which filter size to use

[spillage]

Hi All,

I currently have a canon dslr and now looking at either purchasing a coma corrector for my 150pds  or some filters. I am sure I would be better off with standard filters rather than clip in.

Am I right in thinking that most ccd cameras use 1.25 filters. I only ask because I am not sure if buying 2" filters would be a good choice.

I am just thinking of a possible future upgrade to a ccd and dont want to waste money on 2" filters to have to then go and buy 1.25 ones.

Is there going to be any difference using 2" over 1.25" with a dslr.

Cheers

Spill.

[milosz]

Filter size depends on the sensor size and f-ratio. Given APS-C sensor has around 26-28 mm across and unmounted 1.25'' filters have roughly 31mm, they would have to be pretty close to the sensor to avoid vignetting. So in practice you would go for at least 36mm filters. 2 inch are probably an overkill but won't make any harm.

 

milosz 

[Xplode]

If the choice is between filters or a coma corrector go for the coma corrector, buy a Baader MPCC III, not the Skywatcher coma corrector as it has bad problems with reflections.

If you were to buy and use 1.25" filters on a DSLR it would create a LOT of vignetting because of the long distance between the sensor and the filter. You might even get problems with reflections.

1.25" filters free aperture is around 27mm

[laser_jock99]

I bought a set of 1.25" narrow band filters and ended up having to buy 2" filters a year later.........

Clip filters- again you'd have to re-buy if/when you upgrade to CCD.

[swag72]

I have stuck with 1.25" filters throughout my CCD life... They have been a little restrictive in the last upgrade to a Kodak KAF8300 sensor as the sensor needs the filters very close... to that end I've needed a camera with a built in filter wheel. But I'm happy enough with them and I don't see myself getting a larger sensor... not only would I have to fund new larger filters, but my scopes imaging circle don't cover the large sensors... so that's that

[Waldemar]

1,25" filters really restrict your choice of CCD camera's. If you would want to buy a full frame CCD in the future (or even APS-c) these will cause vignetting. You would far better of saving a bit more and buy 2" filters which will last you a lifetime.
Larger sensors will give you a far wider FOV, but are also more demanding about size of flat imaging circle and filter size. In the end you are the one who has to make the choice. I would allways go for 2" filters... been there, seen it...
of course vignetting can be taken care of with flats, but without extra problems is better and easier. You will encounter enough problems along the way, without having to take care of those also...

[spillage]

Thanks all for you advice.

I will look around for a coma corrector to come up for sale first then look towards buying 2" filters at a later date.

Cheers

Spill.

[Adam J]

I was in this position I bought 2 inch filters with the intent to use them with my DSLR now and a Larger format CCD in the future...though I may also be tempted by a pro cooled full frame CMOS camera if and when they come to the market.

But I went with 2 inch filters because I run my DSLR in a cool box and the t-ring is within the box, as such its not possible to change a clip in filter without removing the camera from the box. I don't like doing that as it lets moist air into the camera and means that you have to re-start the cooling process every time you change the filter. A 2 inch filter just screws onto my coma corrector and it gives me more options in the future. I currently own a 2" CLS, a 8.5nm OIII and a 7nm H-a filter; all work very nicely with my 550D.  

To be honest when I get a CCD i may not even buy a SII, only a few targets respond to it strangely and a am not a fan of the Hubble pallet anyhow. The only issue I see with using 2" filters later on is that you can only get 5 filter wheels. Unless someone knows of a larger one available that I have not seen. It means you cant fit your LRGB and narrow band filters into a single wheel.  

[Waldemar]

7 minutes ago, Adam J said:

The only issue I see with using 2" filters later on is that you can only get 5 filter wheels. Unless someone knows of a larger one available that I have not seen. It means you cant fit your LRGB and narrow band filters into a single wheel.  

I use a Starlight Xpress Maxi Filter Wheel. That holds 9 x 2" filters: https://www.firstlightoptics.com/starlight-xpress-accessories/starlight-xpress-maxi-usb-filter-wheel.html

I have everything in it: LRGB, H-Alpha, OIII, SII, Clear and UHC .

[Star101]

Thanks for this thread. I too have been thinking of getting into NB. And what filter size I need. I have Hyperstar and the adaptor takes a 2" filter. 2" can cost a lot more than 1.25" and was thinking, when imaging the standard way I will have to have 1.25"

Having read this thread, I have decided that 2", although more expensive, is the way for someone such as myself who is starting out in NB to go. I will not be happy with 1.25" filters, knowing some issues would not occur if I had purchased 2" filters instead.

 

[Waldemar]

20 hours ago, Star101 said:

Thanks for this thread. I too have been thinking of getting into NB. And what filter size I need. I have Hyperstar and the adaptor takes a 2" filter. 2" can cost a lot more than 1.25" and was thinking, when imaging the standard way I will have to have 1.25"

Having read this thread, I have decided that 2", although more expensive, is the way for someone such as myself who is starting out in NB to go. I will not be happy with 1.25" filters, knowing some issues would not occur if I had purchased 2" filters instead.

 

Several filter drawer systems are available which can be used with the HyperStar, you will have to have made a dedicated adapter for them to get the right focal distance, but the filterchanging will be so much easier: http://www.gerdneumann.net/english/filterzubehoer-filter-accessories/filter-drawer-system-filter-schubladen-system.html or: https://www.teleskop-express.de/shop/product_info.php/info/p5742_Quick-filter-changer-for-50mm---50-4mm-unmounted-filter---low-profil.html

[Adam J]

On 10/02/2017 at 11:57, Waldemar said:

I use a Starlight Xpress Maxi Filter Wheel. That holds 9 x 2" filters: https://www.firstlightoptics.com/starlight-xpress-accessories/starlight-xpress-maxi-usb-filter-wheel.html

I have everything in it: LRGB, H-Alpha, OIII, SII, Clear and UHC .

