

My plan was either to do eyepiece projection with the Hyperion asph 36 and to get the field of view bolt on a 2" reducer or a different telescope route. Looking at FLO FOV calculator, doing prime focus with a 0.5x reducer, dslr and the skymax180 will not get the whole of M31 in the picture; but doing eyepiece projection using the asph36 and the reducer will. I already have the reducer so no cost issue there but I didn't want to throw a few pennies at the eyepiece if it doesn't work similarly with the telescope. Not interested in buying a massive camera lens, I would rather go the astronomy route.

Thanks for the comparison of images 👍

 

[vlaiv]

2 minutes ago, M40 said:

Thanks for that Louis, I think I need to get a grip of these terms. As I understand it, afocal projection is placing a camera with lens fitted against the eyepiece of the telescope whereas eyepiece projection is removing the lens from the dslr, fitting the camera to the eyepiece and prime focus is fitting the dslr direct to the telescope with no eyepiece or camera lens but possibly a reducer or barlow. 

I also need to understand the numbers that vlaiv has posted. 

My plan was either to do eyepiece projection with the Hyperion asph 36 and to get the field of view bolt on a 2" reducer or a different telescope route. Looking at FLO FOV calculator, doing prime focus with a 0.5x reducer, dslr and the skymax180 will not get the whole of M31 in the picture; but doing eyepiece projection using the asph36 and the reducer will. I already have the reducer so no cost issue there but I didn't want to throw a few pennies at the eyepiece if it doesn't work similarly with the telescope. Not interested in buying a massive camera lens, I would rather go the astronomy route.

Thanks for the comparison of images 👍

 

Maybe best way to go about it is to understand following:

Telescope is projection device - it projects angles to flat (or rather slightly curved) surface of focal plain. Focal length of telescope determines how big that projected image will be.

Take M31 for example - It has about 3° in angular extent and if you want to frame it nicely - you should really capture about 4° of the sky. Here is how to translate that into size in focal plain:

size in mm = tan(degrees / 2) * FL * 2

where focal length is in mm and tan takes degrees as argument (convert to radians if tan takes radians as arguments).

For example, if you want to put Andromeda galaxy nicely framed into 1.25" eyepiece that has 27mm maximum field stop, you need to use focal length of 386mm. Here is an example of that:

In order to know if telescope can deliver nice image of object, you need to know what is fully corrected / illuminated circle for that scope. Most scopes with 2" focuser will have this circle smaller than 47mm as this is maximum field that is supported by 2" focuser. In reality, most scopes have this value at less than about 30-35mm.

This is irrespective of eyepiece used.

All of this means that you want rather short focal length telescope in order to capture M31, regardless of the method of imaging you use - afocal, eyepiece projection or prime focus. Btw, you are right about what each of these mean - afocal is EP + lens, eyepiece projection is EP and camera directly and prime focus is just camera and no eyepiece.

Prime focus is best way to capture images. Here you'll need to match object angular size and sensor diagonal size via above formula to get needed focal length. If you have APS-C sensor that has diagonal of about 27-28mm, again, you'll need telescope around 380mm of focal length to frame M31 nicely.

Skymax 180 has 2700mm of focal length if I'm not mistaken - it is very narrow FOV telescope.

To get good image of M31 you really need something like this:

https://www.firstlightoptics.com/pro-series/sky-watcher-evostar-72ed-ds-pro-ota.html

With reducer. Possibly 80mm F/6 telescope (that is 480mm of focal length) again with reducer.

These scopes use dedicated field flatteners that are sometimes focal reducers. Simple x0.5 focal reducer won't work well with them (edges will be blurry) - this is because focal plain is not straight, it is slightly curved.

[M40]

Thanks for all the info vlaiv. I have been working my way through understanding the EP and that makes sense; I see where you are coming from with the tunnel vision. Between you and Captain Magenta and his topic reverse engineering of the skymax, I have more than enough maths to sort out 😜 Not anywhere near getting to grips with the focal reducer so that's my next challenge.

It has become very clear though that to get the whole of M31 in the frame using this telescope is not really going to happen, so you saved me a couple of pennies on getting an EP that wouldn't work so many thanks for that.

[Captain Scarlet]

On 12/11/2020 at 12:27, vlaiv said:

... I reckon that skymax 180 illuminates about 30-35mm due to having 30mm rear baffle and only 25mm exit hole at the back of the scope. ....

I have recently thoroughly measured my Skymax 180, and it turns out that with the supplied 2" back and diagonal its focal length is close to 2900mm rather than 2700mm. I suspect the 2700mm is left over from the days when it was supplied with a short vb and 1.25" diagonal.

I determined its focal length by taking photos of stars, and in the process also discovered that its image circle is about 40mm. See the pic here, taken with a full-frame Canon EOS 6D:

Edited November 13, 2020 by Captain Magenta

[vlaiv]

34 minutes ago, Captain Magenta said:

I determined its focal length by taking photos of stars, and in the process also discovered that its image circle is about 40mm. See the pic here, taken with a full-frame Canon EOS 6D:

It would be really interesting to measure vignetting and illuminated field up to half of illumination (50% of peak intensity).

I guess that would be interesting to anyone trying to image. Light reduction of more than 50% while correctable with flats - will simply lower SNR too much.

[Captain Scarlet]

Perhaps one could take a photo of a uniformly flat target and measure the SNR at a series of locations across the whole field. One could then infer illumination from the SNR? I might take that photo and do the measurements...

Edited November 13, 2020 by Captain Magenta

[vlaiv]

8 minutes ago, Captain Magenta said:

Perhaps one could take a photo of a uniformly flat target and measure the SNR at a series of locations across the whole field. One could then infer illumination from the SNR? I might take that photo and do the measurements...

I think that perhaps easiest thing to do is just take regular flats.

Then it is fairly easy to get percentage of illumination - by simply measuring ADU values in center (like average of small circular selection) and ADU values in the corner. Divide the two and you'll get illumination percentage.

In principle, SNR loss is square root of reciprocal of illumination percentage at best. For example, if one has 50% illumination at the edge - this means that center receives x2 signal compared to the edge (reciprocal of 0.5 - which is 50%) and SNR of pure signal is square root of signal, so in this case, SNR at edge is sqrt(2) = ~ x1.41 worse at best. It is never that good because we have dark current noise and read noise that don't depend on illumination and stay the same.

[Captain Scarlet]

So yes then. Flats would be the thing. I’ll take some shots and post up, and have a go at doing the calcs myself as well.

Btw out of curiosity ... do you not sleep? 😉

M

[vlaiv]

1 minute ago, Captain Magenta said:

So yes then. Flats would be the thing. I’ll take some shots and post up, and have a go at doing the calcs myself as well.

Btw out of curiosity ... do you not sleep? 😉

M

In about 1h maybe?

I tend to stay up late and usually sleep in in the mornings. Luckily, my job (partially responsible for that sort of sleep cycle) allows me to start late as I work from home.

Btw, I'm 1h ahead of GMT so it's well past midnight here ...