Well, I finally succumbed to the Cat

[900SL]

Interesting word, succumb. It conjours up images of being swallowed up by a couch. Anyway, I digress.

 

Yes, after loving my Samyang 135 for fast w-i-d-e field but being a bit dissatisfied with the framing (and the corner stars on my copy), I finally plumped for a V2 Redcat 51 for my lightweight travel rig and sent yet more depreciation tokens off to @FLO  (send me a good un, chaps )

I'm thinking that although a touch undersampled with a 533MC pro or D5600, a quick run through the regular suspects on Telescopius shows that the framing is pretty spot on for many of the wider nebula targets

Hopefully first light at the end of this month assuming DHL do their usual magic

Later in the year I hope to run it alongside my GT71 as a dual scope rig.

That's all for now

 

 

 

[vlaiv]

12 minutes ago, 900SL said:

I'm thinking that although a touch undersampled with a 533MC pro or D5600

Have you seen the specs for V2 RedCat 51?

MTF below 90% for 30lpmm.

That is line pairs per mm. 30 of them. Per 1000um - so one line pair is 33.33um or line width of 16.66um.

And you think that 3.76um pixel will be under sampled? That is x4.5 smaller pixel size than you need to get below 90% contrast.

Samyang 135 F/2 has better MFT fully open over the size of 533mc:

Grey line is 30lpmm

[900SL]

but, but, what does all that mean Vlaiv? I have no idea what any of that means

My simple understanding is that the image scale of the combination is 3 arc seconds per pixel, which I assumed was undersampled. Obviously it's not that simple  

I guess this is to do with aperture and resolution?

I like the SY 135 but mine must have some decentering because I get poor stars even at f4 out of center

Edited August 12, 2022 by 900SL

[vlaiv]

First, don't get me wrong - I'm not saying that scope is poor in any way.

You'll be hard pressed to find well corrected scope that can illuminate 43mm diameter.

However, such a small aperture, coupled with usual imaging conditions - seeing FWHM 2" and lightweight mount that usually guides around 1" RMS has limit in how much it can resolve - even for diffraction limited optics.

In such conditions - 50mm of aperture can resolve to about 2.4"/px (~3.82 FWHM stars).

This is with diffraction limited optics. When you correct over such large field - sharpness of optics goes down and as result FWHM goes up.

Here - for this scope, in linear units, diameter of Airy disk is 6.6um

(see here: https://www.astropix.com/html/astrophotography/astrophotography-calculators.html#ldad)

However, if you look at this:

which is spot diagram for RedCat 51 V2 - you see that even on optical axis, Geo Radius (which is half of diameter) is ~7.4um - or diameter is 14.8um

That is ~x2.3 larger.

Scope acts as if aperture is x2.3 smaller as far as "sharpness" is concerned (or about 22mm of aperture).

In above calculation 2" FWHM seeing, 1" RMS guiding - this will produce about 6" FWHM stars - and needed sampling rate is closer to 4"/px rather than 3"/px

(you get similar results if you take 3um RMS radius and compare it to 0.95" RMS radius for 51mm aperture - which at 250mm translates to ~1.15um and again 3/1.15 = ~2.6 larger).

In any case - this shows that 3"/px is over sampled for this lens in regular conditions - same conclusion we came to by examining MTF graph.

Again - this is not fault of the scope / lens, but rather problem with small pixels.

[900SL]

Thanks Vlaiv, I slowly learning from you. I will try it end of month on couple of targets on D5600 and see what I get.

[900SL]

@vlaiv so is there any benefit in 2*2 binning? 

[vlaiv]

6 minutes ago, 900SL said:

@vlaiv so is there any benefit in 2*2 binning? 

Depends what you want to achieve.

Most people don't have issue with being over sampled. I personally don't like it, and to me it is better to have sharp image when viewed at 100% zoom.

If you don't mind slightly soft appearance when viewed at 100% - you don't need to do anything in particular.

If you want to have sharp looking image when viewed at 100% - then you have several options. You can debayer with interpolation and then bin x2 resulting image after stacking. This will produce sampling rate that is adequate for the image, but it won't produce expected SNR improvement. For expected SNR improvement to happen - you need statistically independent data (like in mono camera - where each frame is as is - not debayered). With debayering, you interpolate missing values and you create statistical dependence between pixels to some degree.

It is a bit like using stacking same sub several times and expecting to see improvement - you won't see it, you need every sub to be new exposure - you can't just copy one sub 10 times and stack that.

Anyway - you can do it like that - bin x2 with some SNR improvement (but not x2 SNR improvement as you would expect from mono data), or you can use split debayer - which will only reduce sampling rate, but won't perform any interpolation.

If you decide to use mono + filters - then binning is certainly the best option as it will work "as advertised".