Help figuring these star shapes out?

[Backyard Space Dude]

Hello all! 

Brand new SW Esprit 100 + Starizona ApexED L has been sitting gathering dust waiting for clear skies, until the other night where I could finally test it out.

The back focus with the ApexED L and the Esprit 100 is 58mm, I've got that dialled in (including the +1mm optical length for the filter). I know there may be some slight differences in each optical system, but I can't quite figure out what's going on with my system. It looks like maybe I've got tilt as well as incorrect back focus. The astigmatism in each corner looks slightly different.

To add to the confusion my guiding was terrible that night. I think I'll have to try again with better polar alignment and guiding, and tighten up the screws in the optical path! But it would be handy to get some pointers so I don't go in the wrong direction.

Any advice from the great minds of SGL?

 

EDIT: I realised I didn't exactly give much useful information!

This is 1x 600s Ha (7nm) exposure taken with an ASI1600MM P. I took 6x 600s exposures, and they all show the same star shapes in all areas of the image, leading me to believe it's likely a result of somewhere in the optical path.

Here's a crop of all 4 corners:

Edited October 25, 2020 by Backyard Space Dude

[andrew s]

Run it through CCDInspector. It has a free trial period and will give you some data to work with. Regards Andrew 

[teoria_del_big_bang]

I am nowhere near experienced enough to give the best advice but when you zoom in it is a bit of an odd one. The stars are not the normal shape I would associate with back focus issues or even tilt. To my aging eyes the dimmer stars look pretty round but all the brighter starts are kind of teardrop shape pointing towards the  bottom of the image, as if it was something momentary that happened during the exposure rather than something permanently wrong.

What exposure was this ?

Were all exposures the same ?

I would certainly follow your own advice and try again with better guiding and make sure things are tight as they should be.

As I say I am still learning so take this with a pinch of salt but I am following the thread so please reply if you make improvements the next clear night.

But star shapes aside that is a great image really 🙂 so not all bad.

Steve

Edited October 25, 2020 by teoria_del_big_bang
spelling

[Backyard Space Dude]

1 hour ago, teoria_del_big_bang said:

I am nowhere near experienced enough to give the best advice but when you zoom in it is a bit of an odd one. The stars are not the normal shape I would associate with back focus issues or even tilt. To my aging eyes the dimmer stars look pretty round but all the brighter starts are kind of teardrop shape pointing towards the  bottom of the image, as if it was something momentary that happened during the exposure rather than something permanently wrong.

What exposure was this ?

Were all exposures the same ?

I would certainly follow your own advice and try again with better guiding and make sure things are tight as they should be.

As I say I am still learning so take this with a pinch of salt but I am following the thread so please reply if you make improvements the next clear night.

But star shapes aside that is a great image really 🙂 so not all bad.

Steve

Thanks for your reply! I've edited the OP to include the details as I realised I didn't exactly give much detail originally! All exposures where the same, at 600s.

2 hours ago, andrew s said:

Run it through CCDInspector. It has a free trial period and will give you some data to work with. Regards Andrew 

Thanks! I'm on a Mac so not sure it will work, but trying to work around it as we speak.

[Backyard Space Dude]

2 hours ago, andrew s said:

Run it through CCDInspector. It has a free trial period and will give you some data to work with. Regards Andrew 

Thank you for the heads up on this software, it looks to be very powerful! If I'm reading this correctly it looks like I have some serious tilt? The darkest area looks to be in the bottom right. I'm not sure if the numbers are normal but it looks like there's more curvature than I'd have hoped for with the ApexED flattener/reducer?

[Backyard Space Dude]

Update: 

Last night I ensured good balance, tight screws and threads and good polar alignment. Star shapes were still about the same, even on a different area of the sky. They were all just a bit random as the night before.

Having a closer look, it appeared that "cone" shaped stars where more present than anything, so I took that as too much length in the back focus and took out some of the spacer rings. The result was a back focus of roughly 57mm (1.5mm shorter than it should be). After taking an exposure I found more severe star shapes in the corners, but at least they all looked the same - possibly coma but I'm not sure that's technically correct (See attached image 1x 30s luminance). I stuck it through CCD inspector and found HUGE curvature, but at least the sensor was far more central (but still not perfect). To me, it looked like the back focus was far too short.

At about 0130hrs this morning I gave up. After getting some sleep I took the optical path completely apart and tried to configure the OAG differently to allow me to thread the ApexED onto the focuser, rather than using the self centering thumb screws I'm currently using. Unfortunately the guide cam comes too close to the focuser to allow me to thread the adaptor on. I put it all back together as previously but carefully ensuring 58.6mm back focus - which should be absolutely spot on according to Starizona.

I'm due some clear tonight, so I'll give it another go as it is. I'll also remove the guide cam and thread the reducer onto the focuser and see if that improves things. If it does I may have to look at a different OAG or use a finder/guider.

