Star shapes in corners.

[tooth_dr]

Apologies for the poor quality of the photos.  The clouds kept coming in and out of the view really quickly.

What do you think of the corners of the FOV - tilt, spacing or otherwise?  It's a SharpStar94, with the specific FF/FR 08, giving 413mm at F4.4.  I'm using a Nikon D800E, with 4.88um pixels.  This is a 30s shot, ISO400 of around the M45 region, same image stretched differently.

 

 

Thanks

Adam.

 

 

[tooth_dr]

Any one able to comment?

 

Thanks

[Xiga]

On ‎07‎/‎01‎/‎2021 at 15:20, tooth_dr said:

Any one able to comment?

 

Thanks

Did you get a new scope Adam? I had only heard of the Sharpstar Newts, I didn't know they did Fracs too. F4.4 is a nice speed increase over the 80ED's F6.3.

I take it this is uncalibrated? It might be easier to diagnose if it was calibrated.

I would say none of the corners look right if i'm being honest, although a Full Frame camera will certainly be testing things to the max. To my untrained eye, I would say the top-right looks worse than the other 3 corners, so if I had to guess, i'd say you probably have both tilt and spacing issues going on. Perhaps someone more experienced in diagnosing these things can chime in though.

Next time out you could try a more densely populated area to pull in as many stars as you can (open cluster or anywhere near the Galactic plane etc). You could also try a shorter sub (15 secs at ISO 800?) although I highly doubt you had tracking issues with your Mesu.

Good luck!

[tooth_dr]

4 hours ago, Xiga said:

Did you get a new scope Adam? I had only heard of the Sharpstar Newts, I didn't know they did Fracs too. F4.4 is a nice speed increase over the 80ED's F6.3.

I take it this is uncalibrated? It might be easier to diagnose if it was calibrated.

I would say none of the corners look right if i'm being honest, although a Full Frame camera will certainly be testing things to the max. To my untrained eye, I would say the top-right looks worse than the other 3 corners, so if I had to guess, i'd say you probably have both tilt and spacing issues going on. Perhaps someone more experienced in diagnosing these things can chime in though.

Next time out you could try a more densely populated area to pull in as many stars as you can (open cluster or anywhere near the Galactic plane etc). You could also try a shorter sub (15 secs at ISO 800?) although I highly doubt you had tracking issues with your Mesu.

Good luck!

Hi Ciaran

I did indeed buy one of the SharpStar refractors, it's a fairly new model, I think it came out in October or November.   It's supposed to be 55mm backfocus and I'm using a standard M48 T adapter so it should in theory be exactly at the 55mm!

I'll try another sub over the galactic core to get more stars, good idea!  I'm imaging now, so will move to a different target to get more stars in the FOV.

[tooth_dr]

Here are some recent images using the SS94 and the dedicated 94 0.8x FF.

Currently I have the spacing set to 55mm using ZWO spacers that came with the 2600MC.  I think the stars look not quite right. 

With the FF Nikon, the situation is worse.  I've tried 55mm (that is with the standard T-adapter) but have also increased the spacing incrementally to 57mm.  I noticed them some degradation in the shapes increasing the spacing further.

For transparency, I have been in touch with vendor, and have been told that this astigmatism is inherent to the design of the system, and that apart from trying different spacing or using software to correct it, then there is no other remedy.  I dont want to be obsessed with the minutiae of star shapes, so any input would be appreciated, but i am conscious that the total outlay for the scope and flattener was £1497+ postage.

 

Thanks

Adam.

 

 

 

[tooth_dr]

No takers?! I posted on cloudy nights and was advised to stop it down to 80mm.  Hoping to get alternative views here.

 

[david_taurus83]

What reducer did you get with it Adam?

[tooth_dr]

9 minutes ago, david_taurus83 said:

What reducer did you get with it Adam?

I bought the 94EDPH 0.8x reducer, to go with the 94EDPH scope  

https://www.365astronomy.com/sharpstar-2.5-0.8x-reducer-and-flattener-for-full-frame-cameras-for-sharpstar-94edph-telescope-m48-camera-connection.html

[david_taurus83]

It doesn't look too bad on the 2600. Full frame is another matter. Did you get it for the Nikon? The spot diagram definitely doesn't show pin point stars at 20mm from centre though.

 

[tooth_dr]

9 minutes ago, david_taurus83 said:

Did you get it for the Nikon?

I’ll be honest and say yes I bought it specifically for its claims of sharp stars at 44mm.  I may use the 2600MC on it mostly but wanted the option for full frame imaging.

[vlaiv]

8 minutes ago, david_taurus83 said:

It doesn't look too bad on the 2600. Full frame is another matter. Did you get it for the Nikon? The spot diagram definitely doesn't show pin point stars at 20mm from centre though.

To put that into perspective - ~20µm of spot diagram is 6px across - without influence of seeing / tracking.

In comparison 90mm of aperture at F/4.4 has Airy disk diameter of 5.5µm - or x4 less than geometric radius.

Star shapes are not the issue here - it is too small pixel size. If you reduce image by factor of x2 - then you get rather nicer looking stars.

These seem ok, right? That is far corner of ASI2600MC image

 

[tooth_dr]

Just now, vlaiv said:

To put that into perspective - ~20µm of spot diagram is 6px across - without influence of seeing / tracking.

In comparison 90mm of aperture at F/4.4 has Airy disk diameter of 5.5µm - or x4 less than geometric radius.

Star shapes are not the issue here - it is too small pixel size. If you reduce image by factor of x2 - then you get rather nicer looking stars.