Yeah but they want more than the  combined cost of my narrow band filters for the filter Wheel......

Edited February 14, 2017 by Adam J

[ecuador]

As mentioned, you can get away with using 1.25" filters even on APS-C sensors if you manage to get them close, and I manage to do that even for a DSLR that normally "keeps you at distance" - see here for a little DIY - and vignetting left is minimal at the extreme corners. Some filters are quite expensive at 2"... 

Edited February 14, 2017 by ecuador

[Adam J]

11 hours ago, ecuador said:

As mentioned, you can get away with using 1.25" filters even on APS-C sensors if you manage to get them close, and I manage to do that even for a DSLR that normally "keeps you at distance" - see here for a little DIY - and vignetting left is minimal at the extreme corners. Some filters are quite expensive at 2"... 

Yep it definitely worth remembering that a EOS clip filter is just a 1.25 inch filter in a different mount that allows it to be held very very close to the sensor. A fact that I find deeply annoying given that the Clip filters cost so much more than the 1.25 inch equivalents for no real reason.  Problem with the approach you linked though is what do you do if you are using a coma corrector?

Also a slight warning, some filters (normally narrow band filters) are actually directional. So threading them on in the way shown in your link may have unknown consequences to their performance. 

Edited February 14, 2017 by Adam J

[ecuador]

35 minutes ago, Adam J said:

Yep it definitely worth remembering that a EOS clip filter is just a 1.25 inch filter in a different mount that allows it to be held very very close to the sensor. A fact that I find deeply annoying given that the Clip filters cost so much more than the 1.25 inch equivalents for no real reason.  Problem with the approach you linked though is what do you do if you are using a coma corrector?

Also a slight warning, some filters (normally narrow band filters) are actually directional. So threading them on in the way shown in your link may have unknown consequences to their performance. 

Yes, the cost of the clip filters is very annoying, isn't it?

The DIY solution I proposed is for anything that lets you use a regular T-ring. In my case it works fine with my TRF-2008 reducer, my TSFlat2 flattener and even my F/6.3 reducer/corrector. However, it won't do with the Skywatcher correctors that have a 48mm thread and require a 55mm distance (so there has to be an M48 ring on the camera).

As for the directional filters, you missed the part where I show how to attach it in that case (the sensor distance increases slightly - but you can't have everything!).

[spillage]

Thanks for all the info provided. I think I will stick with 2" filters and for now will just get a uv/ir filter and in the future if/when I ditch the dslr for ccd then slowly purchase nb filters.

Can I ask if anyone has or tried or even compared the more expensive uv/ir filters compared to the £15 chinese ones available on ebay. Will I really notice a difference when using a cheap one with my dslr.

[Adam J]

3 hours ago, spillage said:

Thanks for all the info provided. I think I will stick with 2" filters and for now will just get a uv/ir filter and in the future if/when I ditch the dslr for ccd then slowly purchase nb filters.

Can I ask if anyone has or tried or even compared the more expensive uv/ir filters compared to the £15 chinese ones available on ebay. Will I really notice a difference when using a cheap one with my dslr.

I am very confused, what makes you think that you need a UV / IR filter for use with a IR modified 500D perticually when used with a reflector? You only really need a UV/IR at all on a DSLR if you have had a full spectrum mod done on the camera, but even at that point unless you are also using a coma corrector on the 150PDS you still dont need a IR/IR filter....

Edited February 15, 2017 by Adam J

[spillage]

I was under the impression on a post I read a while a go on here that the ir filter will filter out unwanted ir whilst allowing wanted ir onto the sensor. Sure it also mentioned it helps with processing the image.

Maybe it was because it lets all the Ha pass but reduces the ir.

Edited February 15, 2017 by spillage

[Waldemar]

1 hour ago, spillage said:

I was under the impression on a post I read a while a go on here that the ir filter will filter out unwanted ir whilst allowing wanted ir onto the sensor. Sure it also mentioned it helps with processing the image.

Maybe it was because it lets all the Ha pass but reduces the ir.

Correct. UV and IR are unwanted. IR will give you bloated stars.

[Adam J]

8 hours ago, Waldemar said:

Correct. UV and IR are unwanted. IR will give you bloated stars.

It gives bloated stars because of chromatic aberration in a refractor, OP lists a 150PDS which is a reflector and so chromatic aberration is not a problem UV and IR will still be brought to focus. Also unless he has removed both the filters from his astro-mod 500D then the front filter (also the sensor cleaning element) already acts as a UV IR cut filter, its not quite as harsh as a Baader replacement filter but it will work perfectly well with a 150PDS even if he is using a coma corrector.

Having modified my own cameras I can 100% grantee you that an additional UV/IR cut filter is totally redundant in a reflector / coma corrector combination.

The only exception to this is if both the filters where removed from the 500D as opposed to just the rear filter, this is called a full spectrum modification, in this case chromatic aberration would result due to the lens elements in the coma corrector as IR would be passed and detected way out to 950nm rather than just 720nm with the front filter intact. However, the OP states that his 500D is merely an IR mod in his signature.

Edited February 16, 2017 by Adam J

[Stub Mandrel]

It is rarely mentioned, but a shorter focal length scope of the same aperture  used with a smaller sensor with smaller pixels can be cheaper than going for large sensor and big filters.