If anyone has any input I'd greatly appreciate it! I'm kind of just fiddling aimlessly at the moment!

[ollypenrice]

I find it hard to understand how a flattener-reducer can do as Starizona claim.

The reducer provides a 0.65x reduction factor and produces pinpoint star images by correcting for the field curvature and astigmatism present in standard apo doublets and triplets and RC/ACF telescopes.

What is a 'standard Apo doublet or triplet?' They vary in field flatness so how can one flattener perfectly fit all? On top of that, 0.65x will take your Esprit down to F3.6. If you asked me how realistic I thought that was I'd have to say, 'Not very...'  Takahashi, at a prodigious cost, manage it with their reduced FSQs but, even then, they don't do so reliably as plenty of frustrated owners testify on astronomy forums world wide.

Personally, if I wanted a wider FOV than you're getting at 550mm, I'd just use a shorter FL scope.

I'm not being very helpful, I know, but I wonder if you're not seeking the impossible.

Olly

Edited October 27, 2020 by ollypenrice

[Backyard Space Dude]

4 hours ago, ollypenrice said:

I find it hard to understand how a flattener-reducer can do as Starizona claim.

The reducer provides a 0.65x reduction factor and produces pinpoint star images by correcting for the field curvature and astigmatism present in standard apo doublets and triplets and RC/ACF telescopes.

What is a 'standard Apo doublet or triplet?' They vary in field flatness so how can one flattener perfectly fit all? On top of that, 0.65x will take your Esprit down to F3.6. If you asked me how realistic I thought that was I'd have to say, 'Not very...'  Takahashi, at a prodigious cost, manage it with their reduced FSQs but, even then, they don't do so reliably as plenty of frustrated owners testify on astronomy forums world wide.

Personally, if I wanted a wider FOV than you're getting at 550mm, I'd just use a shorter FL scope.

I'm not being very helpful, I know, but I wonder if you're not seeking the impossible.

Olly

Thanks Olly, that does help somewhat as it seems I might be chasing something that’s almost impossible to achieve. I certainly don’t have the patience for it as my attempt last night will validate! 

I repeatedly took the system apart and put it back together going up and down by one spacer/shim at a time. Although I did see a difference in star shape, I still couldn’t get each corner looking decent. I’ll attempt to speak to Starizona and see what they have to say.
 

[gorann]

One of my scopes is an Esprit 100, and I love it, but since it is already a fast refractor at f/5 I would not try to challenge it with a reducer. It is perfect at its native FL and what it can do there. For getting more wide field I whent for a RASA 8 - its 200 mm aperture gathers 4 times more photons and it clearly shows it. To get a faster telescope there is no short cut other than getting a telescpe with a larger aperture, as I think maybe Olly @ollypenrice would agree on.

[Backyard Space Dude]

Thanks Goran. A RASA would have been a fantastic purchase, but I chose to go with another refractor. I may yet choose to purchase the standard flattener from FLO for my Esprit, it's not that expensive. You never know, maybe in the future I'll get enough cash for two rigs!

However, after drafting an email out to Starizona I decided to give it another go. I hadn't really gone about my troubleshooting methodically yet. So here I am, 4th night in and after a couple hours of different focal lengths, orientations, checking focus in each corner with a bahtinov mask and noting down my findings - I've gotten as close to it as I can. 

Top corners look good, lower corners look too far out. This is in all orientations - so it looks like I'll have to grab a little tilt adjuster from FLO to compensate for what seems to me to be sensor tilt. 

For the time being I'm happy, I've seen some decent results with this combination so I'm going to continue to work through! 

[Backyard Space Dude]

If anyone finds themselves in my situation in the future I thought I'd make an update:

I've spent about 6 nights trying to dial this in, and I can say it is a VERY delicate process. When I took delivery of the tilt adjuster, I could only find it going into place directly in front of the filter wheel - which was insane because I couldn't get to the adjustment screws. I figured out a slightly different place for it (behind the reducer) but I still had to take the entire optical train off of the telescope to make adjustments. 

Last night I spent about 6 hours with an extremely laborious process of continually:

- Taking a test exposure to get correct focus

- Taking an in-focus test exposure

- Importing the test exposure into CCD inspector for analysis

- Removing the optical train and making the adjustments on the tilt adjuster/adding or removing back focus

- Putting the optical train back together and onto the 'scope

- Repeat.

I got close a couple of times (I presume to blind luck), but I found that I was over compensating as I didn't know just how critical the spacing had to be. For example, at one point I thought I had to simply bring the right side of the sensor further away, but ended up bringing the left side far closer in by doing so and the result was disheartening to say the least! 

My inexperience with this is really showing, but I'm determined to get round stars edge-to-edge. There are examples of people using the same setup as me and achieving amazing results, so I know it can be done.

At around midnight I managed to get it as close as I could before giving up again. Here are the results (Centred on Caph):

Preview_Caph_1s_Bin1_L_gain139_-18C.tif