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Will an upgrade be worth it. F ratio quandary!


bomberbaz

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Ok been thinking for a while about upgrading my imaging telescope to increase aperture, reduce image size and reduce integration time.

Current gear Evoguide +FF and zwo183MC osc. Evoguide supposedly 240 fl (F4.8) but ASIAIR reports it as 267fl which would give F5.34. 

I do not know if this if this is accurate but assuming it is, it makes things less complicated.

Anyway, I was thinking of getting the SW 72ED + a FF that leaves it at it's native focal length of 420 F5.8.

I have calculated (assuming the evoguide fl is 267) and the 72ed is true to its stated fl, that the 72ed will gather data at around 1.25 the speed of the evoguide, all other things being equal.

Both ota should be ok with regard to sampling from my back garden.

Am I missing anything here?

EDIT

Further to the above, I have it at a 1/3 improvement for the 72ed over the evoguide but only about 8% if the fl of the evoguide is actually 240. !🙄 

Edited by bomberbaz
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I think you'll see a massive difference going up in aperture from the evoguide. From my experience aperture makes a difference, the thing you'll notice more is the resolution difference. If you can bin the camera or get a larger pixel one that will also speed up signal capture. Never really binned the 183, but I know the 294 performs better in terms of signal acquisition.

Note, you'll still be imaging 4-8 hours per image at least unless it's one of the bright common targets. More time will be better if you've got the patience.

Edited by Elp
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48 minutes ago, Elp said:

I think you'll see a massive difference going up in aperture from the evoguide. From my experience aperture makes a difference, the thing you'll notice more is the resolution difference. If you can bin the camera or get a larger pixel one that will also speed up signal capture. Never really binned the 183, but I know the 294 performs better in terms of signal acquisition.

Note, you'll still be imaging 4-8 hours per image at least unless it's one of the bright common targets. More time will be better if you've got the patience.

I have been looking at the 533mc, seems to marry up ok when running it through this and has no amp glow   https://astronomy.tools/calculators/ccd_suitability

Last time of gathering data on deerlick group, the total data grab was 7 hours so happy putting in the time.

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I suspect you're after the focal length.

Have you considered a samyang 135? It's stupidly fast, but obviously aperture/resolution and FL limited.

I think I've explored most speed improvement avenues, and there's no substitute for total imaging time, due to weather though it can be frustrating.

Edited by Elp
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23 minutes ago, Elp said:

I suspect you're after the focal length.

Not exactly sure what you mean by this although I know increasing or decreasing this affects performance both ways.

What I have with the evoguide has served its purpose in getting me into imaging, now I just want to step it up a little.

As you know it is married up to the az GTi so in my efforts to improve performance, I must be wary of weight as I have no wish or intention for that matter of upgrading my mount. The 72ED seems to have the perfect compromise of weight and hopefully performance.

I have now forgot what point I was trying to make here 🤣

Anyway, seems it will work so just need to look at finance 😭 haha

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I use the 72ed on az gti with an zwo asi294mc pro. The amp glow isn't an issue and is so easily calibrated out and I've never seen it on my images. You do tend to get more star bloat with the 72ed but with filters an Astronomik L3 in my case, the stars are tight. (Attached my recent images) 

Lee 

Ngc-2244-The-Rosette-Nebula-13+23-2-23-2hr-51-mins-combined_edit_87756724051712.jpg

IC410 The Tadpole Nebula 24-1-23.jpg

IC 1805 The Heart Nebula.jpg

Edited by AstroNebulee
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42 minutes ago, AstroNebulee said:

I use the 72ed on az gti with an zwo asi294mc pro. The amp glow isn't an issue and is so easily calibrated out and I've never seen it on my images. You do tend to get more star bloat with the 72ed but with filters an Astronomik L3 in my case, the stars are tight. (Attached my recent images) 

Lee 

 

Thanks Lee. Is there enough room in the focus to accommodate a filter drawer. https://www.firstlightoptics.com/zwo-accessories/zwo-2-filter-drawer-m42-m48-v2.html

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20 minutes ago, bomberbaz said:

Thanks Lee. Is there enough room in the focus to accommodate a filter drawer. https://www.firstlightoptics.com/zwo-accessories/zwo-2-filter-drawer-m42-m48-v2.html

Yep I have the 2 inch filter drawer in my setup. I'll attach an image of my setup and a diagram of the components. I still have a little bit of focus tube travel so all ok. I have the elastic band on the filter drawer just as added security for the drawer but the magnets are sufficient enough. My 72ed is the so called longer tube version as opposed to the short tube version as opposed to the new short tube version which is 10mm shorter. This won't be an issue though. 

