Bahtinov mask "Banana" spikes

[woldsweather]

I have the same issue with my GPCAM guide/finder scope. It was new 6 months ago. I have never seen a really sharp image with it. Is this likely to be a collimating issue?

 

Altair GPCAM2 130 Mono Guide Imaging EAA Camera

Altair 10x60mm RACI Finder Scope 

Altair 72 EDF Refractor with CNC Dual Speed R&P Focuser Optical Test Report - Add 1.0x Flattener:Add Flattener, Choose Version:Deluxe CNC (Imaging)

EQM-35 PRO mount

[Yawning Angel]

It presents the same symptom, yes. I’m not sure if it’ll impact your guiding however?

[woldsweather]

PHD won't hold a star at all. No stars show on the PHD screen and SharpStar is very poor at finding stars.

[Yawning Angel]

I’d run it via sharpcap and see how it performs when you up the gain and exposure 

[woldsweather]

I've done that these past 4/5 evenings but I've always given up and switched to Polemaster. Its not in focus and it doesnt come into focus by the normal means. There have never been any stars showing in PHD's screen but as I'm new to this I didn't realise there should be. I have wasted the whole winter 6 month period over this.

[Yawning Angel]

Double post

Edited April 20, 2021 by Yawning Angel

[michael8554]

22 hours ago, woldsweather said:

I have the same issue with my GPCAM guide/finder scope. It was new 6 months ago. I have never seen a really sharp image with it. Is this likely to be a collimating issue?

Altair GPCAM2 130 Mono Guide Imaging EAA Camera

Altair 10x60mm RACI Finder Scope 

Altair 72 EDF Refractor with CNC Dual Speed R&P Focuser Optical Test Report - Add 1.0x Flattener:Add Flattener, Choose Version:Deluxe CNC (Imaging)

EQM-35 PRO mount

According to the SharpCap Control Panel, you are taking 8 second exposures, so the image is extremely over exposed.

Try again at 1 second on a bright star.

It does look like you are close to focus though. 

If it does need collimating, which is what earlier posts suggest is the cure, then it would be worth first checking in the old-school way of looking at the out-of-focus Airy Rings.

With SharpCap in Video mode to give continuous images.

But PHD2 is very forgiving of star shape, witness it's success at guiding coma-shaped stars with OAGs.

I would ditch the Bahtinov Mask and use the HFD reading in the Star Profile window, adjust focus for lowest HFD.

Michael

Edited April 21, 2021 by michael8554

[michael8554]

20 hours ago, woldsweather said:

PHD won't hold a star at all. No stars show on the PHD screen and SharpStar is very poor at finding stars.

The PHD2 screen will show a bright noisy frame when no stars are present, so try gentle pulses in RA, the screen will change to dark with stars when stars are in the FOV.

Michael

[woldsweather]

19 minutes ago, michael8554 said:

According to the SharpCap Control Panel, you are taking 8 second exposures, so the image is extremely over exposed.

Try again at 1 second on a bright star.

It does look like you are close to focus though. 

If it does need collimating, which is what earlier posts suggest is the cure, then it would be worth first checking in the old-school way of looking at the out-of-focus Airy Rings.

With SharpCap in Video mode to give continuous images.

But PHD2 is very forgiving of star shape, witness it's success at guiding coma-shaped stars with OAGs.

I would ditch the Bahtinov Mask and use the HFD reading in the Star Profile window, adjust focus for lowest HFD.

Michael

Yes I'm wishing I hadn't posted that image we tried every conceivable level of gain and low exposure. Just got a fainter image of the above.

[woldsweather]

18 minutes ago, michael8554 said:

The PHD2 screen will show a bright noisy frame when no stars are present, so try gentle pulses in RA, the screen will change to dark with stars when stars are in the FOV.

Michael

My neighbour has many years experience of using PHD and he can't get anything.

[michael8554]

As I said you have got it working in SharpCap and are very near focus, so should work in PHD2.

Do you see the same Altair driver in the PHD2 "Connect Equipment"  window as you see in SharpCap ?

You don't have SharpCap running in the background at the same time ?

Michael

 

[Mushroomrice]

On 18/07/2018 at 22:58, mikeonnet said:

Hi all,

I'm looking for some advice on the 'curved banana' shaped spikes I have. Having used a DSLR connected to a 0.85 focal reducer & Skywatcher 80 refractor, the bahtinov diffraction spikes were perfecly straight including the diaganal lines. However, I am now using a ZWO mono 290 and a filter wheel with the adapters (55mm from the sensor to reducer) but now the lines are curved - see image. I know it's a noisy image but this was just a quick snapshot but my main concern is what is causing the curvature of the spikes.

Is this an issue of the camera not being the correct distance to the reducer? I have only had a quick imaging session of M27 the other day and too be honest, it seemed OK but I just wonder if this 'wobble' will be impacted on all the light frames I will be taking. Any advice please...

I

Hi, I am also experiencing the strange banana pattern with my bahtinov mask. Having read through it seems there are many theories but haven't come across anyone who has experienced this and resolved the issue. Has anybody managed to identify the cause?

[Yawning Angel]

Check the collimation, even on a refractor. The stock focusers can be a bit droopy with a full imaging train hanging off them

Edited August 31, 2021 by Yawning Angel

[Stub Mandrel]

On 30/08/2021 at 12:57, Mushroomrice said:

Hi, I am also experiencing the strange banana pattern with my bahtinov mask. Having read through it seems there are many theories but haven't come across anyone who has experienced this and resolved the issue. Has anybody managed to identify the cause?

I've seen it low down with atmospheric dispersion, each short 'tick' is a spectrum from red to blue. You can actually use it to adjust an atmospheric dispersion corrector, as when it's set right the pattern becomes straight.

I might also he caused by chromatic aberration with a frac.