These seem ok, right? That is far corner of ASI2600MC image

 

I don’t have so much an issue with the 2600MC. It may need a fine tune. But I still don’t think the stars look ok in your image 

[vlaiv]

2 minutes ago, tooth_dr said:

I’ll be honest and say yes I bought it specifically for its claims of sharp stars at 44mm.  I may use the 2600MC on it mostly but wanted the option for full frame imaging.

Well, we need to be careful of such statements.

Yes, perhaps sharp stars in corners of full frame sensor if full frame sensor is using 9µm or larger pixel size.

Again - this is corner of full frame - but reduced x3 in size - I'd say acceptable stars?

[vlaiv]

3 minutes ago, tooth_dr said:

I don’t have so much an issue with the 2600MC. It may need a fine tune. But I still don’t think the stars look ok in your image 

Out of interest, could you point to image with ~90mm F/4-F/5 scope that has nice round small stars to the edge of APS-C sensor?

It seems that many people are now interested in such setups, given that we have decent selection of APS-C and FF sensors available and refractors are also affordable - but somehow I feel that it is tall order to have small frac (a lot of curvature) well corrected to that sensor size. Then again - I don't have much experience on that topic - and maybe it is indeed very viable option?

[tooth_dr]

18 minutes ago, vlaiv said:

 

Again - this is corner of full frame - but reduced x3 in size - I'd say acceptable stars?

Reducing x 3 in size, is that the same as binning 3x3?

[vlaiv]

17 minutes ago, tooth_dr said:

Reducing x 3 in size, is that the same as binning 3x3?

It is very similar.

Binning is one of the way of reducing size - with very special property - it improves SNR by known factor and does not introduce pixel to pixel correlation.

Other types of reducing size also improve SNR - but by smaller factor and not as predictable as binning and they do some things that introduces pixel correlation (more of a math stuff that should not particularly concern you).

For full effect - binning should be applied on linear data. I just wanted to show that if you reduce the size (and lower sampling rate) - you'll get nicer looking stars. Something similar needs to be done with fast lens - they are just not sharp enough to be used with smaller pixels - but if you resize image down (which has same effect as binning regarding sampling / pixel size - you effectively increase pixel size by doing that) - things look nicer.

 

[tooth_dr]

Just now, vlaiv said:

It is very similar.

Binning is one of the way of reducing size - with very special property - it improves SNR by known factor and does not introduce pixel to pixel correlation.

Other types of reducing size also improve SNR - but by smaller factor and not as predictable as binning and they do some things that introduces pixel correlation (more of a math stuff that should not particularly concern you).

For full effect - binning should be applied on linear data. I just wanted to show that if you reduce the size (and lower sampling rate) - you'll get nicer looking stars. Something similar needs to be done with fast lens - they are just not sharp enough to be used with smaller pixels - but if you resize image down (which has same effect as binning regarding sampling / pixel size - you effectively increase pixel size by doing that) - things look nicer.

 

So...I need to be imaging with a ~15um pixel size to get 'nicer looking' stars.  Seems like my £1500 was well spent.

[Xiga]

Hi Adam, I'm sure you researched all the options, so apologies if I'm stating the obvious here. 

Have you thought about sending the SS back and just going for an Esprit 100 instead? Slightly slower, slightly more expensive and slightly higher FL obviously, but at least you know it will perform up to a 40mm imaging circle. 

[tooth_dr]

13 minutes ago, Xiga said:

Hi Adam, I'm sure you researched all the options, so apologies if I'm stating the obvious here. 

Have you thought about sending the SS back and just going for an Esprit 100 instead? Slightly slower, slightly more expensive and slightly higher FL obviously, but at least you know it will perform up to a 40mm imaging circle. 

Definitely not stating the obvious.

I would but am in a slightly awkward position.  When I bought the scope, I wasnt able to get a focal reducer or field flattener for the 94 scope from the same supplier.  I emailed about it, but they dont list it on the product range, and said it would take weeks or months to get it so recommended that I purchase elsewhere.  As the scope doesnt work without either I had to source it from another supplier.  I believe I can still return the scope, but will be left with a £328 focal reducer that wont be easily sold on.  I havent heard back from the focal reducer supplier, but they dont offer returns (14 days only) but they do offer a field flattener for this scope, that brings it to F5.5 also, but at a slightly shorter focal length than the esprit (517mm).  If it is to be believed, the field flattener delivers 'tack sharp stars' across a 44mm imaging circle, so this is a viable option to minimise loses, provided I can get an exchange.

[Xiga]

16 minutes ago, tooth_dr said:

Definitely not stating the obvious.

I would but am in a slightly awkward position.  When I bought the scope, I wasnt able to get a focal reducer or field flattener for the 94 scope from the same supplier.  I emailed about it, but they dont list it on the product range, and said it would take weeks or months to get it so recommended that I purchase elsewhere.  As the scope doesnt work without either I had to source it from another supplier.  I believe I can still return the scope, but will be left with a £328 focal reducer that wont be easily sold on.  I havent heard back from the focal reducer supplier, but they dont offer returns (14 days only) but they do offer a field flattener for this scope, that brings it to F5.5 also, but at a slightly shorter focal length than the esprit (517mm).  If it is to be believed, the field flattener delivers 'tack sharp stars' across a 44mm imaging circle, so this is a viable option to minimise loses, provided I can get an exchange.

Nothing's ever easy in this game Adam is it?! 

Hopefully the supplier steps up and offers the exchange. Offering only a 14 day return period is a bit of a joke, especially when we can wait 2 months for a clear night here in the UK. 

Good luck with the exchange and hopefully the FF works out better for you. 

Ps - I'm seriously thinking of getting an OSC IMX571 camera too. As you gather more data on various targets, I'd be interested to hear your thoughts on how it compares against 8300 data.