Can a RASA 8 be Increased from F2 to F3 or F4

[Catanonia]

OK no doubt this is a unusual question as most sane folks will go for fast F Stop and the RASA8 gives this in spades with F2 @400mm

But can you, or has anyone, put a lens on it to reduce the F Stop and increase the Focal Length ?

This lens element would have to go between the camera on the front and the corrector plate.

Just wondering if it is possible  or if anyone has attempted it ?

 

 

Edited September 7, 2021 by Catanonia

[AKB]

You could, of course, increase its f-number by simply reducing the aperture.  😉

This is actually something which hyperstar users can do to improve star shape… not necessary, of course, with the superior optics of the RASA!

[vlaiv]

I think it would be far easier (and possibly cheaper) to get F/4 newtonian of same aperture than to try to use barlow on RASA. You can pair F/4 or F/5 newtonian with reducing coma corrector to get ~F/3 scope (like PowerNewtonians and Boren Simon astrographs that are at F/2.8 or F/3.2).

[ollypenrice]

I doubt it.  The present final lens group is already very radical and trying to undo its effects would be optically messy, I suspect.

While @vlaiv is here I'd like to come back to an earlier discussion with Catanonia over sampling rate in the RASA.  Although the usual calculation suggests that our RASA 8 with ASI 2600 should be OK working at about 1.8"PP, vlaiv suggested it would be over-sampled. As things stand with our rig, he is absolutely right. It is not going to give us results to present at 100%, though at 66% it is looking good. Further work on collimation and tilt might tighten it up but my instinct is to think it won't. Since we intended our rig to do widefield it's not a problem but I don't think the RASA is going to make a great high res instrument. F2 is what it is and, like all optical configurations, comes with its strengths and weaknesses.

Olly

[tomato]

After reading  the Celestron white paper on the RASA concept, I think choosing another scope would be the way to go if you want a higher focal ratio.

 

[vlaiv]

4 hours ago, ollypenrice said:

I doubt it.  The present final lens group is already very radical and trying to undo its effects would be optically messy, I suspect.

While @vlaiv is here I'd like to come back to an earlier discussion with Catanonia over sampling rate in the RASA.  Although the usual calculation suggests that our RASA 8 with ASI 2600 should be OK working at about 1.8"PP, vlaiv suggested it would be over-sampled. As things stand with our rig, he is absolutely right. It is not going to give us results to present at 100%, though at 66% it is looking good. Further work on collimation and tilt might tighten it up but my instinct is to think it won't. Since we intended our rig to do widefield it's not a problem but I don't think the RASA is going to make a great high res instrument. F2 is what it is and, like all optical configurations, comes with its strengths and weaknesses.

Olly

I have to say that RASA8 is simply most cost effective solution to a particular problem.

Despite the fact that it is very fast optically at F/2 - it is not only solution at that speed (in terms of time to target SNR) - nor the fastest one for that matter - but it is far the cheapest solution.

In this case, "problem" can be stated as - "Give me fastest imaging rig working at ~3.4"/px with FOV that is 3.36° x 2.25°". RASA8 is not the "fastest" solution, maybe also optically not the best, but paired with ASI2600 - it is the cheapest.

Other two solutions that I've found:

Solution No. 1:

Quad setup consisting of:

4 x https://www.teleskop-express.de/shop/product_info.php/info/p12268_TS-Optics-94EDPH---7-Element-Flatfield-Apo-with-94-mm-Aperture-f-4-4.html

+ 4 x ASI2600

It might even be somewhat faster then RASA8. If we account for two mirrors at 97% reflectivity and 46% CO - clear aperture of RASA8 is equivalent to 174.5mm clear aperture (or 4 x 87.3mm)

7 element APO will have 14 glass to air interfaces and if we give each 99.5% transmission (very decent coatings) that is about 93.2% transmission of light in total. That reduces effective aperture from 94mm to 90.7mm of clear aperture.

I'd call that effective tie.

Solution No. 2:

This one will be slightly faster (and will cost many times more )

This time dual setup consisting of:

2 x

https://www.teleskop-express.de/shop/product_info.php/info/p11282_APM-LZOS-APO-Refractor-130-780-mm-with-3-7--APM-Focuser.html

+ 2 x Riccardi x0.75 reducer

+ 2 x ASI6200MC

ASI6200MC has x1.5 larger diagonal than ASI2600 so it needs to be at 600mm of FL to cover the same FOV

780mm x 0.75 = 585mm

Pixels are the same size - so we can bin x3 instead of x2 and get same pixel scale.

But this time we have 2 x 130mm which is roughly equivalent of 2 x 126mm of clear aperture (this time 6 elements - or 12 glass/air interfaces) or 178.4mm

So we have 174.5mm vs 181.4mm vs 178.4mm of respective aperture all working at ~3.4"/px, and people think RASA is fast

(think of quad setup of those LZOS apos instead of dual one - now that is speed!)

Fun, right?

 

[Catanonia]

Knew it was a weird question and I would concur that the cost / practicalities are too high

Get another scope if you want more FL

[Catanonia]

12 hours ago, vlaiv said:

I have to say that RASA8 is simply most cost effective solution to a particular problem.

snip

 

Interesting read and thanks for taking the time to put this together. Spent too long on TS website, but luckily my wallet survived

[newbie alert]

15 hours ago, vlaiv said:

 

 

Pixels are the same size - so we can bin x3 instead of x2 and get same pixel scale

(think of quad setup of those LZOS apos instead of dual one - now that is speed!)

Fun, right?

 

If you bin x2 or x3 it costs you resolution, right?

