LtE in CMO #276

From Kunihiko OKANO

® . . . . . . . . . Date: Sat, 02 Aug 2003 00:40:32 +0900

From: Kunihiko OKANO <>

To: Christophe Pellier <>


Subject: Re: Mars and filters


Dear Christophe Pellier


Thank you for your mail. Your images of Mars are superb.


 The sensitivity of CCD in the range of 400nm-450nm is very important to take images of clouds. If the sensitivity of CCD in this range is low, the effect of tilting may be insignificant. As you pointed out, the tilting of filters also results in the shift of IR leak band, therefore the IR blocker is always required, even if the B filter itself has IR block function.


 My latest idea is to use a plastic filter with an interference B filter. I am planning to use FUJI-film's Tri-Acetate filter SP-4 with the SBIG or IDAS B filter. Unfortunately, I did not test this idea yet because Tokyo is still in the rainy season.


 I attached the charts of band pass of SP-4 and SP-6. The transparency by SP-4 + interference B filter will attain 80% at the peak. The SP-4 may be the best for ST-7ME (high sensitivity in 400nm-450nm range), but the SP-6 will be recommended if the sensitivity in 400nm-450nm range is low, like my ST-5C camera.


 The FUJI's filters are widely available in Japan, but I don't know the availability in France. If you could test this idea, I would like to send you pieces (30mm×30mm) of SP-4 and 6 via air-mail. I am sure that the condition of sky of your observatory is much better than Tokyo.




® . . . . . . . . .Date: Tue, 05 Aug 2003 21:58:09 +0900

From: Kunihiko OKANO <>

To: Christophe Pellier <>


Subject: Re: Mars and filters


Christophe Pellier wrote:


> Dear Kunihiko Okano, many thanks for your answer! Sorry for the delay in my

> answer, I was a bit away those days.

> I'm interested to test your idea. The thing I don't understand is why you're

> thinking about using those Fuji filter in conjunction with a B interference

> filter, the transmission curves of the SP4 and SP6 look already quite good ?


This is because the SP-4 has a second passband in the range of 580nm-700nm, that is, the SP-4 is Magenta filter. Transparency of any Blue plastic filters is usually low (<50%). This is the reason why I use the SP-4 + interference blue.





NB: The Japanese edition of OKANO’s LtE includes more three emails written in Japanese to the Editor: The following are brief summaries.


® . . . . . . . . .Date: Wed, 06 Aug 2003 01:39:44 +0900

Subject: Mars by the use of ST-7ME


Dr OKANO tried to take pictures of Mars by the use of ST-7ME since the rainy season just ended in Tokyo. He believes that the filters IDAS-B+ Fuji SP-4 are fitted to the ST-7ME which is assumed to work sensitive to the range of the blue ingredient. He also added an example by the use of SBIG’s B for comparison. Tokyo’s atmosphere may however be not clear enough to bring a decisive difference. He assures that SBIG’s Planet Master proved to work quite well in choosing the images.




® . . . . . . . . .Date: Wed, 06 Aug 2003 22:28:37 +0900

Subject: Re: Mars by the use of ST-7ME


OKANO admits that the mist component in B becomes weaker if one chooses a good white balance concerning the spc because the widths of passband are divergent in R, G and B.


® . . . . . . . . .Date: Sat, 09 Aug 2003 02:00:10 +0900

Subject: Mars on 7 August 2003


OKANO reports that the transparency condition was good on the night of 7 August, so that the mist turned to be quite obvious in B (he used IDAS-III-B+Fuji SP-4). As such, the transparency is a decisive factor to the B images. Here he adopted the usual RGB method, because the LRGB destroies the very nuance of the white mist.


Kunihiko OKANO (Tokyo, Japan)




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