Recommended a refractor please

[Pitch Black Skies]

I'm looking around for an imaging refractor and can't decide. 

 

I'm using a mono camera, 294MM with LRGB,SHO.

Do I still need a triplet to avoid CA even though I will be focusing on each colour separately? My guess is that problems will arise when shooting L.

I want to image large targets such as m31, m42, Elephants trunk nebula, etc. I find my 130pds just a little too cropped in for these. Collimation is becoming a pain and I'm not sure if I'm really keen on the diffraction spikes anymore.

Currently I'm looking at WO 72GT, SW 72ED, SW 80ED. These are within my budget.

Thanks, 

Alan

Edited August 21, 2022 by Pitch Black Skies

[Lee_P]

I've been using an Askar FRA400, and rate it highly. FLO have them for sale (out of stock though). I've written a review, and have lots of example photos (albeit taken with an ASI2600MC Pro OSC camera, so different to your 294MM).

[newbie alert]

So do you want something to capture m31 in full?

The stellarmira 90 is a cracking scope for the money.. not sure on your budget but it should fit in especially if you buy the reducer

[newbie alert]

Yep sure does 

Edited August 17, 2022 by newbie alert
Changed image to show camera angle at 90

[Clarkey]

You will need a triplet or well corrected doublet. I can vouch for the SM90 but I suspect it is a bit over budget. The Askar FRA300 and 400 seem good and there are a couple of the small Sharpstar scopes that might suit. I think it depends on FL and budget really.

[vlaiv]

9 hours ago, Pitch Black Skies said:

Do I still need a triplet to avoid CA even though I will be focusing on each colour separately? My guess is that problems will arise when shooting L.

Yes, L will be problem and it will result in slightly bloated stars.

If you are really on a tight budget - there is solution for bloated stars in L - Astronomik L3 as luminance filter. It removes a bit of spectrum on far ends that usually cause bloating. It will work with OSC as well, but even better with LRGB approach.

As far as scope goes - here is a good one:

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80-mm-f-6-FPL53-Triplet-Apo---2-5--RAP-Focuser.html

Not sure how much it will cost after import duty and tax - could be comparable in price to WO81? It is triplet though.

Alternative is same scope from AA:

https://www.altairastro.com/starwave-80-ed-r-triplet-apo-travel-refractor-465-p.asp

(but currently out of stock)

[Pitch Black Skies]

How important is the sampling rate and what is a good general guideline to have?

The CCD tool recommends ~ 0.7-2"/pixel for good seeing. With that in mind, the FRA400 gives out 2.4"/pixel on the 294 in bin2.

In the unlocked bin1 mode it gives 1.2"/pixel. Problem is, the full well capacity is very low and file sizes will be huge in that mode. The sub exposure times would probably be painfully slow too.

Similar situation with TS Apo recommended. 

Edited August 21, 2022 by Pitch Black Skies

[vlaiv]

7 minutes ago, Pitch Black Skies said:

How important is the sampling rate and what is a good general guideline to have?

The CCD tool recommends ~ 0.7-2"/pixel for good seeing. With that in mind, the FRA400 gives out 2.4"/pixel on the 294 in bin2.

In the unlocked bin1 mode it gives 1.2"/pixel. Problem is, the full well capacity is very low and file sizes will be huge in that mode. The sub exposure times would probably be painfully slow too.

Similar situation with TS Apo recommended. 

Depends.

There is optimum sampling rate for given conditions - one that will capture all detail available (and let you sharpen your image further) - but will not over sample or use more pixels for the job than necessary.

You can think of it as dividing line - below this line is under sampling - using "less than needed" pixels to record image and above this line is over sampling - using "more than needed" pixels to record the image.

Nothing wrong with under sampling. In fact - you often need to under sample if you want to get wide field image as you simply don't have enough pixels to cover big patches of the sky.

Over sampling is causing two problems. One is aesthetic in nature - and I guess often people simply don't bother with it. Other is problem with speed.

If you use more pixels than needed - you spread light over more pixels, and in return each pixel gets less light - weaker signal, and SNR suffers.

