Are my filters too small?

[AndrewRrrrrr]

Evening All,

Looking for some advice:

I recently upgraded my camera from a 183M to a 26M. But using the same EFW with 1.25" inch filters. This is with a "wave 80" scope (80mm aperture, f6 with a 0.8x reducer so 384mm focal length) 

Sensor to filter distance is 28.5mm.

I thought I might get away with it: according to the "CCD Filter Size" on astronomy.tools website which specifies a minimum filter size of 32mm (1.25" = 31.75mm) 

Attached is the integrated LUM channel and it's corresponding flat. (I might re-do the flats to see if anything has changed, I'm still in the "shake down" phase of the new camera I guess......)

Or maybe it's something else altogether? I haven't attached the heater yet but it wasn't a cold or humid night

.

Opinions welcome, thanks in advance!
 

 

[Albir phil]

4 hours ago, AndrewRrrrrr said:

Evening All,

Looking for some advice:

I recently upgraded my camera from a 183M to a 26M. But using the same EFW with 1.25" inch filters. This is with a "wave 80" scope (80mm aperture, f6 with a 0.8x reducer so 384mm focal length) 

Sensor to filter distance is 28.5mm.

I thought I might get away with it: according to the "CCD Filter Size" on astronomy.tools website which specifies a minimum filter size of 32mm (1.25" = 31.75mm) 

Attached is the integrated LUM channel and it's corresponding flat. (I might re-do the flats to see if anything has changed, I'm still in the "shake down" phase of the new camera I guess......)

Or maybe it's something else altogether? I haven't attached the heater yet but it wasn't a cold or humid night

.

Opinions welcome, thanks in advance!
 

 

Looks like it's vignetteing due to the 1.25 inch filter, that's my opinion other's my have some other explanation 🤔

[adyj1]

When I committed to 1.25" filters I noted from an online resource that the maximum sensor size (across the diagonal) for 1.25" filters is just over 22mm. 

Your APS-C sensor is 28.3mm diagonal... 

Ady

[Adam J]

6 hours ago, AndrewRrrrrr said:

Evening All,

Looking for some advice:

I recently upgraded my camera from a 183M to a 26M. But using the same EFW with 1.25" inch filters. This is with a "wave 80" scope (80mm aperture, f6 with a 0.8x reducer so 384mm focal length) 

Sensor to filter distance is 28.5mm.

I thought I might get away with it: according to the "CCD Filter Size" on astronomy.tools website which specifies a minimum filter size of 32mm (1.25" = 31.75mm) 

Attached is the integrated LUM channel and it's corresponding flat. (I might re-do the flats to see if anything has changed, I'm still in the "shake down" phase of the new camera I guess......)

Or maybe it's something else altogether? I haven't attached the heater yet but it wasn't a cold or humid night

.

Opinions welcome, thanks in advance!
 

 

Yes the 1.25 inch filters will not cover a APS-C sized sensor, you will need 36mm filters for that. The best you can get from 1.25 inch is about F4 with a 4/3 sensor like the ASI1600mm pro or the ASI294mm pro.  You can always just crop it to about that size for now without any negative effects but if you want to use the fill sensor you are going to have to change your filter and filter wheel. 

Adam

[AndrewRrrrrr]

thanks for the advice guys, looks like i best get saving up lol. 

[ollypenrice]

Don't look at a stretched version of your flat to decide. It tells you very little.  What you should do is read off the ADU value of the unstretched flat (ie the linear flat) in the corners and in the middle. I've found that it was perfectly possible to ask flats to correct a 25% drop-off in brightness between centre and corners, so you need to know what your light drop-off is. It may well be that your drop-off is greater than that but, before spending, it would be worth a check.

Is there any way in which you might get your filterwheel closer to the chip? You don't have any spacers between F/W and camera?

Olly

[Anne S]

I had a similar problem when I installed a focal reducer on my 102mm Wave some years ago. A smaller ccd, just a SX694. Completely solved with 36mm filters. When I removed the reducer everything was fine. My ccd was only 16mm diagonal though. I'd like a 26M camera some day and I'm concerned that I'd need to go up to 2 inch filters. My 694 backfocus is 17.5 approx.
 

The reducer steepens the light cone which increases the risk of vignetting.

[Adam J]

1 hour ago, Anne S said:

I had a similar problem when I installed a focal reducer on my 102mm Wave some years ago. A smaller ccd, just a SX694. Completely solved with 36mm filters. When I removed the reducer everything was fine. My ccd was only 16mm diagonal though. I'd like a 26M camera some day and I'm concerned that I'd need to go up to 2 inch filters. My 694 backfocus is 17.5 approx.
 

The reducer steepens the light cone which increases the risk of vignetting.

There is just no way that a even a F3 would need more than 1.25 inch filters with a SX694, I strongly suspect something else was going on. As I understand it 36mm is fine and used by many people on the 26M with no issues. The only thing I can think is that you must have had the filter a long long way from the sensor. 

Adam

[Adam J]

16 hours ago, ollypenrice said:

Don't look at a stretched version of your flat to decide. It tells you very little.  What you should do is read off the ADU value of the unstretched flat (ie the linear flat) in the corners and in the middle. I've found that it was perfectly possible to ask flats to correct a 25% drop-off in brightness between centre and corners, so you need to know what your light drop-off is. It may well be that your drop-off is greater than that but, before spending, it would be worth a check.

Is there any way in which you might get your filterwheel closer to the chip? You don't have any spacers between F/W and camera?

Olly

I would say that the main indicator that bad things are happening are the reflections and diffraction spikes most likely coming from the edge of the filter mount interrupting the light cone. 

Adam

Edited April 10 by Adam J

[Anne S]

9 hours ago, Adam J said:

There is just no way that a even a F3 would need more than 1.25 inch filters with a SX694, I strongly suspect something else was going on. As I understand it 36mm is fine and used by many people on the 26M with no issues. The only thing I can think is that you must have had the filter a long long way from the sensor. 

Adam

The focal reducer was causing some reflections from off axis stars as well but that only became apparent after I'd replaced the filters. No issues with the new filters and replacement reducer. The scope is an f7, though works at f5.53 with the reducer.

It's good to hear that the 26M should be fine with 36mm filters. I can concentrate on saving up for it!