But yeah give me a 130 LZOS any day of the week

[vlaiv]

1 hour ago, newbie alert said:

If you bin x2 or x3 it costs you resolution, right?

In case of RASA8 and ASI2600 - it does not.

Telescope itself has spot diagram that matches better with ~6.7µm pixel size than with 3.76µm.

In case of RASA8 - you can't get better resolution due to optical compromise made by designers - they wanted to make fast, wide field system that is corrected up to 30mm diameter and they ended up with spot diagram that is quite large for telescope of 8" of aperture.

RASA8 is not diffraction limited optics.

RMS spot size upper limit over the field of telescope is about 4.5µm. That is RMS. Relationship with RMS (sigma) and FWHM is x2.355 so FWHM of RASA8 without any seeing influence or tracking errors is ~10.7µm.

Other two options that I listed will certainly be capable of better resolution. 90mm refractors will certainly easily provide with 2"/px in regular conditions and 130mm one will go at 1.5"/px.

RASA8 from above is capable of about 3.4"/px at best - but again, that is OK for telescope intended use. It is wide field low resolution instrument that is optically fast.

[Catanonia]

17 hours ago, vlaiv said:

In case of RASA8 and ASI2600 - it does not.

Telescope itself has spot diagram that matches better with ~6.7µm pixel size than with 3.76µm.

 

So I am using a ZWO 183MC (colour) with my RASA 8 and always using bin1.  Should I be using bin 2 for even faster captures and not loose resolution ?

 

Edited September 9, 2021 by Catanonia

[vlaiv]

1 hour ago, Catanonia said:

So I am using a ZWO 183MC (colour) with my RASA 8 and always using bin1.  Should I be using bin 2 for even faster captures and not loose resolution ?

 

You will loose "resolution" but you won't loose detail.

Resolution here stands for number of megapixel and level of "magnification" when you show image at 100% of it size - meaning one image pixel to one screen pixel.

Here is an example of what it means to properly sample and over sample image:

I have an image that is sampled at 1"/px - which is oversampling. Actual detail in the image needs only 2"/px:

That is crop shown at 100% of that image.

If I now resample it to 2"/px and show it at 100% (same area crop) - this is what we will see:

All the detail from above image is still here. Image looks sharper and has more contrast - although all we did was just change sampling rate. Now, if you want to look at it enlarged - you can, and you will get exact same image as above:

Indeed - this image is not original one, this is reduced image (one at 2"/px) enlarged to 200% (resampled back to 1"/px) - yet it shows same level of detail.

This tells us that no detail is present in image for sampling rates finer than 2"/px and that is all you need to present data you captured. It looks better when viewed at respective 100% zoom level and if you look at full image (fit to screen) - there will be 0 difference.

If you really want to "zoom in" reduced version - then do so and it will still show same level of detail as oversampled image. You can also try the same with properly sampled image - reduce it x2 and then enlarge it - and you will see the difference because twice reduced version of properly sampled image won't be able to record all the detail.

From SNR point of view - it is better to have properly sampled image than over sampled.

So yes, I'd say that with RASA8 - you actually bin x3 rather than by x2 as it is closer to 6.7µm pixel size if you are working with 2.4µm pixel camera as 2.4µm x 3 = 7.2µm.

 

[Catanonia]

1 hour ago, vlaiv said:

snip.....

From SNR point of view - it is better to have properly sampled image than over sampled.

So yes, I'd say that with RASA8 - you actually bin x3 rather than by x2 as it is closer to 6.7µm pixel size if you are working with 2.4µm pixel camera as 2.4µm x 3 = 7.2µm.

 

wow, so basically you are saying, with the 183 OSC, I should be imaging at least with Bin2 and almost reducing my capture time in 1/2 ?  I could even go bin 3

mmmm, I am planning a 6 panel mosaic and I think I will give this a go as I should be able to get 6 panels done in 1 night.

With the mosaic, I can live with the 1/ 2 resolution

Thank you so much for this.

[Catanonia]

1 hour ago, vlaiv said:

snip

 

So what would be the best ZWO camera in your opinion that would suit the RASA 8 with respect to pixel size / sampling ?

Edited September 9, 2021 by Catanonia

[vlaiv]

1 minute ago, Catanonia said:

So what would be the best ZWO camera in your opinion that would suit the RASA 8 with respect to pixel size / sampling ?

One with the largest sensor size that can be covered by RASA8 corrected circle and highest QE.

Since RASA8 can cover 30mm - that means APS-C sensor size. ASI2600 seems like logical choice as it has higher QE than ASI071MC.

Any ZWO camera you choose will need binning since all have small pixels.

 

[Catanonia]

27 minutes ago, vlaiv said:

One with the largest sensor size that can be covered by RASA8 corrected circle and highest QE.

Since RASA8 can cover 30mm - that means APS-C sensor size. ASI2600 seems like logical choice as it has higher QE than ASI071MC.

Any ZWO camera you choose will need binning since all have small pixels.

 

Shame the 2600 is 17.5 sensor to plate compared to the 1600 and 183 that is 6.5mm .

On a RASA 8 this leaves no room for a filter drawer

Looks a cracking camera and like the fact it is large sensor, good pixel size and has zero amp glow.

[Catanonia]

1 hour ago, Catanonia said:

Shame the 2600 is 17.5 sensor to plate compared to the 1600 and 183 that is 6.5mm .

On a RASA 8 this leaves no room for a filter drawer

Looks a cracking camera and like the fact it is large sensor, good pixel size and has zero amp glow.

Quoting myself, using this https://www.teleskop-express.de/shop/product_info.php?products_id=11535#cs that I already have, I can use my filter and get the 2600 in focus.

Grrrrr, more money