For example - if you sample at 1"/px instead of 1.5"/px (that might be optimal in given case) - you will need to image x2.25 longer to get same SNR. This number is ratio of surfaces of pixels - 1"/px is actually 1" x 1" = 1" squared per pixel. While 1.5"/px is actually 1.5" x 1.5" = 2.25" squared.

That is x2.25 more sky surface per pixel.

Aesthetic problem is related to when you zoom in to 100%. Over sampled image will simply show bloated stars and blurred object instead of pinpoint stars and nicely defined object. Some people are not bothered by this as they never look at image at 100%.

I always look at image at 100% - otherwise, what is the point of capturing it with so many pixels if you are not going to look at it like that?

Astronomy tools CCD suitability is flawed tool. I started thread at one point to discuss this and to maybe get it changed so it gives good values - but nothing came out of it.

Sampling rate does not depend only on seeing - but in general on FWHM of stars in your image - which in turn depends on seeing, mount performance, aperture size and sharpness of the optics (meaning spot diagram of scope + any flattener combination).

In any case - you'll be hard pressed to actually produce 1"/px image. Most setups can only achieve about 1.8 - 2"/px. Some can go down to 1.5"/px -and very few can go below 1.5"/px and only in good conditions.

[Pitch Black Skies]

Can I get an opinion on this telescope please?

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p7085_TS-Optics-70-mm-f-5-Quadruplet-Flatfield-Apo-mit-3-Element-FPL53-Objektiv.html

The sampling rate comes in at 2.75"/pixel paired with the 294MM. Is that anything to worry about? Should I be trying to get closer to 1.5"/pixel? My guess is that I would need a larger telescope to resolve down to 1. 5"/pixel anyway.

Edited August 22, 2022 by Pitch Black Skies

[scotty38]

You can use the smaller 2.3um pixels.......

Edited August 22, 2022 by scotty38

[Pitch Black Skies]

27 minutes ago, scotty38 said:

You can use the smaller 2.3um pixels.......

Will the image be less detailed using bin2?

I used the bin1 mode on M31 a couple of weeks ago. It's much slower to collect light (4 times I think), the full well capacity is much less, there's a reduction in dynamic range, and the file sizes are huge.

Edited August 22, 2022 by Pitch Black Skies

[scotty38]

Agreed on all of that although I've not yet used that mode on my 294m but I was only suggesting it based on your desire of sampling though.

[Pitch Black Skies]

Thanks, yeah I was considering the bin1 mode I don't think it is the best solution. 

[Clarkey]

I don't think you will have an issue at that sampling rate. My FMA180 is much higher with the ASI1600 and I didn't recoil in horror at the pictures. Being wide field it is slightly less critical.

Another option might be the SharpStar 76EDPH f/5.5 Triplet ED APO Telescope with flattener reducer. Very similar but standard triplet and slightly cheaper once import duties are included.

In terms of real world experience I can't comment. I think both would be good wide field astrographs.

[Elp]

I have no issue with my WOZ61. With an apex reducer it goes down to F3.9 at the cost of distorted stars at the edges. It's not the best image in the world but take a look at my M16 image posted recently considering the very short integration time, I was just happy to be able to image it.

The Samyang 135mm is also an excellent, extremely fast piece of kit to use but I suspect may be wider than you're willing to go also at the cost of resolution.

[scotty38]

12 hours ago, Clarkey said:

I don't think you will have an issue at that sampling rate. My FMA180 is much higher with the ASI1600 and I didn't recoil in horror at the pictures. Being wide field it is slightly less critical.

 

I have to agree. With my 294m and my previous 294mc paired with my GT981 I'm at 2.5" which, while it's not the the OP's 2.75, it's never been the cause of any of my issues.... Yes I can find sharper stars in plenty of other folks' images but if I look at mine in isolation and compared with others mine look fine to me. 

[The Lazy Astronomer]

2.5"/px will not be an issue at all. My native resolution is 1.7"/px and based on the measured FWHM of the stacked image I usually bin 2x2 during processing, which takes me up to 3.4"/px with no ill effect to the image.