Lee 

IMG_20230401_203406.jpg

IMG_20230401_203639.jpg

Edited by AstroNebulee
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Thanks Lee. Glad you put that in. Never knew I also would need a rotator too. 

12 minutes ago, AstroNebulee said:

Yep I have the 2 inch filter drawer in my setup. I'll attach an image of my setup and a diagram of the components. I still have a little bit of focus tube travel so all ok. I have the elastic band on the filter drawer just as added security for the drawer but the magnets are sufficient enough. 

Lee 

 

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19 minutes ago, bomberbaz said:

Thanks Lee. Glad you put that in. Never knew I also would need a rotator too. 

 

I have the rotator just to help framing some targets and because it's an all screwed imaging train and I like the 294mc pro orientation in landscape to my scope tube if that makes sense, so the longest side of the sensor is aligned to the top of my scope. (just my ocd I guess) . I've only rotated it once on the soul nebula but never captured enough data on it. Every other target has fitted in perfect, (M31 not rotated attached) I use the RVO rotator as it has less wobble when the thumbscrews are undone to rotate than the skywatcher one has. It's just much better. 

Lee 

M31-Sept-&-Oct-2022_edit_124103626154500.jpg

Edited by AstroNebulee
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1 hour ago, AstroNebulee said:

I have the rotator just to help framing some targets and because it's an all screwed imaging train and I like the 294mc pro orientation in landscape to my scope tube if that makes sense, so the longest side of the sensor is aligned to the top of my scope. (just my ocd I guess) . I've only rotated it once on the soul nebula but never captured enough data on it. Every other target has fitted in perfect, (M31 not rotated attached) I use the RVO rotator as it has less wobble when the thumbscrews are undone to rotate than the skywatcher one has. It's just much better. 

Lee 

 

Had another look at the 72ED and won't bother with the rotator. I rarely rotate anyway but if I need to simply slacked the tube rings. Think that will suffice here too and the less parts on the focal train, the better imho.

Going to have a good think about all my gear next, I simply have too much and need to scale down before I get the 72ED.

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15 minutes ago, bomberbaz said:

Had another look at the 72ED and won't bother with the rotator. I rarely rotate anyway but if I need to simply slacked the tube rings. Think that will suffice here too and the less parts on the focal train, the better imho.

Going to have a good think about all my gear next, I simply have too much and need to scale down before I get the 72ED.

Good idea about rotating with the tube rings 😊, I'll look forward to how you get on Steve, keep us posted. 

 

Lee 

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Your increase in aperture is not going to reduce your integration time per se. It would do if the focal length of the new scope (and the camera) was the same, but your increased focal length means not. In simple terms the F ratio is probably the best indicator of integration time, and you're slightly increasing that.

But yes, with the same camera you get a reduced image size, and an increase in resolution. With the 183, your current resolution is 1.85 arc secs per pixel, and you would reduce that to 1.18 with the 72ED. Seeing will determine whether that really makes a difference in your end results, and you may find there's less difference than you expect, as both of these are good numbers for imaging in UK skies (I'm quite happy with my 1.98 arc secs per pixel at 620mm). However,  I guess the optics may better with the 72 and that's a whole other issue.

Changing camera changes it all again. The 533 has bigger pixels, but not really a bigger sensor (different shape obviously), so in some ways this would be a backward step - back to your 1.85 arc secs per pixel in fact, but the 533 CMOS sensor is newer and performs better. 

That's the problem with this game - there are a lot of variables and they all contribute to where you end up, so it's worth being really clear what you are trying to achieve before pressing the button!

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