Doing my head in with numbers

[Marky1973]

I've been rebuilding my set-up to widen the field I can image narrowband targets with with an initial plan on the Evostar ED72 and AWO183MM but a recent discussion with Uranium/Rob started me thinking more carefully about resolution and Rayleigh limits.....which means I have gotten myself even more confused as I am not sure the scope/camera is the "best" match and then I stumbled across the TS APO65 which looks to be a serious contender to the ED72 - flat-field, quadruplet, 44mm corrected field and less issues about camera back focus (I think) as it has the flattener built in. I know the early ones had problems with pinched optics and astigmatism in the cold, but I believe that was sorted in the later production.

So, I got to looking at the figures for these two scopes.... (I hope I've got this right) - and I included the 0.85 flattener/reducer in the 72 calculations to make it equal

 

Ultimately the under/oversampling on the Rayleigh limits doesn't look "massive" on either scope, but Rob pointed out that we are not sure if the ED72 could cope with the big 1600 chip. I believe the TS65 could with its 44mm corrected field. So, looking at these figures, if I was to stick with the ED72, I should go for the 490EX (expensive!) or, if I went for the TS65, then the 1600mm might be the better bet (just?)

The 1600mm/TS65 obviously gives a wider view than the 72/490ex combination, but there isn't a great deal in it

 

 

I am probably getting too carried away with looking at the numbers but, from this information, I am presuming that either of those two combination would be "okay" and I just need to figure out how much I really want to spend. The 490ex is £1k more than the 1600mm, but I am familiar with CCD imaging and daunted about moving to CMOS and learning all over again....but the 1600mm looks like a nice option with the 65....anyway, not asking anyone to make a decision for me, just making sure that, if I went with either of these options, I wouldn't be creating a set-up that would cause problems down the line.

Thanks for reading (if indeed you still are!! ??)

Mark

 

[spillage]

Sorry I cannot help but I too keep looking at the ed72 to go with my 1600. I seem to have struggled to find out much about these two paired together but know @moise212 has posted here that he has both, but cannot find any images or user comments.

I have never owned a refractor but for the price the ed72 keeps catching my eye. I do wonder though, how long could you life with the flattener. My budget is tight and I have some gear to sell but not enough to include the flattener.

 

Mark.

 

[Marky1973]

I don't think the reducer/flattener is essential.... obviously you will have to put up with curvature at the edges of your images, which may or may not be a problem depending on your targets and how fussy you are.

The point that was made to me was that without knowing the corrected field of the ED72 it is difficult to know if it will be able to cover the whole of the sensor of the 1600....mind you, I struggle with understanding that much, so it's the blind leading the blind! ?

It does look like a lovely scope though....

[pete_l]

I wouldn't worry too much about over/under sampling. Personally I reckon that a factor of 3, either way, doesn't make much difference to the final image once it has been stacked and processed.

But the unobstructed image circle diameter is another matter. Given that this is important for imaging, it is surprising how difficult it is to get this from the manufactures. I asked Meade about this for a top-of-the-line scope I was considering and all they could say was "it should be all right for an APS sized sensor".

While it isn't an issue for small sensors, it does seem to me that using a small sensor is a waste of all that valuable FoV. And of course, a focal reducer can only deliver it's unobstructed field if the scope in front of it can fill the FR's field of view.

[Marky1973]

That's part of the reason why the Ts65 is looking attractive as I believe it should be able to cover the 1600 sensor and my DSLR....

 

[alexbb]

I believe the dedicated 0.85x reducer/flattener is designed to cover an aps-c size sensor, but it's not yet available. I have one universal non reducing OVL flattener that I experimented with together with a Canon 550D, but I didn't figure out the backfocus for it. I expect the backfocus for the dedicated one to be 55mm to fit directly a Canon DSLR and the standard m48-canon adapter.

I did not experiment with the ASI1600 and the 72ed as I have an Esprit 80 too, which of course I prefer.

I will experiment more with the 72ed when I will receive the dedicated flattener, but I'm afraid it won't be available until August maybe.