[nephilim]

If your after a fairly wide FOV & wanting to move away from a reflector, another option would be a lens.  In particular I'd highly recommend the Canon version of the Samyang 135mm f2  ED UMC manual lens. There's a VERY long thread (quite rightly) singing it's praises here

Cost wise its way less than a decent triplet, very friendly weight wise for the mount & it's capable of producing excellent images. I took this with mine.

 

Edited August 25, 2022 by nephilim
Grammer

[Steve Ward]

4 hours ago, nephilim said:

If your after a fairly wide FOV & wanting move away from a reflector, another option would be a lens.  In particular I'd highly recommend the Canon version of the Samyang 135mm f2  ED UMC manual lens. There's a VERY long thread (quite rightly) singing it's praises here

Cost wise its way less than a decent triplet, very friendly weight wise for the mount & it's capable of producing excellent images. I took this with mine.

 

 

Or for an even wider view this new Sigma lens looks amazing ... there's an 85mm f1.4 too.

 

 

Edited August 25, 2022 by Steve Ward

[nephilim]

9 hours ago, Steve Ward said:

Or for an even wider view this new Sigma lens looks amazing ... there's an 85mm f1.4 too.

 

 

@Steve Ward Although a bit too wide for me that looks a very nice lens for those milky way shooters.The manual focus lock switch also sounds good,  it would be nice if this started to become incorporated into Len's with a narrower FOV aswell 

[Uranium235]

Whatever the final choice is, I can strongly recommend that you first look at, and inspect examples of each optic.

And when I say inspect, I really mean it. Ignore whats in the middle and go straight to the corners at 200%. If its eggy, move on (though that can sometimes be down to skill). Take into account the size of the sensor, what you want is something that can comfortably deliver pinpoints over ~22mm diagonal (roughly APS-C).

Also (though it mostly applies to telescopes) check what glass is in the telescope. If its not fluorite or FPL53, again... move on (dont accept anything less). Im usually quite suspicious of manufacturers who dont (or wont) reveal exactly what glass is in there.

[Pitch Black Skies]

On 23/08/2022 at 21:08, The Lazy Astronomer said:

2.5"/px will not be an issue at all. My native resolution is 1.7"/px and based on the measured FWHM of the stacked image I usually bin 2x2 during processing, which takes me up to 3.4"/px with no ill effect to the image.

Would it not be better for you to bin at data capture instead? It would reduce your imaging time.

[The Lazy Astronomer]

23 minutes ago, Pitch Black Skies said:

Would it not be better for you to bin at data capture instead? It would reduce your imaging time.

Makes no difference with CMOS as the binning is in software even if done at point of capture, so may as well capture at bin1 and bin in post based on measured FWHM of the stack. I always hold out hope for some excellent seeing and the chance to leave it at bin1 in post, but alas, it is yet to happen 😔

[Pitch Black Skies]

Whoops, I must have got the wrong end of the stick somewhere 😅.  I thought by using bin 2 I was making the camera faster.

TS80 with 294MM 

Bin 1 = 0.99"/pixel

Bin 2 = 1.99"/pixel

Using aperture^2 x pixel scale^2 I get

Bin 1 = 6272

Bin 2 = 25,344

I thought by using Bin 2 I was going to make the system 5x faster.

So are you saying that a typical 5min sub will be the same brightness on either Bin?

I have spent the last 3 days crunching numbers trying to find the optimum setup over various scopes and bin factors.. 😬🤣

Edited September 6, 2022 by Pitch Black Skies
Typo

[The Lazy Astronomer]

Ah, ok, the 294MM is a special case (I should know this really, as an owner of the 294MM myself!!). 

When I say I shoot bin1, I mean using the originally 'locked' pixel size of 4.63um, not the 'unlocked' 2.31um sub-pixels, so l actually do shoot at bin2, but in my head it's bin1 (apologies for confusing the situation 😁).

With the 294MM, bin2 is done on-camera.