~2000mm ota options

[powerlord]

Hi chaps,

Im enjoying shooting stuff with my old C8 at this sort of FL, but of course it's not really made for imaging. I can use the 6.7 flattener/corrector but then I'm down to 1340.

What ota would folk suggest either new or second hand for imaging around 2000 (4000 with a Barlow)?

Be good to get some ideas from what folk are actually using rather than just picking out stuff from FLO that meets the specs.

Mainly using with Asi1600 and asi533mc. Eq6r-pro.

Budget.. Maybe 1500 tops?

[The Lazy Astronomer]

Can I ask why you want such a long focal length? With your camera's pixel size, that would give you a sampling rate of around 0.4"/px, which is well below what the atmosphere is going to allow for long exposure deep sky work.

[bottletopburly]

A 1000mm fl 200pds using the zwo533mc bin 2x2 works out ok at about 1.22 pix if memory serves me well .

Edited September 3, 2021 by bottletopburly

[alacant]

10 hours ago, powerlord said:

imaging around 2000

Hi

I think the imaging version of the C8 is called 'Edge'. 

If you've good enough atmospheric conditions, it maybe worth a look.

The longest we go is 1200mm. With DSLRs, but only on the occasions when the atmosphere supports us. 

Edited September 3, 2021 by alacant

[Clarkey]

Assuming you are looking at planetary imaging at this FL then a CC would be and option. Good for imaging and visual and less of a dew magnet than and SCT. I image with an RC8 which is excellent once properly collimated. Should be fine with you camera and mount combo. Also within budget.

[michael8554]

It's really not clear whether the OP want's to do Planetary - "4000mm with Barlow"  ?? 

Or DSO's ?

Or both ?

ASI1600 plus Filters and ASI533mc suggests it's DSO's.

The 4/3rds ASI 1600 probably uses more than the coma-free area of the C8's imaging circle.

So I expect a Celestron C8 Edge HD, a Meade 8" ACF, or an 8" Ritchie-Chretien should do the job.

Michael

 

[vlaiv]

What would be intended usage?

In any case - I think that selecting telescope based on focal length is not very good way to do it.

If you are interested in planetary - then look for largest aperture that you can mount and afford. Here correction of the field of telescope is not important as you'll be using only central part of the field. F/ratio and focal length are not important - you'll be adjusting those with use of barlow lens depending on pixel size of your camera.

If you are interested in DSO - I suggest you set realistically your working resolution first. Do you want to image at 1.5"/px, 1.3"/px or perhaps try 1.2"/px and do you have the mount and the skies for it? Then look at your cameras and see what sort of focal lengths will give you your target sampling rate - either natively or with some way of binning.

In the end - go with largest aperture for selected focal length that will give you corrected field large enough to cover your sensor (that you can mount and afford of course).

[powerlord]

I'm only going with my eyes and my results. DSO only.

I'm guiding at around 0.3" rms. And as far as I can see the limit I'm hitting is the limitations on my old C8 (it's just not right - maybe corrector still not right or something). Every indication is that with my asi1600 I could do 2000mm okish. Since that works out at .39"/pixel.

I've got a 200p, and not actually tried that yet - it's such an awkward thing. I really like the C8, and will be TRYING to get it properly looked at - Viking over in Halesworth is quite near me and offer servicing for all OTAs - so I think I will give that a try first.

stu

[vlaiv]

2 minutes ago, powerlord said:

. Every indication is that with my asi1600 I could do 2000mm okish. Since that works out at .39"/pixel.

ASI1600 can do 0.39"/px just fine. Question is what are you loosing by doing so?

Let me show you something.

This above is image of M51 taken with 8" RC and Atik 314+ at 0.82"/px (taken from here: http://www.astrosurf.com/topic/134977-m51-au-rc8/ ). On that image I superimposed image that matches 0.82"/px with full resolution.

You can clearly see that 0.82"/px image taken with RC is not as sharp / detailed as image that matches 0.82"/px.

0.82"/px is simply wasted on level of detail produced with said scope and mount.

Now have a look at same thing but this time at 1.6"/px (I just binned both images x2 and made same flip animation):

Now there is virtually no difference in the detail in galaxy except that one image has tighter stars. This means that actual resolution of that image is closer to 1.6"/px and definitively not as low as 0.82"/px

You want to image at twice as high resolution at 0.4"/px - and that is fine except for two things:

1. You simply won't record detail to match that resolution

2. You will be wasting your imaging time since you need to achieve certain SNR and going higher in resolution means that you are spreading light over more pixels and each pixel receives less signal. SNR will drop. For each doubling in resolution (like using 0.8/px instead of 1.6"/px) you need to quadruple imaging time to reach the same SNR.

If you go for 0.4"/px over say 1.6"/px - you'll need to image for x16 as long in order to achieve same SNR (for same aperture size of course).

[powerlord]

Thanks Vlaiv, I do get you. The unknown is I suppose what skies are like here. Plus that I just don't think my C8 is giving its best just now.

Once I have my C8 fixed, I can do a simple test - same target - C8 straight, vs C8 with 0.67 reducer/flattener.  That should show whether its worth imaging without the reducer or not surely ? If, adjusted to the same pixel size, there is no more detail in the reducer version, then I stick with my c8/reducer ? We're getting into winter, and it seems to me imaging time is going to be less of an issue - and I'd like to focus (sic) on some smaller targets.

stu

[vlaiv]

13 minutes ago, powerlord said:

If, adjusted to the same pixel size, there is no more detail in the reducer version, then I stick with my c8/reducer ?

If you work at same sampling rate with and without focal reducer - in principle you will see no difference at all as far as detail and time taken is concerned.

Only difference will be in size of FOV.

Say you work with ASI1600 natively at 2000mm - for 0.39"/px and of course since that is hugely over sampling - decide to bin x3 your result for ~1.18"/px. Adding focal reducer will get you to ~1340mm and that means ~0.58"/px. Again you are oversampling so you can bin x2 this time - and you'll end up with ~1.17"/px.

There you go, in both cases you'll be around 1.18"/px with 8" of aperture - same "speed" in both cases. If reducer is optically good - there will be no difference in the detail as you did not change - your aperture, nor your mount nor your skies. All three important ingredients will stay the same regardless of your use of focal reducer.

[powerlord]

well, we'll see. time isn't a variable here. no binning. lets just see which shows more detail. snr aside. it should be pretty obvious whether there is a benefit or not I think.