If everything goes smooth also with the 72 and the flattener, I consider buying another ASI1600 for a dual setup.

[Marky1973]

That would be interesting to know - if the reducer does mean the ED72 could cover the 1600mm sensor, that might have an influence on my decision (or make it even harder) - I have both the ED72 and reducer on order....which is convenient... ?

[Jonesdee]

Hi Mark

I have recently bought an ASI 1600MM Pro with the ZWO mini filter wheel and 31mm unmounted filters. I was surprised to find that with my Altair 80mm ED-R refractor (2" focuser and tube) I am only just avoiding vignetting.  Initially I mounted the Filter Wheel the wrong way round, so that the carousel was slightly further away from the sensor and vignetting was clearly evident on all corners.  

If you were to go for the 1600MM, then depending on the scope, you may need to crop your images anyway.  It certainly made me wonder if I should have gone for 36mm filters in case I ever move to a camera with a larger sensor.

Possibly worth factoring into your numbers game

Cheers

Dave

[alexbb]

1 hour ago, Jonesdee said:

Hi Mark

I have recently bought an ASI 1600MM Pro with the ZWO mini filter wheel and 31mm unmounted filters. I was surprised to find that with my Altair 80mm ED-R refractor (2" focuser and tube) I am only just avoiding vignetting.  Initially I mounted the Filter Wheel the wrong way round, so that the carousel was slightly further away from the sensor and vignetting was clearly evident on all corners.  

If you were to go for the 1600MM, then depending on the scope, you may need to crop your images anyway.  It certainly made me wonder if I should have gone for 36mm filters in case I ever move to a camera with a larger sensor.

Possibly worth factoring into your numbers game

Cheers

Dave

To a certain degree there will be vigneting in most scopes if not in all.

What's important, is to have the image optically corrected to the corners. The vignetting will be evened out by calibrating with flats. I imaged at F/4 and 1.25" filters with no issues.

[Marky1973]

22 hours ago, Jonesdee said:

Hi Mark

I have recently bought an ASI 1600MM Pro with the ZWO mini filter wheel and 31mm unmounted filters. I was surprised to find that with my Altair 80mm ED-R refractor (2" focuser and tube) I am only just avoiding vignetting.  Initially I mounted the Filter Wheel the wrong way round, so that the carousel was slightly further away from the sensor and vignetting was clearly evident on all corners.  

If you were to go for the 1600MM, then depending on the scope, you may need to crop your images anyway.  It certainly made me wonder if I should have gone for 36mm filters in case I ever move to a camera with a larger sensor.

Possibly worth factoring into your numbers game

Cheers

Dave

Hi Dave

Yeah, thanks - more numbers! ? 

Looking at the filter size calculator on Astronomy Tools, my 1.25" filters should be okay...fingers crossed.....so much to think about, but I think I am close to settling on the 72 and the 1600. Just as long as I don't start looking at other scopes/combinations in the meantime! ?

Cheers

Mark

 

[Marky1973]

On 17/06/2018 at 11:32, moise212 said:

I believe the dedicated 0.85x reducer/flattener is designed to cover an aps-c size sensor, but it's not yet available. I have one universal non reducing OVL flattener that I experimented with together with a Canon 550D, but I didn't figure out the backfocus for it. I expect the backfocus for the dedicated one to be 55mm to fit directly a Canon DSLR and the standard m48-canon adapter.

I did not experiment with the ASI1600 and the 72ed as I have an Esprit 80 too, which of course I prefer.

I will experiment more with the 72ed when I will receive the dedicated flattener, but I'm afraid it won't be available until August maybe.

If everything goes smooth also with the 72 and the flattener, I consider buying another ASI1600 for a dual setup.

Yeah, I have been in touch with Steve at FLO and he says the same thing about the reducer being designed to cover an APS-C sensor, which helps with my decision - slight undersampling, but to a much lesser degree than my old set-up, so I'm going to try and ignore it.... ?